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Module 7 Transcript

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©2010 ASRT. All rights reserved. CT Basics: Module 7

CT Basics: Procedures Module 7

1. Title Screen Welcome to CT Basics Module 7 – Procedures. This module was written by Hal C. Davison, B.S., R.T.(R)(CT). 2. License Agreement

3. Objectives

After completing this module, you will be able to:

Properly position a patient and select appropriate scan parameters for common CT examinations.

Explain why different window widths and levels are selected.

List the required imaging planes for each procedure.

List the information that should be noted on each scout and scan image.

Review images for quality and accuracy to ensure no further imaging is necessary before the patient leaves the CT suite.

4. Common Examinations

This module will give you a basic understanding of common CT examinations so that you’ll feel comfortable starting your career in computed tomography. The examinations we’ll explore in this module are:

Routine head.

Routine neck.

Routine chest.

CT angiography (CTA) of the chest.

Routine abdomen and pelvis.

CTA of the abdomen and pelvis.

Routine thoracic and lumbar spine.

Routine extremity. Once you begin working in a CT department, you’ll see many more types of CT examinations and

discover that CT has a valuable role in diagnostic imaging. Remember as you work through this module that protocols vary from facility to facility. The procedures described in the module are conventional procedures for an average adult. Also, there are many manufacturers of CT scanners.

Depending on the vendor, different terms can mean the same thing. For instance, as you learned in Module 2, a scanogram, scout and topogram all refer to the same two-dimensional image that is acquired first and is used to set up the CT scan. Let’s get started on your first CT examination.

5. Routine Head Procedure

The head CT examination is ordered often and is performed in many clinical settings. Routine head CT exams without contrast are performed for various indications, such as headaches, intracranial hemorrhage, stroke and trauma. Contrast is injected to demonstrate the brain and its blood vessels. Exam indications requiring contrast include metastatic disease, aneurysm and arteriovenous malformation.


Begin by confirming your patient’s identity and obtaining verbal consent before the start of the examination. To ensure a quality exam, have the patient remove all radiopaque objects from the head and upper neck region, such as glasses, earrings, hair pins and hats. Next, tell the patient what to expect and be sure to instruct him or her to be very still for the duration of the examination.

6. Routine Head – Positioning

Position the patient head first and supine on the CT table with the arms crossed over the body. Once the patient is settled, make sure the patient’s head is straight to eliminate rotation of the midsagittal plane and tilting side to side. Next, have the patient tuck his or her chin toward the chest in a position that is not too difficult to maintain for the entire examination. This position helps bring the orbitomeatal line, or OML, parallel to the axial scan plane.

7. Practice Question 8. Routine Head – Positioning

Turn on the laser light on the CT gantry. The goal is to bring the patient’s head exactly to the center of the scan field. This is known as isocenter. Table height is positioned at the level of the external auditory meatus, or EAM, and the table is zeroed with the laser light at the patient’s mental point for a caudocranial scout protocol or top of the head for a craniocaudal scout protocol.

9. Routine Head – Scout Image

After ensuring that all patient identifiers, facility demographics and the exam protocol selection are correctly entered at the scanner console, begin with the scout or topogram image. The scout image, usually a lateral projection, must display anatomy from the inferior aspect of C1 through the vertex of the patient’s skull.

This lateral scout image or localizer helps the CT technologist to properly select slice levels, scan range and gantry angle. When looking at the lateral scout, you should check that your positioning was accurate. Check for rotation or tilt on the lateral skull image. Also make sure that you removed all radiopaque objects.

10. Routine Head – Scan Mode

As you recall from Module 3, the scanning modes are axial and helical. The routine head examination is acquired in the conventional or axial mode. Axial acquisition of data provides greater detail with less noise to better demonstrate delicate anatomy within the brain. Often, helical acquisition is needed for vascular cases requiring postprocessing or to minimize motion.

Image quality is somewhat compromised so that the examination can be completed quickly when necessary. Click on the buttons to see the different scanning modes in action.

11. Routine Head – Scan Field of View

Scanner platforms or the layout of protocol parameters vary because many systems are tailored for a facility’s preferences. CT technologists must be aware of all functions and workings of each selection, even though the specifics of exam protocols might not change often. There are times when you must adapt to a given situation to provide accurate, high-quality images.

The scan field of view selected is a head setting of 25 cm with a display or reconstruction field of view of 25 cm. If patient positioning is compromised, a scan field of view, or SFOV, of 50 cm is acceptable with a trade-off of image quality.

12. Routine Head – Scan Parameters


Slice selection should include anatomy from the foramen magnum through the vertex of the skull with the gantry angled parallel to the OML. Sometimes you won’t be able to angle the gantry correctly because of patient condition or situation, but tilt the gantry as much as the system allows. Technical factor selection for a routine head CT scan should remain constant for the majority of patients, with the exception of pediatric patients.

The selected kilovolt peak (kVp) is approximately 120, with the milliampere seconds (mAs) in a range of 200 to 600, depending on the scanner and its detector configuration. Each acquired slice is 5 mm thick, with the table moving in 5 mm increments between slices. Slice thickness often is reduced for trauma situations or facility protocol through the posterior fossa.

13. Routine Head – Pitch

For routine head examinations using a spiral scan mode, the pitch setting is applied as the patient travels continuously through the scanner and images are acquired. Pitch, the table speed per rotation, is increased for a single detector row configuration vs a multidetector configuration. Multidetector row pitch can be decreased because the same patient anatomy is imaged in a shorter period of time. Single detector row pitch may range from 1 to 1.7, and multidetector row pitch ranges from .7 to 1.0.

14. Routine Head – Algorithms

Image reconstruction algorithms or kernels are constant for most examinations. For the soft tissue of the brain, a standard algorithm is applied. The window width, or WW, range is 80 to 100, with the window level, or WL, set at 20 to 40. To visualize bony detail, a sharp or bone algorithm is applied. The WW range is 2000 to 2500, and the WL is set at 200 to 300.

15. Routine Head – Image Annotation and Archiving

It’s good practice to annotate additional data on the display, such as which series is precontrast or postcontrast. Other members of the health care team may view the images, and this information is helpful for those not accustomed to viewing CT images with or without contrast.

