CT Flashcards
What “generation” are most CT scanners?
3rd generation - means the x-ray tube and the detectors spin around the patient in synchrony.
What is a 3rd generation CT scanner?
The x-ray tube and the detectors spin around the patient in synchrony.
Under what conditions are CT tubes designed to operate?
Reasonable voltages (between 80-140 kV) Very high tube currents - up to 1000 mA.
Difference between regular diagnostic x-rays and CT for mA, kVp, and focal spot
Regular Dx:
mA: 200-800
kVp: 50-120
Focal spot size: 1-1.2 mm
CT:
mA HIGH up to 1000
kVp: 80-120
Focal spot size: 0.6-1.2 mm
What filter mechanisms are used in CT?
Cu or Al (6 mm)- filter the x-ray beam.
Heavily filtered beam can have a half value thickness of up to 10 mm Al.
Bow Tie filters: compensate for uneven attenuation of the beam by the patient- attenuate less in the center and more on the edges - made of low Z materials - like Teflon (to reduce hardening differences).
What is a Bow-tie filter?
Compensate for uneven attenuation of the beam by the patient- attenuate less in the center and more on the edges - made of low Z materials - like Teflon (to reduce hardening differences).
Compensate for uneven filtration
Reduce scatter
Reduce dose
How is the x-ray tube anode-cathode axis positioned in CT?
Perpendicular to the imaging plane to reduce heel effect.
What is used for scatter reduction in CT?
Collimator is used at both the x-ray tube as well as the detector - shapes the x-ray beam - “defines the section thickness on a single slice.” - the collimator also reduces some scatter.
Use “anti-scatter septa” for additional scatter reduction
What defines the section thickness on a single slice?
Collimator that shapes the x-ray beam and reduces some scatter.
What does “filtered” in filtered back projection mean?
Sharpening of the projection data prior to back projection
Modern set ups use an “iterative reconstruction” math program that allows more noise, so you can have less dose at the same diagnostic quality.
What does multislice mean?
The numbers of detectors in the Z direction (head to toe) determines the number of slices that can be simultaneously acquired.
The MDCT can acquire images with “isotropic resolution” - can do non-axial reconstructions w/o stretching pixels.
What does “isotropic resolution” mean?
The MDCT can acquire images so that we can do non-axial reconstructions w/o stretching pixels.
What determines minimal slice thickness in modern CT?
Detector element aperture width.
How do modern scanners adjust dose?
Scout image- data in the scout is used to estimate density and adjust accordingly.
Continuous modulation as the scan happens
What is a “ray”?
A measure of total x-ray attenuation along a line from the focal point to a single detector
What is “projection”?
All rays at a given angle of the x-ray tube.
It’s a series of rays that pass through the patient - at the same orientation.
What is a “sinogram”?
Bunch of squiggly lines that represent the data from all the projections of all the tube angles (0-360)
What kind of x-rays are used with CT?
Highly filtered, high kV (average energy 75 keV).
What is the matrix size for CT? Each pixel is?
The matrix is 512 x 512 with each pixel representing 4096 possible shades of gray (12 bits)
2^12 = 4096
What is the relationship between pixel width and height to voxels?
They are the same. Pixel W x H = Voxel W x H.
The difference is a voxel has a 3rd dimension (depth), which represents the slice thickness.
A voxel is a cube, a pixel is a square.
How do you calculate pixel size?
FOV / Matrix Size
How do you improve spatial resolution?
Make the pixels smaller (matrix larger)
Pixel size = FOV / Matrix
Is mAs the same on CT as it is on plain film?
The conventional definition of radiographic mAs is not useful in spiral CT.
You have to use “effective mAs” - tube current (mA) x length of time that a given point in the patient is in the beam - exposure time.
Exposure time is going to be related to the collimated beam width and table speed.
What happens to your images if you turn down the mAs?
Images get noisier.
What are the fundamental components you can change with pitch?
Dose or spatial resolution
Overlap your scanning beams, have better spatial resolution but more dose.
Uncoil the beams, your dose becomes less - but your spatial resolution suffers.
What is pitch?
Distance the tables moves during the time of a single revolution of a tube divided by the beam width.
Pitch = Table Movement / Beam Width
If the movement of the table is less than the beam width you end up with a fraction (number less than 1).
