*Multi-detector CT Cerebral Angiography.

Ehab Abou Elfotouh Elaryan. Lecture Of Radio-diagnosis.

Al-Azhar University.

In recent years, rapid advances in computed tomographic (CT) technology and image postprocessing software have been made.

CT angiography was improved substantially by increasing scan speed and decreasing section thickness and emerged as a powerful tool in neurovascular imaging.

Nowadays, spiral CT systems with acquisition capabilities of up to 64 sections per gantry rotation are introduced in clinical practice.

Assessment of vascular studies based on axial images alone is not straightforward; two-dimensional (2D) and three-dimensional (3D) visualization methods are routinely used to obtained images comparable to those acquired with catheter angiography.

Multi-detector CT Angiography:

Optimal image quality depends on two factors:

A- CT angiography technique (scan protocol, contrast material injection protocol, image reconstruction methods).

B- Data visualization technique (image post-processing).

Multi-detector CT Angiography:

A- Technique: Scan speed : For evaluation of the basal intracranial arteries, a scan

range of approximately 100 mm needs to be covered.

Examination of the whole length of the carotid arteries from the aortic arch to the circle of Willis requires a scan range of approximately 250 mm.

The arterio-venous transit time in cerebral bed equal about 5 second.

Multi-detector CT Angiography:

A- Technique: Scan speed :

With four–detector row CT at a collimated section width of 1 mm, a pitch of 1.5, and a gantry rotation time of 0.5 second, the volume of cerebral artery can be covered in about 9 seconds. This is not fast enough to avoid venous overlay.

With 16–detector row CT at a collimated section width of 0.75 mm, a pitch of 1.5, and a rotation time of 0.5 second, the same range can be covered in 3 seconds, well beyond the arterio-venous transit time.

Multi-detector CT Angiography:

A- Technique: Scan speed: At examination of the whole length of the carotid

arteries from the aortic arch to the circle of Willis:

The scan time would be 21 seconds for four–detector row CT.

7 seconds for 16–detector row CT.

4 seconds for 64–detector row CT (64 × 0.6 mm, pitch of 1.3, 0.33-second rotation time).

Multi-detector CT Angiography:

Spatial Resolution: The smallest distance between two points in the object that can be

differentiated as separate details in the image, generally indicated as a length or a number of black and white line pairs per mm (lp/mm).

The small caliber of cervical and intracranial vessels requires the highest spatial resolution in all three dimensions.

In-plane spatial resolution is predominantly determined by detector geometry and the convolution kernel.

It is not substantially improved in scanners with increasing detector row numbers.

Multi-detector CT Angiography:

Spatial Resolution: The major advantage of more detector rows is higher through-plane

resolution by reducing the width of a single detector row from 1–1.25 mm (four–detector row CT) to 0.5–0.6 mm (64–detector row CT).

Typical in-plane resolution with application of a CT angiography protocol:

16 × 0.75-mm detector configuration, 120 kV, 100 mAs field of view of 120 mm, medium sharp convolution kernel is 0.6 mm and through-plane resolution is 0.7 mm.

64 × 0.6-mm detector configuration, 120 kV, 140 mAs field of view of 120 mm, medium sharp convolution kernel is 0.6–0.7 mm and through-plane resolution is 0.5 mm.

Multi-detector CT Angiography:

Contrast Material Injection: In order, to obtain high-quality CTA images, high

concentration of contrast in the vessels is necessary. Short scan times require short contrast material

injection. Technique-related factor: To deliver an appropriate

amount of iodine, injection rates of 4–5 mL/sec and highly concentrated contrast medium (iodine, 350–370 mmol/mL) are preferable.

Type of injection and volume of contrast material may also effect Ct contrast enhancement.

Multi-detector CT Angiography:

TYPE OF INJECTION: I- Intra-venous contrast agent administration, including

three methods: A- Fixed scan delay technique (15-45s). B- Test bolus injection technique. C- Automated bolus-tracking technique (Smart

Prep, CARE Bolus, and Sure Start).

*Individual timing of contrast material injection (bolus tracking or test bolus injection) is mandatory to take advantage of phase-resolved image acquisition.

Multi-detector CT Angiography:

Multi-detector CT Angiography:

Multi-detector CT Angiography:

TYPE OF INJECTION: I- Intra-arterial contrast agent administration: Invasive method. Performed with a combined angiography and CT

unite. High concentration of contrast material can be

obtained in intra-cranial arteries without consideration the appropriate timing of injection.

Need small amount of contrast material.

Multi-detector CT Angiography:

Image Reconstruction: To reduce image noise, images may be reconstructed slightly

thicker than the detector collimation, for example with a 0.75-mm section thickness from a data set acquired with 0.6-mm detector collimation.

Overlapping image reconstruction should always be performed to improve 3D post-processing.

The reconstruction algorithm influences the spatial resolution in plane.

The ideal algorithm would combine low image noise and sharp edge definition, maintaining good low-contrast resolution.

Multi-detector CT Angiography:

Image Reconstruction: Soft algorithm reduce image noise and allow

smooth surfaces with rendering techniques, improving the visualization of aneurysms and vascular malformations.

Sharper algorithm improve edge definition and reduce blooming effects from calcifications, necessary for stenosis measurements, at the expense of higher image noise.

Multi-detector CT Angiography:

Image Post-processing Techniques:

Several image processing techniques for CT angiography are currently being used clinically.

Image processing involves traditional operations such as:

A- Multi-planar reformation (MPR) . B- Maximum intensity projection (MIP). C- Surface and volume rendering associated with

bone subtraction.

Multi-detector CT Angiography:

Multi-planar Reformation ( MPR ):

MPR creates views in different planes without loss of original CT information.

Only 2D views can be generated. If the CT data meet the

requirements of isotropy, spatial resolution is similar to the original source images.

Both diameter reduction and area reduction can be measured, and no information is suppressed in the final image.

Multi-detector CT Angiography:

Multi-planar Reformation ( MPR ): A variant of MPR is curved

planar reformation. Curved planar reformation

provides a 2D image that is created by sampling CT volume data along a predefined curved plane.

This technique is employed to display tortuous structures.

Multi-detector CT Angiography:

Maximum Intensity Projection ( MIP ):

MIP images are created by displaying only the highest attenuation value.

The depth information along the projection ray is lost to visualize the spatial relationship of various structures.

The volume has to be rotated and viewed from different angles.

Multi-detector CT Angiography:

Multi-detector CT Angiography:

Volume Rendering: is a visualization technique that

creates a 3D impression and provides densitometric information.

Visualization of CT angiography data with volume rendering is based on transfer functions that map measured intensities to colors and opacities.

Opacity values on a spectrum from 0% to 100% (total transparency to total opacity) are assigned along artificial rays that pass through the data.

Multi-detector CT Angiography:

Volume Rendering: Separation of different tissue types (ie, bone, contrast-

enhanced vessels, soft tissue) can be performed and can be color encoded.

Multi-detector CT Angiography:

Multi-detector CT Angiography:

Multi-detector CT Angiography:

Multi-detector CT Angiography:

Multi-detector CT Angiography:

Multi-detector CT Angiography:

THANKS FOR ALL.