CT Flashcards
Spatial resolution phantom
noise CT phantom
CT brain image has level of 2 HU and window of 2 HU. Altering what parameter will improve contrast?
increasing the width HU will improve contrast
detection of a large low contrast object can be improved by:
- smoothing algorithm
- thicker slices
- increasing mA
The term “effective mAs” refers to
The term “effective mA” or “effective mAs” is used in helical CT scanning and is the mAs/pitch. As the pitch increases with all other settings remaining constant, the number of x-ray photons contributing to the slice data will decrease (effective mAs). The effective mAs determines the dose to the slice (CTDIvol) and signal to noise.
How does pitch affect effective mAs?
The term “effective mA” or “effective mAs” is used in helical CT scanning and is the mAs/pitch. As the pitch increases with all other settings remaining constant, the number of x-ray photons contributing to the slice data will decrease (effective mAs). The effective mAs determines the dose to the slice (CTDIvol) and signal to noise.
what generation are most CT scanners?
3rd generation
which means the xray tube and the detector spin around the patient in syncrony
what is the typical CT tube current?
1000 mA
(regular general radiography is 200-800 mA)
large or small focal spot on the CT xray tube anode?
large focal spot to handle all the power
what filter is typically used in CT?
copper or aluminum (6mm) to filter the xray beam
what is a bow tie filter?
used to compensate for the uneven attenuation of the beam by the patient and attenuate less in the center and more on the edges
They are made of low Z materials (like Teflon) to reduce hardening differences
where are collimators positioned in CT scanners?
both the xray tube and detector to shape the xray beam
also reduces some scatter
what is filtered back projection?
an analytic reconstruction algorithm designed to overcome the limitations of conventional back projection; it applies a convolution filter to remove blurring. It was, up until recently the primary method in cross-sectional imaging reconstruction.
It utilizes simultaneous equations of ray sums taken at differing angles of a sine wave to compute the values of attenuation coefficients within a cross section.
what is minimal slice thickness determined by?
detector element aperture
what is a ray
a measure of total xray attenuation along a line from the focal point to a single detector
what is a projection?
all rays at a given angle of the xray tube (a series of rays that pass through the patient)
what kind of xrays are used with CT?
highly filtered, high kV (average energy 75 keV)
what is the matrix size for CT? each pixel is?
matrix is 512 x 512
each pixel representing 4096 possible shades of gray (12 bits)
2x
212 = 4096
what is the relationship between pixel width and height to voxels?
that are the same
Pixel W x H = voxel W x H
voxel has a third dimension, voxel is a cube and pixel is a square
How do you calculate pixel size?
pixel size = FOV/matrix
what is a “kernel”?
convolution algorithm, refers to the process used to modify the frequency contents of projection data prior to back projection during image reconstruction in a CT scanner
How do you improved spatial resolution in CT?
Decrease pixel size
Pixel size =FOV/Matrix
what happens to the images if you turn down mAs?
less mAs = more noise
(decreased SNR)
what is pitch?
the distance the table moves in a single revolution
pitch = table movement/beam width
pitch > 1
table moved fastr than the beam and there is a gap between slices
this gap means spatial resolution suffers
pitch <1 affect on spatial resolution and dose
table moves slow and slices overlapped
spatial resolution improves but so does DOSE
typical pitch of cardiac CT
0.2 (slices overlap, high dose)
sometimes a pitch < 1 is referred to as
“over scanning”
whta i the formula to calculate HU
HU = 1000 x (attenuation of material - attenuation of water)/attenuation of water
What is the relationship between HU and Xray attenuation?
When HU increases by 10 HU, xray attenuation increases 10%
does photon energy (keV) mess with the HU?
Yes
changes in photoelectric interaction are going to change xray absorption.
In general since PE dominates at lower E –> low keV will create a HIGHER HU. The closer you approach the k-edge of a given substance, the mover impressive the increase in attenuation (therefore HU) will be
for example, IV contrast will have a HU of around 100 HU at 140 kV, which will increase markedly to 400 HU at 80 kV (where the average energy will land flush on its k-edge)
how does filtration affect HU?
filtering removes low E photons so increases the average E of the beam and lower you HU
HU = 1000 x (attenuation material - attenuation of water)/attenuation of water
what is a classic artifact that can happen due to filtration and beam hardening
“cupping artifact”, where things appear less dense in the center and more dense along the periphery
what type of scanning results in better spatial resolution and less partial volume artifact?
axial scanning
whta is partial volume artifact?
partial volume artifact
occurs when tissues of widely different absorption are encompassed on the same CT voxel producing a beam attenuation proportional to the average value of these tissues.