If the movement of the table is more than the beam width you end up with a whole number (a number more than 1)
What is a “pitch of 1”?
No overlap between slices
What is a “pitch of greater than 1”?
Table moved faster than the beam - have a gap between your slices - spatial resolution suffers, but get less dose.
What is a “pitch of less than 1”?
Table moved slow and slices overlapped - increased spatial resolution, but dose increases.
Sometimes referred to as “over-scanning”
Formula for Hounsfield Units?
HU = 1000 x (attenuation of material - attenuation of water) / attenuation of water.
What is the relationship between HU and x-ray attenuation?
When HU increases by 10 HU, xray attenuation increases by 1%.
Does changing the keV mess with the HU?
Yes.
Number is based on attenuation (blockage/absorption) of xrays. Change in the photoelectric interaction are going to change x-ray absorption. PE predominates at lower energy, low keV will createa higher HU.
Closer you get to the k-edge of a given substance the more impressive the increase in attenuation (and therefore HU) will be.
Will filtration change the HUs?
It can.
Filtration changes the “quality” of the beam - average energy - kind of like changing the kV.
What is “level”?
Level is the midpoint of the gray scale display (the “center”).
Want it at the attenuation of the thing you are interested in.
Width is selected based on what you are comparing - things with different densities, want a wide width
Brain W/L
W 80, L +40
Lung W/L
W 1500, L -400
Abdomen W/L
W 400, L +50
Bone W/L
W 1600, L +500
Overview of steps in make image in CT?
Detectors rotating around the patient which are collecting raw attenuation.
Data is processed in an algorithm (either filter back or iterative reconstruction).
A filter or “kernel” is applied - either smoothing or sharpening the data.
Advantages of Axial acquisition over helical?
Better spatial resolution on the z-dimension since full image sets are taken. No partial volume effect along the axis.
The artifacts of partial volume with helical CT are noticed more along a curved surface - skull.
Advantages of helical acquisition over axial?
Faster.
Post acquisition flexibility in the selection of slice location and lower probability of anatomic discontinuities between adjacent slices containing moving anatomy in the chest/abdomen scans.
What happens if you increase beam width?
Reduced scan time (larger coverage with 1 turn)
Reduced motion artifact (less scan time)
Increases partial volume (more divergent beam)
Does NOT change the radaition dose (mAs is unchanged, even though scan time is less, a larger area of tissue is scanned at the same time).
Does widening the beam width change the radiation dose?
No.
mAs is unchanged, even though scan time is less, a larger area of tissue is scanned at the same time
What happens if you change the kVp?
Does goes up - no AEC, like conventional radiography.
Image noise will increase
Iodinated contrast is more conspicuous at lower kVp (such as 80 kVp), as the average energy of the x-ray beam will be closer to the k-edge of iodine.
What is done in CT after raw data is collected?
Create a projection
Need to figure out the linear attenuation coefficients of each pixel in the image matrix.
Back projection - original way - value of attenuation is calculated for each pixel along a ray - traced through the patient.
Filtered back projection - modern way - Math filter is applied prior to calculating the back projection.
Iterative Reconstruction - “forwarded” information is compared to actual information and differences are used to correct the image - can correct for noise so you can use a lower dose.
What is back projection?
Original way of creating a projection from raw data.
Value of attenuation is calculated for each pixel along a ray - traced through the patient.
What is Filtered Back Projection?
Modern way of creating a projection from raw data.
Math filter is applied prior to calculating the back projection.
What is Iterative Reconstruction?
“Forwarded” information is compared to actual information and differences are used to correct the image - can correct for noise so you can use a lower dose.
What is done in CT after a projection is created from raw data?
Mathematical Filter is applied (Kernel).
Reconstruction algorithm - tradeoff decision between spatial resolution and noise for each kernel. Selection is based on clinical parameters.
“Smooth” kernel - images with low noise, but with reduced spatial resolution.
“Sharp” kernel - Images with higher spatial resolution but increased noise
What is prospective gating in Cardiac?
“Step and shoot” - R-R interval
Reduced radiation b/c the scanner isn’t on the whole time.
No functional imaging
Always axial, not helical
What is retrospective gating in Cardiac?
Scans the whole, time then back calculates
Can do functional imaging
Higher radiation (use of low pitch - increases dose).