The routine head CT scan is acquired in the axial plane and, therefore, is filmed or archived in the axial plane for review and interpretation. As mentioned before, spiral acquisition with thin slices allows postprocessing in additional planes for advanced studies such as CTA of the intracranial vessels.

16. Routine Head – Contrast

Injection parameters for routine head CT examinations requiring IV contrast don’t vary much from facility to facility. Contrast is infused at 1 mL per second for a total of 100 mL, with an initial delay of 3 to 4 minutes. The brain acts like a sponge, and a slow delivery rate over a longer period of time is required.

17. Routine Head – Image Review

Once you’ve completed the scan, take a short time to review the images to ensure quality and completeness. You should check to make sure that you covered all anatomy, there are no motion artifacts and the patient was positioned for symmetrical imaging of the head. For accurate interpretation, the patient must not be rotated or tilted. See if you can identify the anatomy on this CT scan. Move your cursor over the image to identify important anatomy.

18. Practice Question

19. Routine Neck Procedure


This section introduces you to basic CT examinations of the neck. As you gain more experience in CT, you’ll see that multidetector row CT allows for angiographic studies of the neck, which now are performed in place of conventional angiography. In this module, we discuss exams of the neck that demonstrate all anatomy within the neck region.

20. Routine Neck – Positioning

To begin the examination, always confirm the patient’s identity and examination and make sure that verbal consent has been obtained. Next, remove all radiopaque objects in the head and neck region such as glasses, necklaces, earrings, hair accessories and removable dental work. Unless contraindicated, routine neck CT exams usually are performed with a contrast medium, so it’s important let the patient know what to expect when contrast is injected.


Position your patient on the CT table head first and supine, with the patient’s arms at his or her sides. Having the arms at the sides, versus crossed on top of the body, lowers the shoulders as much as possible in the scan range. Once the patient’s head is in the cradle or support device, instruct the patient to be very still for the duration of the scan. Also, tell the patient that he or she will be told not to breathe or swallow during the scan to help minimize motion. It’s also a good idea to inform the patient that it may seem like nothing is happening after you acquire the scout image, but you are busy at work setting up the scan and he or she must remain still.


Using the laser light, the table height is centered to the EAM and is zeroed at the top of the head for a craniocaudal scout. Once again, depending on the scanner vendor and departmental protocol, the table also could be zeroed at the mandibular rami, with the range extending superiorly 200 mm and inferiorly 200 mm from that zeroed point. In any case, the scout scan range must include the top of the head down through the midchest.

23. Routine Neck – Scout Image

Depending on departmental protocol, the topogram or scout for a neck CT scan often is acquired in two planes, an AP and a lateral. After the scout images have been acquired, it’s time to set up the scan. The routine neck CT is performed in a spiral scan mode to capture all the anatomy within the neck at uniform peak contrast enhancement. A 50-cm head scan field of view is used, with a display field of view, or DFOV, set at 22 to 25 cm. These settings can be adjusted according to patient size in certain situations.

24. Routine Neck – Scan Mode

The scan range or slice levels should be adjusted to include anatomy from the level of the EAM to the carina, with no angulation of the CT gantry. Streak artifacts are common when patients have permanent dental work. Additional scan groups of 7 to 10 slices around the teeth and mouth can be acquired to capture information that is lost because of the streak. Once again, departmental protocols are tailored specifically to radiologists’ preferences and may vary slightly.

25. Routine Neck – Scan Parameters

Technical factor selection for a routine neck CT scan are standard and only need to be altered for very large or very small patients. Remember that having the patient correctly centered within the scan field is vital, as many vendors use the scout images to establish technical factors. With dose


modulation enabled, the scanner automatically sets the mAs between 220 and 400, varying the quantity during the scan to deliver the proper amount.

You’ll notice a lower mAs setting with 4- to 16-slice helical scanners requiring longer tube rotation times and scan times. Higher mAs settings are registered with 64-slice helical scanners requiring shorter tube rotation times of 0.5 seconds. The inversely proportional relationship between mA and time also applies in CT scanning.

The kVp, or quality of the x-ray beam, should be set to 120. Increasing the kVp is acceptable for larger patients to help obtain quality images through the thick shoulder area.

26. Routine Neck – Pitch

Slice thickness generally is 2.5 to 3 mm, with the same spacing or increment. The pitch ranges from 0.7 to 1.2, depending on detector configuration (collimation) and how many detectors are sampled for image reconstruction. An increase in pitch results in more z-axis coverage but also decreases resolution. Generally, a good number to remember is in the 0.8 range, as radiologists want images with the greatest resolution. The algorithm or reconstruction filter should be set to standard or soft to best demonstrate all anatomy within the neck region, including blood vessels, muscles and nodes.

27. Routine Neck – Additional Parameters

The window width is set at 350 to 400, with the window level set at 40. The matrix is 512 x 512. Some scanners can display a matrix of 1024 x 1024 or 1,048,576 picture cells. It’s very helpful to add the annotations “with contrast” or “without contrast,” especially when imaging the neck. Scans are acquired and displayed in the axial plane.

28. Routine Neck - Contrast

The injection of contrast media is very important in scanning the neck region. Some departmental protocols may require both a precontrast and postcontrast scan to help differentiate tumors or abscesses from normal anatomy.

Injection parameters require a total volume of approximately 75 to 100 mL of contrast media, injected at a rate of 2 to 3 mL per second. Injection delays range from 35 to 80 seconds from the start of the contrast injection to the start of the scan. Helical scanning is very fast, so the goal is to avoid “out running” the contrast and acquire the scan with peak contrast enhancement, both venous and arterial. Longer delays are required for faster scanners. Some protocols include “steps downs” or phases with decreasing injection rates.

29. Routine Neck – Image Review

After the scan is complete, review the images for quality, ensuring that the contrast was delivered properly, no motion exists and delayed imaging is not needed. Be sure the patient tolerated the exam well and has had no adverse reaction to the contrast, then give him or her your facility’s discharge instructions. See if you can identify the anatomy on this CT scan. Move your cursor over the image to identify important anatomy.