The latest generation of CT scanners with an associated reduction in the volume of a voxel has substantially reduced the occurrence of this artifact.
less stair step arifact is seen with
helical scanning (axial has worse stair step artifact)
how does increased beam width affect time?
larger coverage with 1 turn
how does increased beam width affect motion artifact?
reduces motion artifact
how does increased beam width affect partial volume?
increases partial volume (more divergent beam)
unlike conventional radiography with automated exposure control, what does increased kVp do to dose in CT?
increases radiation dose to the patient since there is NO automated exposure control in CT
how does increasing kVp affect noise?
increasing kVp, image noise will decrease
iodinated contrast will be more conspicuos at what kVp in CT?
80 kVp (average E of the beam which is 1/2 to 1/3 of the kVp will be near the k-edge of iodine)
what are the advantages of a “sharp” kernel
better spatial resolution
tube currents used for CT fluoro
low tube currents (20-50 mA) to minimize radiation dose
is prospective cardiac gating a helical or axial acquisition?
axial!!
ideal HR for cardiac imaging
50-65 bpm
which has a higher dose: retrospective or prospective cardiac CT?
retrospective. Scans the whole time and has a low pitch
noise in CT changes by a facotr of
square root of N
example 1: if mA goes from 200 –> 400 mA, SNR= 1/root 2 or 2/1.4 - so 40% increase in SNR
example 2: if mA goes from 200 –> 800 mA, SNR= 4/root 4 or 4/2 - so DOUBLE the increase in SNR
*need 4x the photon flux to double the SNR
factors that improve SNR in CT
- higher mA
- longer rotation time
- higher kVp
- larger slice thickness
- large pixel (pixel size = FOV/matrix size)
- decreased pitch
increased mA affect on quantum mottle
decreased quantum mottle
in conventional radiography and CT, HVL of soft tissue in a human is assumed to be:
3 cm
HVL difference between conventional radiography, CT, and mammo
conventional: 3cm
CT: 3-4 cm
mammo: 1cm
HVL in mammo is less due to the higher energ beam quality
is contrast resolution better in XR or CT?
CT!
How does bismuth shielding of the orbits affect HU just deep to the shield?
Beam hardening caused by the bismuth shield can result in elevated CT numbers (Hounsfield units) directly deep to the shield. Other techniques exist, such as organ-based tube current modulation, with the same level of dose saving and the same or superior image quality. The American Association of Physicists in Medicine (AAPM) recommends that these alternatives are carefully considered and implemented when possible.
Changing the mA of a head CT from 200 mA to 400 mA will have what affect on SNR?
Increasing the mA will increase the signal-to-noise ratio (SNR) by the square root of 2 (1.4). However, absorbed dose and heat deposition on the anode disc increase linearly with mA and thus will increase 2x. Contrast resolution will increase due to increased SNR.
Changing the mA of a head CT from 200 mA to 400 mA will have what affect on absorbed dose?
Increasing the mA will increase the signal-to-noise ratio (SNR) by the square root of 2 (1.4). However, absorbed dose and heat deposition on the anode disc increase linearly with mA and thus will increase 2x. Contrast resolution will increase due to increased SNR.
Changing the mA of a head CT from 200 mA to 400 mA will have what affec ton heat deposition in the anode disk?
Increasing the mA will increase the signal-to-noise ratio (SNR) by the square root of 2 (1.4). However, absorbed dose and heat deposition on the anode disc increase linearly with mA and thus will increase 2x. Contrast resolution will increase due to increased SNR.
Dose modulation refers to what?