What is CT Fluoro?
Near real time imaging, with the CT image constantly updated (6 per second).
Low tube currents (20-50 mA) are used to minimize radiation doses.
How does Dual Energy CT work?
Scan is acquired using both 140 and 80 kVp (instead of 120).
The HU of each pixel is obtained at both energies. The image is dirtier, but you can do all kinds of stuff like characterize the material (what is the renal stone made of), or do a virtual non con.
It all has to do with different atomic numbers absorbing photons differently.
What contributes to noise?
Noise is data that contributes nothing useful to the image.
Quantum mottle and scatter are factors.
What is Signal to Noise?
Quantitative method for comparing the signal vs noise.
Signal changes directly with increased x-ray flux. (2x x-rays = twice signal).
Noise changes by a factor of square root of N - twice the x-rays = square root of 2 or 1.4 times the noise.
200 -> 400 mA, SNR = 2/square root of 2 = 2/1.4 = 40% increase in SNR
200 -> 800 mA, SNR = 4/square root of 4 = 4/2 = double SNR.
Increase in how much of the photon flux is needed to double the SNR?
4x
Noise goes down by square root of N.
What factors increase SNR?
Higher mA Longer rotation time Higher kVp Larger slice thickness Larger pixel (pixel size = FOV/Matrix Decreased pitch
What filters will worsen noise?
High-resolution or “edge enhancement” filters - although spatial resolution improves.
Smoothing filter will make the noise less noticeable (at the expense of spatial resolution).
What is contrast resolution?
Ability to discriminate between structures with very similar attenuation characteristics (CT numbers).
How is CT spatial and contrast resolution compared to mammo and plain film?
CT spatial resolution is worse.
Contrast resolution is better - b/c minimal scatter reaches detectors (tight collimation - both pre-patient and pre-detector) and by windowing - tight maximizes contrast resolution.
Why is contrast resolution better in CT?
Minimal scatter reaches detectors (tight collimation - both pre-patient and pre-detector) and by windowing - tight maximizes contrast resolution.
What is the limiting factor in contrast resolution?
Noise
What will improve contrast resolution?
Any factor that reduces the amount of visibility of noise (mA, kVp, slice thickness, matrix size, FOV, and reconstruction filter).
What is Spatial Resolution?
Ability to distinguish between small objects that are close together.
Line pair resolution in CT is 0.5 to 1 Lp/mm (about 11 Lp/mm in mammo).
What factors affect spatial resolution?
Focal spot size - larger focal spot = object details are spread out over several detectors - degrades and blurs the image. Smaller focal spot = better spatial resolution.
Magnification - more mag = blur
Detector Aperture Size - as detector size is reduced, the cranial-caudal resolution (z-axis) increases.
Number of Projections - more projections = more data = better resolution.
Reconstruction slice thickness - thinner the detector elements aperture, better spatial resolution in the Z direction
Spatial resolution as a function of pixel size and display FOV.
Spatial resolution as a function of Pitch - increase pitch, increase width of the slice sensitivity profile (SSP).
Patient motion = blurring
Spatial resolution related to filters- sharp filter = better spatial resolution (trade off is more noise) - Smooth filter = crappy spatial resolution (trade off is less noise).
How does focal spot size affect spatial resolution?
Larger focal spot = object details are spread out over several detectors - degrades and blurs the image. Smaller focal spot = better spatial resolution.
How does magnification affect spatial resolution?
More mag = blur
How does detector aperture size affect spatial resolution?
As detector size is reduced, the cranial-caudal resolution (z-axis) increases.
How does number of projections affect spatial resolution?
More projections = more data = better resolution.
How does reconstruction slice thickness affect spatial resolution?
Thinner the detector elements aperture, better spatial resolution in the Z direction
What is Display FOV?
Space defined by the user based on anatomy size to be displayed.
Always less than (or equal to) scan FOV.
What happens to spatial and contrast resolution when you hold matrix size constant and decrease FOV?
Decrease pixel size.
Increased spatial resolution but increases contrast resolution (less photons per box)
What happens to spatial and contrast resolution when you hold matrix size constant and increase FOV?
Increase pixel size.
Decreases spatial resolution but increases contrast resolution (more photons per box).
Factors that affect spatial resolution?