31. Routine Cervical Spine Procedure

Computed tomography of the cervical spine is often ordered in trauma situations because of its ease on the patient and speed in difficult situations. By providing high quality images, CT will compliment plain film radiographs if something questionable is seen. CT provides a thorough look at each vertebra in the axial plane and can provide excellent full column reformats in the coronal and


sagittal planes. Indications may include trauma, degenerative disease, or post myelogram imaging. Many times patients are referred to CT when MRI cannot be performed for different reasons. 32. Routine Cervical Spine – Positioning

In trauma situations, be sure not to move the patient’s neck in any way and also have assistance moving the patient from the stretcher to the CT couch. A cervical collar must be placed prior to scanning the trauma patient. Prior to starting the exam, instruct the patient to be very still for just a few minutes. The patient may breathe normally, but they must not move their head, neck, or shoulders.

All radiopaque objects such as earrings, glasses, and necklaces must be removed prior to the start of the examination. Scan time is very fast with today’s scanner technology, and many times the technologist will want to check the images after reconstruction to be sure no motion is apparent. Motion during data acquisition will impact not only the axial reconstructions, but also the coronal and sagittal reformats.

Subtle fractures could be missed if motion is present. The patient enters the scanner head first with their arms at their side and extended as low as possible to lower the shoulders below the lower cervical region. 33. Routine Cervical Spine – Scout Image

For the cervical spine scout or localizer image, center the patient evenly on the table and then zero the scanner at the EAM for a protocol that will scan from superior to inferior. Raise the height of the table to a level in which the lateral laser light bisects the mid-coronal plane along the cervical region.

Some departmental protocols may include both an AP and lateral scout or localizer images to use to set up the scan. This will help to establish the smallest possible DFOV while centering the spine. Check to make sure no radiopaque objects, including removable dental work, are visible when the scout or localizer is complete.

34. Routine Cervical Spine – Scan Parameters

The scan mode is spiral with the scan field of view (SFOV) set to 50 cm and the display field of view (DFOV) set to 12 cm. The DFOV may vary depending on each patient and situation. No gantry angle is required with the scan range starting at the skull base and ending at T2 inferiorly. Check both the AP and lateral scouts to center the scan range side to side and anterior to posterior.

35. Routine Cervical Spine - Pitch

Except for pediatric and very large patients, technical factors usually are consistent. The kVp is set to 120-140 with the mAs ranging from 250-350 utilizing the auto mA or care dose function. Depending on scanner detector configuration, the collimation will vary from scanner to scanner and will be tailored to departmental preference.

Routine examinations of the cervical spine generally will require a reconstruction set to 2mm. A thinner set of axial slices should be used for reformatting into the coronal and saggital planes. Trauma protocols should reconstruct each slice at 1mm thickness and spacing to make sure no subtle fractures are missed. The pitch will range from .75 to 1.25.

36. Routine Cervical Spine – Additional Scan Parameters

Reconstruction algorithms or kernels will be set to standard (soft) and an additional set of bone or sharp reconstructions will be required for trauma exams. The matrix size is set to 512 X 512. Viewing the cervical spine in a standard algorithm requires a window width of 300-350 and window level of 40-50. The bone algorithm window width is 2000 with the window level set to 200.


Post processing into the coronal and sagittal planes are common in most departmental protocols. Once the scan is complete check the images to be sure no motion exists and all required levels have been imaged.

37. Routine Cervical Spine – Image Review

After the scan is complete, review the images for quality, ensuring that there are no motion artifacts and that all required anatomy has been visualized. See if you can name the anatomy on this CT scan. Move your cursor over the image to identify important anatomy.


39. Routine Chest Procedure

Another common CT examination is a scan of the chest. The chest or thorax CT can provide images of the lungs, their vasculature and structures within the mediastinum. Indications can include bronchiectasis, pulmonary embolism, a mass, pleural effusions, fever or nodules, to name a few.

40. Routine Chest – Positioning

To begin, always be sure to confirm the patient’s identity and examination, and ensure that verbal consent has been obtained. Next, remove all radiopaque objects in the chest and upper abdominal region, such as necklaces, brassieres and zippers.

41. Routine Chest – Positioning

Begin by positioning the patient on the CT table supine with the head entering the gantry first. Some institutions place the patient supine and feet first into the gantry because many other CT body exams require the patient to be positioned feet first. Having the patient feet first in the scanner has some advantages: the technologist can easily see the patient throughout the examination, the injector and contrast line can be positioned in a convenient location and some patients may feel more at ease. Both methods are acceptable and work well.

The patient should raise and extend his or her arms overhead in a comfortable position. Beam hardening artifacts occur if the arms are not raised over the head. Center the patient within the scanner using the laser light by raising the table height to a level that bisects the midcoronal plane.

Remember, this adjustment is important for scanners using dose modulation. Check to ensure the patient is not angled when lying on the table; the xiphoid tip and sternal notch can be used as references. Zero the scanner with the laser light at the top of the shoulders.

42. Routine Chest – Patient Instructions

Many chest CT examinations require the injection of contrast media, so it’s important to tell the patient what to expect when the contrast is injected. Before you leave the scan room, tell patients that they will be asked to hold their breath during the scan. With today’s faster scanners, imaging the chest in one breath hold is possible for most patients whose breathing is not compromised. Breath holds are done on full inspiration to fully expand the lung volume.

43. Routine Chest – Scout

Once the patient is registered in your system, begin with the scout or localizer image. The localizer is acquired in an AP or PA scan plane and should look very similar to an AP chest radiograph. This image should demonstrate anatomy from the top of the shoulders through the lung bases and into the upper abdomen.


A thorax scan is acquired in the spiral or helical scan mode, which allows all the anatomy to be captured in a single breath hold. Another benefit to spiral scanning is the ability to postprocess images into thinner slices if needed. Collimation settings that sample many detector channels are required to reconstruct thinner images.

44. Routine Chest – Scan Mode

Some CT thorax exams require a detailed look at the lung parenchyma and possible underlying conditions. A high-resolution chest study can provide these images. This requires a conventional or single-slice scan mode in which the CT technologist sets the scanner to take images at select locations, rather than the entire chest.

45. Routine Chest – Scan Parameters

Now that you have the localizer or scout image, you can set up the scan. The scan field of view is set to large or body at 50 cm. The display field of view varies depending of the patient’s size. Set the smallest possible DFOV to include all lung and soft tissue around the chest and axillary region. Use your scout or localizer to narrow the DFOV when appropriate.

