CT REVIEW

Operational Modes First Generation – Pencil beam Single radiation detector Translate rotate motion of 180 translations with 1 degree each 5 min to image Only head 2 nd Gen – Fan shaped beam Mult detectors 30 second scan Head and body (1975 1 st whole body)

First Generation –

Pencil beam

Single radiation detector

Translate rotate motion of 180 translations with 1 degree each

5 min to image

Only head

2 nd Gen –

Fan shaped beam

Mult detectors

30 second scan

Head and body (1975 1 st whole body)

3 rd Gen – Fan beam Several hundred detectors based on rotate/rotate motion 1s scan time or less] Ring artifacts Slip Ring technology leading to spiral CT 4 th gen Fan beam 1000’s of detectors Fixed detectors in 360 degree EBCT – Fast imaging 100 ms or less Electron gun vs. tube

3 rd Gen –

Fan beam

Several hundred detectors based on rotate/rotate motion

1s scan time or less]

Ring artifacts

Slip Ring technology leading to spiral CT

4 th gen

Fan beam

1000’s of detectors

Fixed detectors in 360 degree

EBCT –

Fast imaging 100 ms or less

Electron gun vs. tube

Slip Rings Faster imaging No interscan delay (indexing) Continuous data acquisition (spiral) Less dose

Faster imaging

No interscan delay (indexing)

Continuous data acquisition (spiral)

Less dose

CT Gantry All CT imagers have – Console Computer (very fast with multiprocessing) Generator (high or low voltage) xrays are pulsed. All use high frequency. Gantry (has x-ray tube, detector array, generator tilts 30 degree) Filtration- hardens beam ave energy goes up (bowtie filter) DAS located btwn detector array and computer Converts analog signal to digital (ADC) Analog image projection recorded by each detector element is received by the DAS and transferred to ADC

All CT imagers have –

Console

Computer (very fast with multiprocessing)

Generator (high or low voltage) xrays are pulsed. All use high frequency.

Gantry (has x-ray tube, detector array, generator tilts 30 degree)

Filtration- hardens beam ave energy goes up (bowtie filter)

DAS located btwn detector array and computer

Converts analog signal to digital (ADC)

Analog image projection recorded by each detector element is received by the DAS and transferred to ADC

Detectors Efficiency determines max tube loading and pt dose Number of detectors and concentration help determine quality A group is called an array Utilized scintillation crystal PM tube Types – solid state used today used to be gas (xenon) xenon had fast response and no afterglow but only 50% detection Only about 1mm wide Total detection efficiency depends on the number of detectors and how tightly they are packed Mult. Detector arrays allow the collectio of two or more data sets simultaneously

Detectors

Efficiency determines max tube loading and pt dose

Number of detectors and concentration help determine quality

A group is called an array

Utilized scintillation crystal PM tube

Types – solid state used today used to be gas (xenon) xenon had fast response and no afterglow but only 50% detection

Only about 1mm wide

Total detection efficiency depends on the number of detectors and how tightly they are packed

Mult. Detector arrays allow the collectio of two or more data sets simultaneously

Collimators – prepatient vs post patient (pre detector) PrePatient controls pt dose & ST in conventional CT Postpatient controls slice thickness (spiral CT) Thickness range .5 – 10mm Source – High intensity x-ray tube capable of rotating 10000 rpm with large (up to) 2mm focal spot Tube mA 200 – 800 effects noise Tube potential kVp 120-140 (increases rad. Intensity to detectors, lowers pt dose)

Collimators – prepatient vs post patient (pre detector)

PrePatient controls pt dose & ST in conventional CT

Postpatient controls slice thickness (spiral CT)

Thickness range .5 – 10mm

Source –

High intensity x-ray tube capable of rotating 10000 rpm with large (up to) 2mm focal spot

Tube mA 200 – 800 effects noise

Tube potential kVp 120-140 (increases rad. Intensity to detectors, lowers pt dose)