Varying mA as the x-ray tube rotates around the patient and along the long axis to provide the best compromise between image quality and dose
Dose modulation adjusts the mA depending on the attenuation of the patient. Initially, systems only modulated along the z-axis to increase mA in projections with higher attenuation (shoulders) and lower mA in areas of decreased attenuation (neck/chest). Now some scanners can also modulate dose as the tube rotates around the patient and can be organ based to decrease dose to radiosensitive organs (eyes/breast). mA is adjusted based on a reference value and will also adjust for body habitus of the patient. Adjusting kVp depending on body habitus and exam is also a method to reduce dose, but is not referred to as dose modulation.
dosimtery phantom
Standard CT dosimetry phantoms consist of cylindric acrylic phantoms with holes for dosimeter insertion at various locations. The 2 sizes are 16 cm in diameter to represent heads and small pediatric bodies and 32 cm in diameter to represent adult bodies.
what is “z-axis variation”?
tails of radiation along the edge of the area being scanned. If multiple scans are being performed these tails add up with the original scan
https://tech.snmjournals.org/content/35/4/213
difference between dose distribution in Body Ct vs head CT
In Body CT, middle gets less dose compared to the periphery (surface is about twice the central dose
In head CT the central and surface dose is similar
CDTI = CT Dose Index; expressed in mGy
Weighted CTDI (CTDIw) is calculated by 1/3 central CTDI + 2/3 peripheral CTDI
What is the difference between CTDIw and CTDIvol?
Weigthed CTDI takes into account the difference between the dose at the periphery (about double the center) and dose at the center. It is calculated by 1/3 the central CTDI + 2/3 the periphery
CTDIw is expressed in mGy (no volumetric or length data - JUST DOSE)
Volume CTDI (CTDIvol) is obtained by divided CTDIw by the pitch (CTDIvol = CTDIw/pich)
strategies to reduce dose to the breast
do the scan at reduced mA (not ideal due to increased noise)
mA modulation **** preferred method
shield the breasts with bismuth (but results in artifact and degraded image)
also…bismuth shielding results in falsely elevated HU directly deep to the shiled
bismuth shielding results in_____________directly deep to the shiled
bismuth shielding results in falsely elevated HU directly deep to the shiled
bismuth shielding has been done to shield orbits or breasts.
Compared to a conventional radiography x-ray tube, a CT x-ray tube typically has:
a higher heat capacity
CT x-ray tubes have to deal with more heat deposition due to higher mAs and longer run times compared to conventional radiography x-ray tubes. They typically are designed to have higher heat capacities through larger and thicker anode disks with graphite backing &/or more effective heat dissipation.
The term “filtered” in filtered back projection refers to
Sharpening of the projection data prior to back projection
Filtered back projection refers to back projection of projection data where blurring was removed by a deconvolution process. Standard back projection would result in blurred images that would not be suitable for diagnostic imaging. This filtering does not remove effects of beam hardening or other common imaging artifacts. Different reconstruction kernels vary in how much sharpening results from the convolution process, resulting in less sharp images (soft tissue kernel) or sharper images (bone kernel).
Z-filtering can be used to maintain
slice thickness at the expense of decreased signal-to-noise ratio.
Numerous measurements are obtained within slice plane (up to 13, in this example) to form thicker slices. Combining many measurements to form thicker slices is referred to as z filtering.
relationship between noise and mA
inverse relationship
as you increase mA –> decreasing noise
noise = 1/ sq rt(mA)
A patient is coming back for a follow up CT. YOu looked at a prior CT, and it was pretty noisy. How would you change the mAs to reduce noise by a facotr of 2?
Tube current is inversely proportional to noise. Increasing the mAs by a factor of 4 will yield half of the noise (1 / sq rt of 4).
assuming a rotation time of 0.3 seconds and mA of 700, what is the effective tube current time product if the pitch is 0.2?
1050 mA
Effective tube current time product is obtained by multiplying the rotation time by the mA and then dividing by the pitch.
So in this case 0.3 x 700 = 210 mA
210 mA is then divided by 0.2 –> giving 1050 mA.
in CT, how do you calculate an estimated effective dose in mSv?
DLP x a conversion factor
a change in contrast flow rate from 6mL/s to 4 mL/s would result in what?
reduced iodine flux
the iodine flux is the nubmer of iodine molecules administered per unit time and is related to the flow rate and the iodine concentation of the contrast agent. A higher flow rate will result in more molecules of iodine given per unit time and a greater amount of enhancement.
you peform a cardiac CTA using retrospective gating to evaluate cardiac function. In order to minimize dose, you use tube modulation. How does this work in retrospective gating to reduce dose?
patients who undergo retrospective gating will be imaged through systole and diastole. Tube modulation minimizes dose during systole but provides enough dose to calculate function and maximizeds dose duering DIASTOLE to evaluate the corononary arteries.