46. Routine Chest – Scan Parameters

Place your scan lines to start above the lung apices and extend inferiorly through the lung bases into the upper abdomen, stopping at the upper poles of the kidneys. No gantry angle is required. If your departmental protocol includes a lateral scout, fine tune your scan lines anterior to posterior to ensure a centered image. These adjustments should not replace properly center the patient within the scanner.

Technical factor selection is important for image quality and especially for radiation dose. The kVp is set to 120 for patients of average size. The kVp can be adjusted up for large patients or if a patient cannot raise his or her arms out of the scan range. Pediatric patients require lower kVp. The mAs usually is set at between 180 and 250. As mentioned before, the dose modulation should be enabled and used for routine chest CT examinations.

47. Routine Chest – Pitch

Standard image acquisition is set to a 5-mm slice thickness, with a 5-mm interval or spacing, avoiding any gap in the lung anatomy imaged. The pitch varies depending on scanner type and collimation selected, and should be within a range of 0.8 to 1.35. The reconstruction algorithm is set to standard. If high-resolution images are required, select a bone or sharp-detail algorithm. Matrix size is generally 512 x 512.

48. Routine Chest – Additional Scan Parameters

Two different window widths and window levels are used when viewing a chest CT exam. A WW of 1500 and a WL of -600 display the lungs and their air-filled spaces, while a WW of 350 and the WL of 40 display the mediastinal and other soft-tissue structures. Many scanner vendors have window width and window level presets that vary slightly.

Departmental protocols differ, but be sure to annotate whether or not contrast was injected for display purposes. The routine chest CT scan is acquired in the axial plane and, therefore, is filmed or archived in the axial plane for viewing and interpretation.

49. Routine Chest – Contrast

If indicated, injected contrast media is very helpful in differentiating between normal and abnormal anatomy. An injection delay of 20 to 30 seconds is required to allow the contrast to circulate from the venous to the arterial blood system. Common injection flow rates are 2 to 3 mL per second.


Check the IV access to ensure it is adequate and can handle the rate. Many facilities allow slower injection rates for patients with limited venous access.

50. Routine Chest – Image Review

After the scan is completed, review your images for quality. Check to ensure that the patient held his or her breath, no motion is visible, all anatomy is included and contrast is patent in the chest’s vascular structures. See if you can identify the anatomy on this CT scan. Move your cursor over the image to identify important anatomy.


52. CT Chest Angiography

Many patients arrive in the emergency department with sudden, sharp chest pain on one side and shortness of breath. Or, a postsurgical inpatient may experience shortness of breath that correlates with a low oxygen saturation rate. These scenarios are examples of when a CTA study of the lung vasculature may be ordered to rule out a pulmonary embolism, or PE.

CTA is frequently done instead of conventional angiography because it is quick and less invasive. Current CT scanners can acquire thin-section images in a short time following a bolus injection of contrast media. In addition, CTA for PE is very sensitive.

53. CTA – Chest

This section of the module also discusses CTA procedures for the thoracic aorta. CTA of the thoracic aorta may be clinically indicated to rule out a dissection due to trauma, for surgical mapping or for aneurysm evaluation. Although CTA of the chest and CTA of the thoracic aorta both image the chest area, they require different scan protocols to capture the contrast at its peak enhancement within the vessels of interest.

54. Chest CTA – Positioning

As with any CT exam, you should: Confirm the patient’s identity. Ensure that the patient is dressed appropriately for the exam. Remove radiopaque objects within the scan region of interest. Obtain verbal consent. Answer the patient’s questions and explain exam instructions. Screen for contraindications to contrast. Check lab values. Make sure that IV access is available for contrast injection. And confirm that the patient has been fasting for at least 4 hours.

Positioning and centering for this exam are the same as for the routine chest CT. The patient lies supine and enters the gantry feet first while extending his or her arms overhead in a comfortable position. Center the patient within the scanner using the laser light by raising the table height to a level that bisects the midcoronal plane.

Remember, this adjustment is important for scanners using dose modulation. Check to ensure the patient is not angled when lying on the table; the xiphoid tip and sternal notch can be used as references. Zero the scanner with the laser light at the top of the shoulders because the localizer image scans the patient in a craniocaudal direction beginning at this location.

55. Chest CTA – Contrast

Once you have completed preprocedure screening for contrast media and the patient has been informed of the common side effects associated with contrast, the IV must be evaluated closely before starting the scan. Simulate the injection with a syringe of saline flush to make sure it will hold throughout the injection. This exam requires rapid injection of all the contrast media, so a proper IV line is critical.


Ideally, a large-bore IV catheter such as an 18 gauge should be placed in the patient’s right antecubital vein. This placement allows a bolus of contrast to reach the right side of the patient’s heart quickly, before traveling to the lungs. Finally, tell patients that they will have to hold their breath on full inspiration.

For patients with compromised breathing, instruct them to hold their breath as long as possible and then let it out very slowly when they must. If a patient can’t perform a breath hold, tell him or her to breathe very slowly and shallowly during the scan.

56. Chest CTA – Scout

Once the patient is registered in your system, you can begin with the scout or localizer image. The scout should demonstrate anatomy from the top of the shoulders through the lung bases and into the upper abdomen. CTA of the chest is acquired in the spiral or helical scan mode, which allows all the anatomy to be captured in a single breath hold. A 16- or 64-slice CT scanner can scan the chest in 10 to 12 seconds.

57. Chest CTA – Scan Parameters

After acquiring the localizer or scout image, you can set up the scan. The scan field of view is set to large or body at 50 cm. The display field of view varies depending on the patient’s size. Set the smallest possible DFOV to include all lung and soft tissue around the chest and axillary region. Use your scout or localizer to narrow the DFOV for each patient. The scan range is set from the top of the lung apices through the lung bases.

Some CT departments have protocols that scan inferiorly to superiorly. With slower scanners, this is necessary to catch the distal pulmonary artery branches at peak enhancement so as not to miss smaller emboli. No gantry angle is needed for this exam. Technical factor selection is built into each protocol, but sometimes adjustments are necessary. The kVp is set to 120, with the mAs varying from 180 to 250 using the dose modulation function. Once again, centering within the gantry is very important to ensure the patient receives the proper amount of radiation for a quality scan.