Image quality Contrast resolution (contradicts spatial resol) Distinguish between similar tissues Based on atomic number, mass density Improved when scatter is reduced Contrast is improved by: Large pixel size (smaller matrix) High mAs (reduces noise) Thick slices Low pass filter Measured with MTF Spatial resolution Image noise Linearity/uniformity

Contrast resolution (contradicts spatial resol)

Distinguish between similar tissues

Based on atomic number, mass density

Improved when scatter is reduced

Contrast is improved by:

Large pixel size (smaller matrix)

High mAs (reduces noise)

Thick slices

Low pass filter

Measured with MTF

Spatial resolution

Image noise

Linearity/uniformity

Spatial Resolution Ability to reproduce small objects having high subject contrast. Degree of blurring. Matrix size and FOV are principle effect on SR. Matrix is array of cells (made of picture elements – pixels) Larger matrix better SR (trade off is larger recon time) Each pixel is a 2 dimensional rep of a volume element (voxel) FOV – diameter of the reconstructed image FOV increased - matrix size is constant, pixel size increases and SR goes down Display FOV can be equal or less than SFOV

Ability to reproduce small objects having high subject contrast. Degree of blurring.

Matrix size and FOV are principle effect on SR.

Matrix is array of cells (made of picture elements – pixels)

Larger matrix better SR (trade off is larger recon time)

Each pixel is a 2 dimensional rep of a volume element (voxel)

FOV – diameter of the reconstructed image

FOV increased - matrix size is constant, pixel size increases and SR goes down

Display FOV can be equal or less than SFOV

Slice Thickness – Thinner give better resolution and help with rapidly changing anatomy Thin – less partial volume Thin – higher dose because of increased overlap High Freq convolution gives edge enhancement. Possible increased SR Measured in lp/mm (combined line and interspace) Larger pixel size poorer spatial resolution Measured with PRF, ERF, LSF combine to create MTF (MTF of 1 is absolutely perfect image) As increased value blur decreases SR improved by: Small detector size Small pixel size Thin slices High freq convolution filter Smaller focal spot

Slice Thickness – Thinner give better resolution and help with rapidly changing anatomy

Thin – less partial volume

Thin – higher dose because of increased overlap

High Freq convolution gives edge enhancement. Possible increased SR

Measured in lp/mm (combined line and interspace)

Larger pixel size poorer spatial resolution

Measured with PRF, ERF, LSF combine to create MTF (MTF of 1 is absolutely perfect image) As increased value blur decreases

SR improved by:

Small detector size

Small pixel size

Thin slices

High freq convolution filter

Smaller focal spot

MAGNIFICATION Targeting – when using original image projection data results in improved spatial resolution Photo Zoom – easier and faster but less spatial resoultion

Targeting – when using original image projection data results in improved spatial resolution

Photo Zoom – easier and faster but less spatial resoultion

CT #s Generally +1000 (bone) - -1000 (air) Based on linear attenuation Higher HU brighter the pixel Range is known as window width (used to vary the contrast) Reduce the width increases the contrast Level is the central value (based on density) Windowing refers to manipulation of WL and WW to optimize the image contrast

Generally +1000 (bone) - -1000 (air)

Based on linear attenuation

Higher HU brighter the pixel

Range is known as window width (used to vary the contrast)

Reduce the width increases the contrast

Level is the central value (based on density)

Windowing refers to manipulation of WL and WW to optimize the image contrast

Post Processing MPR – multi planar reconstruction from axial slices MIP Maximum Intensity Projection Selects voxels along a row or column in a volume of intrest with the highest CT number or specified range for display Does not provide depth information SSD – Shaded-Surface Display are surface rendered images ie facial reconstruction QCT – Quantitative CT – compares vertebral bone with a standard phantom to assay bone mineral content VR – Volume Rendering Best to see deep anatomy but requires most processing