Focal spot (smaller spot = better)
Detector width (smaller detector = better)
Nyquist Limitations “Sampling” (oversampling = better)
Reconstruction filter (ex- bone “sharp’ algorithm gives a higher spatial resolution)
Factors that affect Contrast Resolution?
Number of x-rays (mAs, kV, pitch). More dose (less mottle) will improve contrast resolution
Slice thickness (thicker = more x-ray quanta = less noise)
Reconstruction Method (iterative > filtered back)
Reconstruction filter (soft tissue > bone)
How is dose distributed in CT vs regular Dx imaging?
In Dx, the entrance skin dose is much larger than the exit skin dose.
CT, scanner spins, the dose is symmetrically distributed.
Center is gonna get less in body.
Head - middle is the same
How is the dose varied in body CT and head CT in the scan plane?
Body scans: surface is about twice the central dose
Head scans: central and surface are very similar.
How is dose varied in the Z-axis?
Along the length of the patient (or phantom) - get “tails of radiation along the edge of the area being scanned.
Profile of radiation is not limited to the primary area being imaged.
Multiple scans are performed these tails add up with the original scan.
What is CTDI?
CT Dose Index
Radiation dose, normalized to beam width.
Weighted CTDI - 1/3 the central CTDI + 2/3 peripheral CTDI (exprsessed in mGy)
Volume CTDI - dividing weighted CTDI by the pitch.
What is weighted CTDI?
1/3 the central CTDI + 2/3 peripheral CTDI (exprsessed in mGy)
Weighted CTDI is the intensity being used and can relate to mottle.
What is Volume CTDI?
Dividing weighted CTDI by the pitch
What are the relationships between pitch and dose?
Pitch of 1 = dose similar to axial scanning
Pitch < 1 = increased dose b/c of overlap
Pitch > 1 = decreased dose b/c energy is more spread out
Pitch of 2 halves the dose.
Pitch of 0.5 doubles the dose.
What is DLP?
Dose Length Product
CTDI-Vol x length of the scan in cm.
What is “Effective Dose” for CT?
Effective Dose = k x DLP.
“k” is a body part constant.
Effective dose is going to be in Sv.
What are CTDI numbers based off of?
Phantoms.
If patient is larger than the phantom, the dose is over estimate.
Patient smaller, then dose is under estimated.
How does CTDI dose change if the patient is bigger or smaller than the phantom?
Bigger = dose overestimated.
Smaller = dose underestimated.
What are the ACR established diagnostic CT reference values?
CTDI vol:
75 mGy for Head
25 mGy for adult abdomen
20 mGy for Peds Abd (5 year old)
ACR established diagnostic CT reference values for heads?
75 mGy
ACR established diagnostic CT reference values for adult abdomen?
25 mGy
ACR established diagnostic CT reference values for Peds Abd?
20 mGy (5 year old)
What is the risk of radiation induced cancer per dose?
5% per Sv = adult
Up to 15% per Sv for child
About 1/10th that for someone older than 50
What are the pediatric considerations for CT?
Recommended that you reduce the mAs.
Reduced techniques are possible b/c x-ray penetration is greater in children
Dose reduction in head CT are more modest than peds belly.
What are strategies to reduce dose to the breast?
Do the scan at reduced mA
Use milliampere modulation (adjust based on density) - preferred method
Shield the breasts with bismuth - get artifact and degraded image (beam hardening can falsely elevate H.U. directly deep to the shield.
Dose of 1 chest CT is about equal to how many PA and Lat CXR?
100
What the effective dose of CT of the extremities?
Very low (<1 mSm) b/c they don’t contain any radiosensitive organs.
What is embryo dose in CT AP?
30 mGy
When is individual dose monitoring mandated?
If occupational dose is favored to be greater than 10% the annual dose limit (500 mrem)
How will increasing the kVp change the CTDI-w on both a head and body phantom?
Increase both
What is beam hardening?
Lower energy photons are removed preferentially from the beam as they pass through an object - leaving a “harder beam” with an increased average energy.
Two artifacts: Cupping and Dark bands/streak
What artifacts are associated with beam hardening?
Cupping and Dark bands/streak
What is Cupping Artifact?
Associated with beam hardening
X-rays passing through the middle of a uniform shape (like a head) are hardened more than those traveling through the periphery (a shorter path). The harder the beam, the slower the rate of attenuation.