58. Chest CTA – Section Thickness and Pitch

The section thickness must be very thin, with no spacing between sections in order to image possible small pulmonary emboli. A maximum of 2-mm slice thickness with a 2-mm interval is ideal. However, some radiologists prefer image overlap to prevent any missing information. In this case, scanning at 2 mm with 1-mm spacing is sufficient and also provides additional reconstruction options. The CTA protocol for the thoracic aorta uses very similar technical factors and slice thickness. The detector configuration or collimation may vary from scanner to scanner, but reconstructing thin images that overlap allows quality 3-D and maximum intensity projection, or MIP, postprocessing.

The pitch varies depending on the scanner type and detector configuration, but should be within a range of 0.75 to 1.35, with an ideal pitch less than or near 1.0 for the highest quality. If your departmental protocol requires a CTA exam of the entire aorta through the bifurcation in the pelvis, the pitch may need to be adjusted higher to compensate for increased anatomical coverage. The reconstruction algorithm is set to standard or soft. Matrix size is generally 512 x 512.

59. Chest CTA – Additional Scan Parameters

Two different window widths and window levels are used when viewing a chest CT exam. A WW of 1500 and a WL of 600 displays the lungs and their air-filled spaces. To visualize the opacified vasculature of the lungs and the aorta, a WW of 500 and WL of 60 provides optimum contrast.

60. Chest CTA – Labels


When setting up the scan, label or annotate the different series within an entire exam, such as precontrast, postcontrast or delayed images. This is very helpful for CT exams that require different scans at specific time intervals following contrast injection. Postexam processing varies among departments, depending on radiologist preference.

For a CT scan investigating suspected pulmonary embolism, it’s common to include the axial set filmed in both the lung and mediastinal windows and an oblique series of each pulmonary artery and lung.

61. Chest CTA – Postprocessing

CTA of the thoracic aorta requires more postprocessing of data to accompany the axial set. A common protocol includes an AP, lateral and oblique sagittal MIP of the thoracic aorta, as well as 3-D rendering of the aorta.

62. Chest CTA – Timing

You’re ready to image the entire chest through the lung bases, but now comes the most important part in the protocol. You want to scan the intended vasculature at peak enhancement, but how do you know when to start the scan after injection? In the past, scans were started after an average delay following the injection. This technique still is used sometimes today. However, it may not work for all patients because of differences in cardiac output or IV size and location.

Injected contrast media takes different times to reach peak enhancement in the anatomy of interest. Vendors of multidetector row CT scanners have developed a software program that triggers the start of the scan automatically, eliminating the guesswork in scan delays.

With CTA, you get one chance to scan after the injection, and the blood vessels of interest must be fully enhanced. The scan starts once the Hounsfield (HU) unit or CT number reaches its threshold within the corresponding vasculature enhanced with contrast media. The CT technologist must properly select and scan one slice at a predetermined level. This predetermined level often is on or very close to the blood vessel or vessels of interest. For a proper angiography study, the scan should start as soon as the contrast is most concentrated and dense

63. Chest CTA – Timing

The bolus tracking mechanism has different names, depending on the CT scanner model and vendor. Names include Smart Prep, Bolus Tracking, CARE Bolus, or Sure Start. The image used to track or monitor the contrast is taken after the scout or localizer image and before the scan. The scan delay hinges on this monitoring phase.

64. Chest CTA – PE Study

For a pulmonary embolism study, the region of interest (ROI) monitoring or bolus tracking location should be the main pulmonary artery, as shown in the image on the left. The image on the right displays the incorrect location for the monitor.

A threshold of 90 to 100 HU must be met to trigger the start of the scan with an injection delay post-threshold of 3 to 5 seconds. Inject 75 to 100 mL of contrast, followed immediately by 30 to 50 mL of saline administered at 4 mL per second.

65. Chest CTA – Thoracic Aorta

For a thoracic aorta study, the ROI monitoring or bolus tracking location should be the ascending aorta, as shown in the image on this page. A threshold of 100 to 150 HU must be met to trigger the start of the scan, with an injection delay post-threshold of 3 to 5 seconds. Inject 100 to 125 ml of contrast, followed immediately by 30 to 50 mL of saline administered at 4 mL per second.


66. Chest CTA – Image Review Once you have completed the study, check the patient to make sure he or she feels well

following the contrast injection. Next, review the images to ensure that the vasculature is opacified, all anatomy is imaged and no motion is visible. See if you can identify the anatomy on this CT scan. Move your cursor over the image to identify important anatomy.


68. Routine Abdomen/Pelvis Procedure

The abdomen and pelvis CT examination is ordered frequently, and you’ll quickly become efficient at performing it. Abdomen/pelvis CT scans are performed on outpatients and inpatients, as well as on an emergency basis. Indications include diverticulitis, abdominal pain, trauma or a mass. The examination can evaluate many organ systems in just one scan.

With the aid of IV and oral contrast, all the major organs, blood vessels, bowel and mesentery within the abdomen and pelvis can be imaged. Physicians turn to abdomen/pelvis CT scans for many answers.

69. Routine Abdomen/Pelvis – Positioning

To begin the examination, always confirm the patient’s identity and the examination and ensure that verbal consent has been obtained. Next, remove all radiopaque objects that lie within the lower chest, abdominal and pelvic regions, such as brassieres, belt buckles and zippers. Position the patient supine and feet first entering the CT gantry.

Once again, some facilities scan the patient entering the scanner head first. The patient’s arms need to be raised overhead to ensure they do not interfere with the anatomy to be scanned and degrade image quality.

70. Routine Abdomen/Pelvis – Positioning

For the scout or localizer image, demonstrate the anatomy that needs to be covered to set up your scan. For an abdomen CT only, without the pelvis, use the axilla and iliac crest landmarks as positioning aids for starting and stopping the scan. Starting a few inches below the axilla ensures that the top of the liver is included in the scout. If the order includes the pelvis order, be sure the patient will travel through the scanner past the symphysis pubis.

Inform patients that they will be required to hold their breath on inspiration and that they must not move between the scout and the actual scan. Some institutions prefer patients to hold their breath on expiration instead of inspiration. Center the patient within the scanner using the laser light by raising the table height to a level that bisects the midcoronal plane.