MPR – multi planar reconstruction from axial slices

MIP Maximum Intensity Projection

Selects voxels along a row or column in a volume of intrest with the highest CT number or specified range for display

Does not provide depth information

SSD – Shaded-Surface Display

are surface rendered images ie facial reconstruction

QCT – Quantitative CT – compares vertebral bone with a standard phantom to assay bone mineral content

VR – Volume Rendering

Best to see deep anatomy but requires most processing

Spiral CT Continuous source rotation Pt couch moves as xray rotates No interscan delay Volume of data in one breath hold Can overlap leading to better post processing techniques Less contrast media Less dose Interpolation- method is either 360 degrees or 180 degrees (allows for scanning at a higher pitch) which is commonly utilized. Computes a value based on known values on each side

Continuous source rotation

Pt couch moves as xray rotates

No interscan delay

Volume of data in one breath hold

Can overlap leading to better post processing techniques

Less contrast media

Less dose

Interpolation- method is either 360 degrees or 180 degrees (allows for scanning at a higher pitch) which is commonly utilized. Computes a value based on known values on each side

Pitch See formulas Contiguous spiral pitch = 1 Can result in partial volume effect when object is contained in adjacent slides Overlap is a pitch less than 1 Extended (gaps) pitch more than 1 Dose – as pitch increases patient dose is reduced approximately proportional to 1/pitch Ie. Pitch .5 dose x2 Pitch 1 dose x1 Pitch 1.5 dose x 0.7 Couch speed should not exceed slice thickness in order to obtain best compromise between image quality and image volume.

See formulas

Contiguous spiral pitch = 1

Can result in partial volume effect when object is contained in adjacent slides

Overlap is a pitch less than 1

Extended (gaps) pitch more than 1

Dose – as pitch increases patient dose is reduced approximately proportional to 1/pitch

Ie. Pitch .5 dose x2

Pitch 1 dose x1

Pitch 1.5 dose x 0.7

Couch speed should not exceed slice thickness in order to obtain best compromise between image quality and image volume.

Noise Graininess of an image Is higher with spiral than conventional regardless of pitch Noise is less for 360 degree interpolation Conventional CT 100% 180 inter pol 125% 360 inter 85% Increase in slice thickness results in less noise and lower dose Increase in pixel size results in less noise Anything that reduces CT noise will improve contrast resolution

Graininess of an image

Is higher with spiral than conventional regardless of pitch

Noise is less for 360 degree interpolation

Conventional CT 100%

180 inter pol 125%

360 inter 85%

Increase in slice thickness results in less noise and lower dose

Increase in pixel size results in less noise

Anything that reduces CT noise will improve contrast resolution

Linearity Ability of the CT to assign the correct Hu to a given tissue, measured with a 5 pin water phantom. When water = 0, bone 1000 and air -1000 the ct has perfect linearity

Ability of the CT to assign the correct Hu to a given tissue, measured with a 5 pin water phantom.

When water = 0, bone 1000 and air -1000 the ct has perfect linearity

Formulas Heat Units Single phase - 1HU = 1kVp x 1mA x 1s Three phase – 1HU = 1.4 x 1kVp x 1mA x 1s Example: how much heat energy is produced by 15 100ms images acquired at 130 kVp/600mA? 15 x 0.1s x 130 kVp x 600 mA = 0.2 MJ Voxel size (mm 3 ) = FOV/matrix x slice thickness Example: what is the voxel size for a 2mm slice thickness imaged with a 320 x 320 matrix over a 20cm FOV 200 mm/ 320 x 2mm = 1.25 mm 3

Heat Units

Single phase - 1HU = 1kVp x 1mA x 1s

Three phase – 1HU = 1.4 x 1kVp x 1mA x 1s

Example: how much heat energy is produced by 15 100ms images acquired at 130 kVp/600mA?

