Computed TomographyAn overview of Radio-Physics.

Dr Sushil Patil. 1st Year, Dept of Radio-

Diagnosis, NSCB Med College,

Jabalpur.

Principle:

Internal structure of the object can be reconstructed from multiple projections of the object.

Tomography = Tomos ( slice) + graphein ( to

write)

Slit scan projection of Patient’s body

Digitizing Image received by computer

Reconstructing image using Mathematical Algorithms

History

Godfrey N Hounsfield(1970-71), Research eng. with Electro-Musical Instrument Ltd.(EMI): Invention of Computed Tomography

Godfrey Hounsfield & Cormack…1979 Nobel prize for Medicine…For Discovery of CT.

CT Generations

• Time reduction is predominant reason for introducing newer generations.

Changes down the Generations: Fan shaped beam Increasing No of detectors Ring of detectors.

1st Gen CT ScannerOriginal EMI unit

• Linear scan and Rotate 1°rotation and 180 linear scans i.e. one linear scan

Gantry rotation by 1°

Again linear scan• Pencil like X-ray beam.

• Paired Detectors.

2nd Generation

Fan shaped beam Linear scan and Rotate i.e. 30⁰rotate and 6 scans. i.e. one linear scan

Gantry rotation by 30°

Again linear scan Multiple Detectors… up to 30 detectors …

decreased no. of rotatory steps from 180 to 6.

3rd Gen CT Scanner

• Rotate- Rotate scan• Fan Shaped Beam• Detector array…up to

300 detectors

4th Gen. CT Scanner:

• Rotate- Fixed scan• 360⁰ ring Stationary

Detectors ...decreased calibration

requirements.

Generations Projection X R Tube-Detector movement

Detectors Scan time

First (EMI) Head OnlyPencil Like beam

Translate (linear) and rotate 180 scans,1⁰rotation

Paired 4.5-5 min.

Second Single ProjectionFan Shape Beam

Translate (linear) and rotate 6 scans , 30⁰ rotations

Up to 30 in linear array

10-90 sec

Third Single ProjectionFan Shape Beam

Rotate-Rotate Both @ 360⁰

Up to 300 in curved array

2-10 sec

Fourth Single ProjectionFan Shape Beam

Rotate-FixedTube 360⁰ Rotation

600-2000 placed in 360⁰ ring

2-10 sec

Helical MULTIPLE proj.Fan Shape Beam

Rotate-FixedTube 360⁰ Rotation

placed in 360⁰ ring

Spiral (Helical) CT:

Advances in SLIP RING TECHNOLOGY made possible invention of spiral CT.

Slip Ring Tech. Consists of brushes that fit into grooves to permit current and voltage to the X ray tube to be supplied while the tube is in continuous rotation around the gantry.

Spiral CT

Spiral is a misnomer… Helical is the proper term… since tube rotates in helical path with constant radius.

( spiral path would indicate progressive decreasing radius)

Spiral CT

The patient table is also moved slowly in the gantry while tube is in motion.Thus data comprising of continuous helical scan of the patient is acquired.

Pitch = Table increment mm/sec Section thickness mm

Pitch <1.0 imply data oversampling > 1.0 imply some data is being missed.

Two general advantages of increasing pitch are:1. faster scanning2. reduced dose (the radiation is less concentrated)The major limiting factor associated with increased pitch is reduced image detail in the direction the body is moved.

Spiral CTADVANTAGES:

• Much shorter total scan time…less contrast media.

• Scan can be completed in One breath hold….decrease motion artifacts.

• With adjusting pitch, area of interest can be oversampled.

Conventional CT Scanner

• Parts- Gantry, X ray Tube, Detectors, Computer

• Image Reconstruction• Image Display- Window level, Window width.• Image Quality- Quantum Noise, Resolution• Artefacts• Patient Exposure.• RECENT ADVANCES in CT.

PARTS Of CT Unit

Gantry: • Movable form of CT unit containing X Ray tube and detectors.

• Gantry frame maintains alignment of tube and detectors.• Gantry aperture for movement of patient for scanning.

Table:

Made up of Carbon graphite to decrease the beam attenuation. Tabletop has certain weight limits. Top is motor driven to allow pt movement through gantry aperture and also vertical movement.

X Ray Tube…

Earlier-

• stationary anode• oil cooled

• 2x16mm focal spot• 120 kVp and 30mA• 80x80 matrix size

Now-

• Rotating anode• high heat loading and

dissipating capabilities• 0.6-1.2 focal spot • 120 kVp and Up to

1000mA • with 512x512 matrix

generally.