Finally, zero the scanner with the laser light positioned just a few inches below the axilla because the patient will travel in the craniocaudal direction.

71. Routine Abdomen/Pelvis – Scout

The resulting AP scout or localizer should demonstrate an image that looks like an abdomen radiograph. Check to make sure the patient is straight on the table, no adjustments need to be made and all radiopaque objects have been removed.

72. Routine Abdomen/Pelvis – Scan Parameters


The scan mode is spiral, because the abdomen and pelvis can be scanned in one breath hold of approximately 13 to 15 seconds with today’s faster scanners. If the patient can hold his or her breath for this short length of time, misregistration of anatomy and motion are eliminated. Be sure your patient is prepared and properly coached before the exam as you need his or her help to obtain high-quality images.

The scan field of view is set to large or body at 50 cm. The display field of view is patient dependent and varies based on the patient’s size. Set the smallest possible DFOV to include all the abdominal cavity and soft tissue. The scout or localizer is used to narrow the DFOV for each patient. It also is a good idea to use the same DFOV each time a patient returns for a CT scan.


For an abdomen CT, set your scan range to start above the dome of the liver and end at the iliac crest. This range should ensure that all major abdominal organs are included. If the order also includes the pelvis, extend your scan lines through the symphysis pubis, which will include the entire bladder and rectum. Slight protocol variations are common from department to department.


No gantry angle is needed for this exam. The kVp should be set at 120 but can be lowered for pediatric patients. The average mAs used is between 200 and 250. Slice thickness should be set at 5 mm with an interval also set at 5 mm.


The pitch for a routine abdomen/pelvis CT scan should be set between 0.56 and 1.2. Standard or soft algorithms should be used, depending on the anatomy of interest. A window width set to between 350 and 400 is recommended, with a window level between 40 and 60. The matrix size for a routine abdomen/pelvis CT is generally 512 x 512.

76. Routine Abdomen/Pelvis – Annotation

Annotate each series, indicating without contrast, with contrast or delayed scans. There are protocols that include multiphase scans through certain organs, such as the liver or kidneys, at specific postcontrast time intervals. This following section reviews scan information for a routine abdomen/pelvis CT. However, there are many protocols tailored specifically for certain abdominal organs that won’t be discussed in this module.

77. Routine Abdomen/Pelvis – Abdomen Reconstruction

The abdomen CT scan is viewed in an axial plane of 3- to 5-mm thickness. Reconstructing thinner slices through certain organs such as the adrenal glands may be included in your facility’s protocol. Because of the ease of use of current scanner platforms and postprocessing workstations, coronal and sagittal reconstructed images often are included with the axial set.

78. Routine Abdomen/Pelvis – Contrast

Oral and IV contrast agents commonly are used for abdomen/pelvis CT examinations. Oral contrast helps to opacify the stomach and gastrointestinal tract, just as in GI studies performed with fluoroscopy. The patient should drink the oral contrast before arriving for the exam to allow the entire GI tract to be fully opacified. Some departmental protocols include additional swallows of contrast just before scanning to fill the stomach and duodenum.



If IV contrast is indicated and the patient is cleared to receive it, an injection is administered after the localizer or scout image is obtained and the scan is set up. IV contrast enhances blood vessels and organs within the abdomen and commonly is injected at 2 to 3 mL per second for a total of 100 to 150 mL.

Modified protocols are required for pediatric patients and patients with decreased renal function requiring low osmolar or nonionic contrast. The total dosage given to pediatric patients is based on their weight.


A delay following the injection of contrast is needed to allow the contrast to circulate and enhance the anatomy of interest. Routine abdomen/ pelvis CT exams require a delay of 65 to 75 seconds postinjection. If you’re scanning only the pelvis, a delay of 90 seconds is adequate.

81. Routine Abdomen/Pelvis – Image Review

After the scan is complete, be sure to check on your patient and review images for quality. This check should be done to make sure no motion is visible, all the anatomy is covered, the timing of contrast-enhanced imaging was correct and delayed imaging is not required. See if you can identify the anatomy on this CT scan. Move your cursor over the image to identify important anatomy.


83. Abdomen/Pelvis CT Angiography

In recent years, CT has evolved to become a reliable tool for evaluating vasculature within the abdomen and pelvis. As mentioned previously, multidetector row CT is replacing conventional angiography. Postprocessing of raw data into multiplanar reconstructions and reformats provides excellent images for radiologists and surgeons. CTA of the abdomen and pelvis is used for imaging the aorta, celiac truck and the hepatic, renal, mesenteric, iliac and femoral arteries, along with their associated organs.

84. Abdomen/Pelvis CTA – Contrast

Successful CTA scans depend on contrast media and the timing of scanning after contrast administration. Individual patient variables include the size and location of the IV and the patient’s cardiac output. However, with bolus tracking software, the scans should be successful regardless of these factors.

The concept and application of the software are identical to the PE chest CT exam previously discussed in this module. The software helps acquire high-quality images by scanning at exactly the right moment when the vessel of interest is full of contrast and at peak enhancement.

85. Abdomen/Pelvis CTA – Positioning

Positioning and set up for CTA are identical to the routine abdomen/pelvis exam already discussed. The patient enters the CT scanner supine and feet first with arms extended overhead. Ideally, a large-bore IV catheter such as an 18 gauge should be started in the right antecubital vein.

Keep the patient’s arms straight and extended, using support if necessary, so as not to impede the flow of contrast. Inform the patient that the contrast will be injected at a high rate and he or she may be uncomfortable for a short time.

86. Abdomen/Pelvis CTA – Scan Parameters


The AP scout or localizer image should demonstrate anatomy from the midchest through the symphysis pubis. The scan mode is spiral, with the scan field of view set to body or 50 cm. The display field of view is patient dependent and varies with the patient’s size. Set the smallest possible DFOV to include the entire abdominal cavity.


Even though the scan may target specific vasculature or organs, the scan range still must cover all anatomy included in the protocol or as indicated. If an abdomen CTA is ordered, cover the anatomy from just above the dome of the liver to the iliac crest. If the entire aorta needs to be scanned, the range should start just above the liver and extend down through the symphysis pubis.