15 x 0.1s x 130 kVp x 600 mA = 0.2 MJ

Voxel size (mm 3 ) = FOV/matrix x slice thickness

Example: what is the voxel size for a 2mm slice thickness imaged with a 320 x 320 matrix over a 20cm FOV

200 mm/ 320 x 2mm = 1.25 mm 3

Image Display Pixel size = FOV/Matrix Example: what is the pixel size if the FOV is 20 cm with a 512 x 512 matrix? pixel size = 200mm/512 = 0.39mm Pitch Spiral CT pitch = patient translation/360 Slice thickness

Image Display

Pixel size = FOV/Matrix

Example: what is the pixel size if the FOV is 20 cm with a 512 x 512 matrix?

pixel size = 200mm/512 = 0.39mm

Pitch

Spiral CT pitch = patient translation/360

Reconstruction Filtered back projection leading to a digital matrix which can additionally be post-processed Based on multiple projections Creates pixels each of one gray scale based on HU and linear attenuation of tissues Computer must be fast for extensive data set Ie 512x512 matrix needs to solve 262,144 equations simultaneously ADC / DAC

Filtered back projection leading to a digital matrix which can additionally be post-processed

Based on multiple projections

Creates pixels each of one gray scale based on HU and linear attenuation of tissues

Computer must be fast for extensive data set

Ie 512x512 matrix needs to solve 262,144 equations simultaneously

ADC / DAC

Algorithms (sets of instructions) Analytic Iterative Back projection (combined with filtering Convolution) w/o filter would be blurred Convolution filter sometimes called a kernel Interpolation – used in CT reconstruction to determine slices in spiral scanning

Algorithms (sets of instructions)

Analytic

Iterative

Back projection (combined with filtering Convolution) w/o filter would be blurred

Convolution filter sometimes called a kernel

Interpolation – used in CT reconstruction to determine slices in spiral scanning

Artifacts Spiral Artifacts Specifically – Breakup and Stair Step, both occur due to reformatting interpolated transverse images to MPR’s of coronal or sagittal Motion both voluntary or involuntary Appears as streaks Metal artifacts – occur because of incomplete projection profiles and appears as streak or star shaped Partial Volume- reduced by using thinner slice thickness at expense of noise.

Spiral Artifacts Specifically –

Breakup and Stair Step, both occur due to reformatting interpolated transverse images to MPR’s of coronal or sagittal

Motion both voluntary or involuntary

Appears as streaks

Metal artifacts – occur because of incomplete projection profiles and appears as streak or star shaped

Partial Volume- reduced by using thinner slice thickness at expense of noise.

Beam Hardening – appears as dark ring inside cranial bone and cupping at the center of image. Due to false low linear attenuation coefficient and false low ct number. Ring artifacts – occur in third gen CT because of detector malfunction

Beam Hardening – appears as dark ring inside cranial bone and cupping at the center of image. Due to false low linear attenuation coefficient and false low ct number.

Ring artifacts – occur in third gen CT because of detector malfunction

Dose Generally higher in CT than diagnostic or fluro Typical CT 5000mrad vs 100 mrad in diagnostic Radiation dose is inversely proportional to pitch in spiral CT Dose is higher with thinner slices due to overlap Patient dose can be reduced but at expense of noise Measured with CTDI and uses a pencil ionization chamber CTDI = to multiple scan ave. dose (MSAD) if the slice thickness is equal to the couch incrementation Highest radiation dose is near the patient and due to scatter Practice ALARA (minimize time, max dist, shield)

Generally higher in CT than diagnostic or fluro

Typical CT 5000mrad vs 100 mrad in diagnostic

Radiation dose is inversely proportional to pitch in spiral CT

Dose is higher with thinner slices due to overlap

Patient dose can be reduced but at expense of noise

Measured with CTDI and uses a pencil ionization chamber

CTDI = to multiple scan ave. dose (MSAD) if the slice thickness is equal to the couch incrementation

Highest radiation dose is near the patient and due to scatter

Practice ALARA (minimize time, max dist, shield)