Detectors

• Measure the transmitted radiation.

• Requirements: High Stabilty. Fast Response Time. Wide Dynamic Range= Measurable largest signal Measurable smallest signalTwo Types:1. Scintillation Detectors2. Ionization Chambers

Scintillation Crystals and Photomultiplier Tubes:

• Convert X Ray Photon energy into Electrical Signals. •Crystals function just like an Intensifying screen.• Crystals material: Sodium Iodide, Calcium Fluoride• Photomultiplier tubes replaced by Silicon Photo iodides - smaller, more stable, lower cost.

XENON Gas filled Ionization Chambers:

Photon entering detector ionizes gas atom into +ve and -ve ions. -ve ion(electron) moves to anode, producing current (output signal) in anode.•Xenon- heaviest inert gas therefore more density. compressed to increase density.

Scintillation Crystals

Ionization Chambers

• Near 100% efficiency

• Large size

• Long after glow of NaI crystals

• Detector Crosstalk: when photon strikes detector, gets partially absorbed and then enters adjacent detector and is detected again. Leads to decreased resolution.Can be used in 4th gen scanner

Less efficiency -60% due to low density of absorbing material.Small size

Negligible lag time.

COMPUTER:

CT console provides access to software program that controls data acquisition, processing and display.

Image Reconstruction:

• Cross sectional layer of the body is divided into many tiny blocks called VOXEL… … a 3D element

Image Reconstruction:

Each Voxel has been traversed by numerous X ray photons and the intensity of the transmitted radiation is measured by detectors.

The Degree of Attenuation of X-ray beam depends on• Composition of tissue in the path • Thickness of section• Quality of the X-ray beam

Image Reconstruction Algorithm use following formula to calculate Attenuation Coefficient for each Tissue block or Voxel.

• N= No. Of Transmitted Photons• N₀= No. of Initial Photons• e= Constant (2.718)• µ = Attenuation Coefficient• X= Tissue thickness.

Algorithms for Image reconstruction:

After solving Thousands of equations determine linear attenuation coefficient of each voxel is determined.

Algorithm is a mathematical method to solve these equations.

• Back Projection Method• Iterative Methods• Analytical Methods: Two Dimensional Fourier Transformation Filtered Back projection

CT number or Hounsfield No. These numbers are calculated by comparing linear attenuation coefficient of each pixel to the linear attenuation coefficient of water. CT No. Of Various Body Tissues: Bone average +1000 Bone Cortical +80 Liver +40-70 Cerebellum +30

Blood +13-18 CSF +15 Water 0 Fat -100 Lungs -150-400 Air -1000

Image Display

• The viewed image is then reconstructed as a corresponding matrix of picture elements as PIXELS... a 2D element as shades of grey color.

Display Matrix: 256x256 , 512x512 or a Voxel a

Pixel 1024x1024 pixel sizes but generally 512x512 pixels is used.

Window Width and Window Level

• Human eye can recognize 256 shades of Gray. So we have to image -1000 to +1000 CT numbers i.e. 2000 numbers in 256 shades of gray.

• How this is possible??? We usually select a CT number that will be about the

average CT Number of the tissue being examined.

We usually select a CT number that will be about the average CT Number of the tissue being examined.

• i.e.+200 HU for bone and computer is programmed to assign one shade gray to each of the 128 numbers above and 128 numbers below this Baseline CT number .

• Here +200 is WINDOW LEVEL and Range above and

below i.e. 128 is called WINDOW WIDTH.

128

+200 HU for Bone

128

• So in practice Different window level and window width is employed to obtain maximum information for different body tissues.

i. e . Bone window Lung window etc.

Image Quality

Quantum Mottle (Noise):

• Precision is measure of background or matrix uniformity.

• Deviation from this uniformity represents Quantum Mottle or Statistical fluctuation.

• It is function of how much photons are absorbed effectively by each voxel of tissue.

Factors affecting Noise:• Radiation Dose. More mA= More the Photons absorbed=Less Noise• Voxel size Small voxel size = capture less photons = more noise.

Decreasing Slice Thickness (To Improve Detail)

Small Voxel size More Noise INCREASED RADIATION

DECREASE noise .

Contrast

• Most CT images can demonstrate contrast difference of as little as 0.4% as compared to minimum of 10% in routine Xray.

Resolution

Spatial Resolution

• Spatial resolution is ability of the CT scanner to display separate images of two objects placed close together.

To increase Resolution, Patient Radiation Dose should have to be Increased.