Occasionally, only the pelvis is needed; in that case, the range starts just above the iliac crest and ends at the ischial tuberosities. Again, departmental protocols vary and may be slightly different from what is mentioned in this module.


CTA requires no angulation of the gantry. Postprocessing allows reformatting from many angles if needed. The kVp is set at 120 with the mAs ranging from 225 to 275 for most average patients and for equipment using dose modulation. The collimation or detector configuration varies depending on the scanner and how many channels it’s equipped with.

Most protocols use all possible channels, so the reconstruction thickness can be very thin with overlap for high-quality reformatted images and 3-D postprocessing. Common axial reconstructions for viewing and archiving are set at 2-mm thicknesses with 1-mm spacing. The pitch varies depending on the scanner type and detector configuration, but should be within a range of 0.75 to 1.35, with an ideal pitch of less than or near 1.0 for the highest quality.

89. Abdomen/Pelvis CTA – Image Reconstruction

The reconstruction algorithm is set to standard or soft, with a matrix size 512 x 512. Ideal window widths range from 550 to 700, with window levels between 40 and 70. Annotate precontrast scans as well as all postcontrast series and any specific delay series obtained. CTA exams have hundreds of images and different series postcontrast and with postprocessing, so being descriptive is vital.

90. Abdomen/Pelvis CTA – Postprocessing

Scanning the patient is just the first half of the examination these days in CT. Postprocessing the raw or scan data follows departmental and standard protocols. Common imaging planes to store in the picture archive and communication system, or PACS, include axial images (either at 2-mm or 5-mm slice thickness); images reformatted into the coronal, sagittal and oblique planes; and 3-D volume rendering. Even though you acquire the scan using the smallest detector channels, it’s not necessary to send all those images through for interpretation. Thin overlapping slices are needed for quality postprocessing. Individual departmental protocol will dictate specifics.

91. Abdomen/Pelvis CTA – Timing

The most important parts of the CTA exam are the contrast media and the timing of the scan. The aorta is the largest artery in the body, carrying blood at a high rate of speed. Sending a bolus of iodinated contrast media through the aorta means you get one chance to catch it at the right moment. Until recently, average delay times were used to start the scan at the anatomy of interest.

This practice led to both good and poor studies and was not always reliable. Remember, contrast cannot be injected continuously or reinjected into patients because of the effect it has on the


kidneys. Different CT vendors developed software that helps the CT technologist scan patients at just the right time.

This bolus tracking mechanism monitors the contrast injection and triggers the scanner to start when the vessel of interest reaches a predetermined contrast threshold. To do this, the technologist takes one slice or image approximately 2 cm above the vessel or organ of interest. Using this slice, the technologist places the ROI within the descending aorta.

Trigger thresholds range from 125 to 150 HU. Once the aorta enhances with contrast and reaches the trigger threshold, the patient is moved into position and the scanning starts after a delay of 3 to 5 seconds. Most of this delay is accounted for by the table automatically moving the patient a short distance.

To catch a large-diameter artery at peak enhancement in a , the contrast media needs to be injected at a rate of 4 mL per second for a total of 100 to 125 mL. For scanners equipped with a dual head injector, the contrast is followed by 30 to 50 mL of saline to help reduce the total amount of contrast used and to decrease beam hardening artifacts. This extra bolus of saline also helps push the contrast farther along in the circulation.

92. Abdomen/Pelvis CTA – Image Review

After the scan is complete, check on the patient to make sure he or she tolerated the exam and contrast injection well. Review the images before postprocessing to ensure the aorta and associated vessels are fully opacified and all anatomy was covered without motion. See if you can identify the anatomy on this CT scan. Move your cursor over the image to identify important anatomy.

93. Practice Question 94. Routine Thoracic and Lumbar Spine Procedures

Computed tomography of the thoracic and lumbar spine is a great complement to plain-film radiographs. CT provides a thorough look at each vertebra in the axial plane and can supply excellent full-column reformatted images in the coronal and sagittal planes. Indications for these examinations include trauma, degenerative disease and postmyelogram evaluation.

95. Routine Thoracic and Lumbar Spine – Positioning

Before starting the exam, give the patient brief instructions on what to do for a successful examination. The patient must remain perfectly still until the CT technologist re-enters the room. The scan time may only take 10 to 15 seconds with today’s faster scanners, but the patient must not move until all images are checked.

For both the thoracic and lumbar spine exams, the patient enters the scanner feet first and supine with arms extended overhead. Important anatomical landmarks used for positioning include the top of the shoulders, iliac crest, xiphoid process and the symphysis pubis.

96. Routine Thoracic and Lumbar Spine – Positioning

For the thoracic spine scout image, center the patient by zeroing the scanner at the top of the shoulders. The protocol should stop the scout at the top of the iliac crest. Be sure the patient is straight and centered on the scanner. Raise the height of the table so that the lateral laser light runs along the posterior one-third of the body.

97. Routine Thoracic and Lumbar Spine – Scout

The lumbar spine scout is set to start at the xiphoid process and end at the symphysis pubis. This setting should provide extra vertebrae to count to ensure proper level selection. Many departmental


protocols include both AP and lateral scout or localizer images to set up the scan. These images help establish the smallest possible display field of view while centering the spine.

For an exam requiring both the thoracic and lumbar regions, the scout parameters should start above the top of the shoulders and extend through the symphysis pubis. Check to make sure no radiopaque objects are visible on the scout or localizer image.

98. Routine Thoracic and Lumbar Spine – Scan Parameters

The scan mode is spiral, with the scan field of view set to 50 cm or head. The display field of view varies, with some departments requiring the same DFOV for all patients. Except for pediatric patients and those with tall, large builds, most patients’ vertebrae are very similar in size. A good DFOV range is 12 to 20 cm. No gantry angle is required for a thoracic scan starting at C7 and ending at L1. The lumbar scan range starts at T12 and ends at the bottom of the sacrum. Check both the AP and lateral scout images to center side to side and anterior to posterior.

99. Routine Thoracic and Lumbar Spine – Scan Parameters

Except for pediatric and very large patients, technical factors do not vary. The kVp is set at 120 to 140, with the mAs ranging from 250 to 350 using the dose modulation function. Slice thickness is set at 2 to 3 mm, with the spacing or indexing at 2 to 3 mm. A thinner set of axial reconstructions should be used for reformatting into the coronal and sagittal planes.