The center of the image appears darker than the peripheral portions.
What is Dark Bands/Steak Artifact?
Occurs in the setting of two dense objects. X-rays that pass through one are less attenuated than those that pass through both.
Results in dark bands and streaks between the two objects.
Classic location is bone or where a dense contrast was used.
How do you fix beam hardening?
Filtration - pre-harden the beam before it hits the patient, and/or the addition of a bow-tie filter.
Calibration correction- use a phantom to allow the detector to compensate for hardening effects.
Correction software- iterative correction algorithm
Avoidance- Tile the gantry or position the patient to avoid areas that cause hardening.
What is partial volume artifact?
Two ways:
“Partial volume effect” - a dense object protrudes partially into the width of an x-ray beam. Results in divergence of the beam and manifests as shading artifacts adjacent to said object.
CT voxels are 3D cubes. Have a dense thing taking up half the cube, and a sparse (low attenauting) thing in the other half, the machine will average the two together giving you something that has intermediate density.
How do you fix partial volume?
Make slices thinner.
If noise is a problem, acquire thin slices then generate thicker slices by adding them together.
What is Photon Starvation?
High attenuating areas (classically the shoulders) can result in photon starvation manifesting as streaking - seen when the beam travels horizontally - through the greatest area of attenuation.
How do you fix photon starvation?
Two ways:
Automatic tube current modulation- increases the dose through the area of greater attenuation you can add enough photons to overcome this effect.
Adaptive filtration- correct the attenuation profile “smooth the data” in the high attenuation portions.
What is under sampling?
Insufficient number of projections used to reconstruct the CT - diminish quality and result in mis-registration artifacts. Two types:
View aliasing: have under sampling between projections. See fine stripes radiating from the edge (but at a distance from) a dense object. Fixed by acquiring the largest possible number of projections per rotation - slowing the rotation speed.
Ray Aliasing: Have under sampling within a projection. See strips appearing close to the structure. Fixed by using specialized high resolution techniques - manufacturer employed.
What are the two types of under sampling?
View aliasing: have under sampling between projections. See fine stripes radiating from the edge (but at a distance from) a dense object. Fixed by acquiring the largest possible number of projections per rotation - slowing the rotation speed.
Ray Aliasing: Have under sampling within a projection. See strips appearing close to the structure. Fixed by using specialized high resolution techniques - manufacturer employed.
How does metal cause artifact?
Beam hardening, partial volume, aliasing, and having density ranges higher than what can be handled by the computer.
Metals with high Z (iron, platinum) tend to have more artifacts than those with lower Z (titanium)
Fix: remove, increase kVp (sometimes works). Use thinner slices. Certain interpolation software can help.
What is over-scanning?
Pitch < 1
What is over-ranging?
Scanning above and below the target, to collect additional data for a helical scan.
How is over-scanning different from over-ranging?
Over-scanning = Pitch < 1
Over-ranging = Scanning above and below the target, to collect additional data for a helical scan.
What is incomplete projection?
Parts of the patient are hanging outside the field, but still attenuating x-rays this messes with the computer’s math.
Arms hanging down or having the IV contrast on or near the patient.
What is ring artifact?
Calibration error or defective detector on third generation scanner will cause errors in angular position - circular defect.
What is helical artifact in the axial plane?
Helical interpolation - around the top of the skull (anatomy changing rapidly in the Z direction). Higher pitch = worse.
Minimize by reducing the variation in the Z direction - use a low pitch, use 180 instead of 360 when possible, using thin sections instead of thick.
Why head CTs are still commonly done with axial scanning over helical.
What is helical artifact in multi-section?
Distortion is more complicated on multi-section scanners, with a classic “windmill” appearance where several rose of detectors intersect.
Worsens with increased helical pitch.
A “Z-filter” is done to reduce the severity of windmill artifacts.
What is stair step artifact?
Seen as stair step on the edges of a multi-planar reformatted image, when you have a wide collimation of non-overlapping intervals.
Less severe with a helical scanner - getting some overlap.
Fixed by thin slices.
What is Zebra Artifact?
Reformat artifact, which can occur from helical data - secondary to helical interpolation process (increases noise along the Z axis).
Stripes most pronounced on a 3D image.
Effect is most significant away from the axis of rotation.