Contrast Resolution

• Contrast Resolution is ability of system to

display as distinct image of areas that differ in

density by small amount.

Spatial Resolution

• Expressed in mm or Line pairs per cm depending on manufacturer.

• i.e. Resolution of 0.5 mm= 10 lines pair/ cm= 10 lines of 0.5mm separated by 10 spaces of 0.5mm

• Modern scanner have resolution up to 15 lines pair per cm.

Spatial ResolutionDepends on:

• Scanner design- Xray tube focal spot size, Detector size, Magnification.• Computer Reconstruction• Display matrix

i.e. increasing the matrix size or Improve Spatial decreasing individual pixel size Resolution

Contrast Resolution

• It is ability of system to display as distinct image of areas that differ in density by small amount.

background noise = Contrast Resolution

ARTIFACTS: Artifacts is the discrepancy between the CT Numbers in reconstructed image and true attenuation coefficient of the object.

Motion Artifacts:•Image displays an object in motion as streak in direction of motion.

i.e. motion of gas in stomach, metallic object.

• To Prevent: Remove metallic objects. Gantry angulation i.e. to avoid dental filling.

Ring Artifacts

• Seen Rotate Rotate geometry

scanner.

• Result of the miscalibration of

single detector.

• With sum effect of multiple

projection in 360 degree results

in annular or ring artifacts.

Partial volume Effect:• Since the data from entire section (voxel width) is averaged together to form the image, any object in the section with width less than the section thickness may get obscured.• To prevent: can be overcome by obtaining slightly overlapping section.

Beam Hardening:

• It results of the attenuation of the beam as it passes through the patient since the low energy photons are rapidly absorbed.

• As a result CT number of posterior structures may be much different than similar density anterior structures.

• Seen as Dark bands or Cupping artifacts if lower CT numbers than anticipated.

Beam Hardening.

In CT head in 360 tube rotation, Centre of brain will always get partially attenuated Xray beam as compared to periphery. Prevent- Generally reconstruction programs anticipate and correct for this variation, but it is not precise.

Capping artifacts if reconstruction algorithm overcompensates.

CONE BEAM Artefacts: Imp in Multislice CT as detector width, All the Xray beam pass through the patient not exactly parellel but instead at an Cone Angle at the Periphery of detectors. Cone angle greatest at periphery of detector array.

Cone beam artefacts

• Seen in 8 slice CT onwards.

• To prevent this Cone beam Reconstruction

Algorithm is used.

Patient Exposure

Indices:

CTDI- Computed Tomography Dose Index MSAD- Multiple scan Average Dose. (Dev.By Centre for Devices and Radiological Health,

FDA-US)

• X-ray Chest…0.02 mSv• CT Head…….2.0 mSv………100 CxR equivalent• CT Chest…….8.8 mSv……...400 CxR equivalent• CT Abd/Pelvis…10 mSv……500 CxR equivalent

Recent Advances In CT

Multi Slice (Multi Detector) CTSpiral CTDual Source CTElectron Beam CT

Single slice CT Multi Slice CT

Single row of detectors Multiple Rows of detector rings

Multislice CT

• Multiple Rings of detectors are used.• 2 Slice CT images 2 body slices in 1 Tube Rotation.• 2, 4, 8, 16, 32, 40, 64 Slice CT are in use. • The major benefit of multi-slice CT is the increased

speed of volume coverage.

• This has particularly benefited CT angiography techniques - which rely heavily on precise timing

Dual Source CT Two X-ray tubes oriented at 90 degrees to one another. Dual Detector System. Scan in half the time of a standard CT. Scan Time < 1 sec Acquires cardiac images from single heartbeats. Contrast Dose required is less.

Electron Beam CT

Electron beam CT Vs X-ray Tube in Conv. CT

X-ray tube -large , stationary, partially surrounding imaging circle.

In EBCT an electron beam is electro-magnetically steered towards an array of tungsten X-ray anodes that are positioned circularly around the patient.

EBCT

• This motion can be very fast as compared to mechanical rotation of the Xray tube in Conventional CT Scanner.• Total scan time-50 to 100 milliseconds

• Scan the beating heart. Adv for CARDIAC CT.

• The Coronary Calcium Score relates to the extent of coronary plaque disease- substantial diagnostic and prognostic implications.

Inverse Geometry CT

• IGCT reverses the shapes of the detector and X-ray sources.

• An array of X-ray sources • A point detector.

Advantages :• Avoid Cone beam artefacts. • Less Scatter Radiation.

Still in Experiemental stage

Thank You...