Trauma protocols may be set to acquire the scan at 1- to 2-mm slice thickness and spacing to make sure no subtle fractures are missed. The pitch ranges from 0.75 to 1.25.

100. Routine Thoracic and Lumbar Spine – Algorithms

Reconstruction algorithms or kernels are set to standard, or soft, and an additional set of bone or sharp may be requested. Some departments use the bone algorithm for trauma indications only. The matrix size is set to 512 x 512. Viewing the thoracic or lumbar spine in a standard algorithm requires a window width of 350 to 400 and window level of 40 to 50.

The bone algorithm window width is 2000, with the window level set to 200. Keep in mind that some departments vary the window width and window level setting to suit radiologists’ preferences.

101. Routine Thoracic and Lumbar Spine – Annotation

For each series saved, annotate helpful information such as specific levels, reconstruction algorithms or imaging planes. The additional information helps physicians or clinicians who are not familiar with CT imaging. Postprocessing into the coronal and sagittal planes is standard in most departmental protocols.

102. Routine Thoracic and Lumbar Spine – Annotation

IV contrast is rarely, if ever, used for spine imaging. However, many patients who undergo myelograms have CT scans afterwards. Once the CT scan is complete, check the images to make sure no motion is visible and all required levels have been imaged. See if you can identify the anatomy on this CT scan. Move your cursor over the image to identify important anatomy.


104. Routine Extremity Procedure

Scanning extremities, joints and bones is becoming common in CT because multidetector scanners produce images with superb detail and high contrast resolution. And, CT imaging is easy for the


patient because the scan time is very short. Frequently, extremity CT scans are ordered to follow up plain films or for surgical planning purposes.

This section of the module discusses scanning the ankle joint and associated bones as an example of CT extremity imaging. Exams and protocols tailored to specific bones and joints vary somewhat from facility to facility.

105. Routine Extremity – Positioning

For this exam, the patient enters the gantry supine and feet first. The ankle to be scanned remains extended and straight while the opposite leg is bent up and out of the scan field. Before scanning, move the patient slowly through the scanner to make sure the bent knee doesn’t hit the gantry. Support and immobilize the leg, ankle or both with weights or other devices to prevent the patient from moving during the scan.

Also, center the affected extremity on the table as much as possible by shifting the patient slightly to the opposite side. Tell the patient that he or she must remain perfectly still throughout the examination. Check to make sure that the patient is as comfortable as possible before starting.

106. Routine Extremity – Scout

Once the extremity is positioned, center it within the gantry using the distal tibia, fibula and calcaneus as anatomical landmarks. The scout image parameters must show the distal tibia and fibula through the bottom of the foot. Most protocols have both the AP and lateral scouts built in. These two scout images should resemble plain-film AP and lateral radiographs of the ankle.

The scan mode is spiral, with the scan field of view set to 25 cm and the display field of view set near 12 cm or as close as possible to the ankle to include soft tissue. Once again, departmental protocol may have an established DFOV for extremity exams.

107. Routine Extremity – Scan Parameters

The scan should be set up to collect axial data from the distal tibia and fibula through the calcaneus. In some cases, indications may dictate different starting and stopping points. No gantry angle is needed for this exam, and the matrix size is set to 512 x 512. The kVp is set at 120 for pediatric patients and 140 for adult patients, with the mAs ranging from 180 to 220.

Slice thickness needs to be very thin with 50% overlap for the greatest detail and smooth reformatting. For example, reconstruction thickness of 1.25 mm with an interval of 0.6 mm provides quality images for interpretation. A thinner set of reconstructions in the axial plane of 0.75 mm by 0.4 mm are used for postprocessing reformats only.

108. Routine Extremity – Scan Parameters

The pitch varies somewhat, depending on the scanner and the number of detector rows, but should range from 0.5 to 1.0. Using more detector rows results in more anatomy covered for each tube rotation. Both bone and soft tissue algorithms or kernels are used for viewing each series.

The bone algorithm series uses a window width of 2000 and window level of 200. The soft tissue algorithm window width is set to 400 and window level is set to 40. Indicate left or right extremity on the images for clarity.

109. Routine Extremity – 3-D Reconstruction

Extremity computed tomography images can be reformatted in the coronal and sagittal planes during postprocessing. In addition, CT technologists can create a 3-D reconstruction of the image data set so that the anatomy of interest can be moved in multiple planes and rotated as an independent object. Use the slider bar on this animation to rotate the reconstructed image data.


110. Routine Extremity – Image Review Postprocessing commonly includes coronal and sagittal reformatted images to accompany the

axial scans. No contrast media is needed for this exam. When the scan is complete, make sure no motion is visible and all anatomy was covered. See if you can identify the anatomy on this CT scan. Move your cursor over the image to identify important anatomy.

111. Practice Question 112. Section Review

If you would like to review any of the specific sections covered in this module, click the corresponding button on this page.

113. Conclusion

This concludes CT Basics Module 7 – Procedures. You should now be able to

Properly position a patient and select appropriate scan parameters for common CT examinations.

Explain why different window widths and levels are selected.

List the required imaging planes for each procedure.

List the information that should be noted on each scout and scan image.

Review images for quality and accuracy to ensure no further imaging is necessary before the patient leaves the CT suite.

114. Bibliography

Bushberg JT, Seibert JA, Leidholdt EM Jr, Boone JM. The Essential Physics of Medical Imaging. 2nd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2002.

Bushong SC. Computed Tomography. New York, NY: McGraw-Hill; 2000. Curry TS III, Dowdey JE, Murray RC Jr. Christensen’s Physics of Diagnostic Radiology. 4th ed.

Philadelphia, PA: Lea & Febiger; 1990. Fishman EK, Jeffrey RB Jr. Spiral CT Principles, Techniques, and Clinical Applications. New York,

NY: Raven Press; 1995. Hofer M. CT Teaching Manual. 3rd ed. New York, NY: Thieme Medical Publishers; 2008. Kelley LL, Petersen CM. Sectional Anatomy for Imaging Professionals. St. Louis, MO: Mosby;

1997.

115. Development Team 116. Final Slide

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