CT

Question 1

Answer 1

Spatial resolution phantom

Question 2

Answer 2

noise CT phantom

Question 3

CT brain image has level of 2 HU and window of 2 HU. Altering what parameter will improve contrast?

Answer 3

increasing the width HU will improve contrast

Question 4

detection of a large low contrast object can be improved by:

Answer 4

• smoothing algorithm
• thicker slices
• increasing mA

Question 5

The term “effective mAs” refers to

Answer 5

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.

Question 6

How does pitch affect effective mAs?

Answer 6

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.

Question 7

what generation are most CT scanners?

Answer 7

3rd generation

which means the xray tube and the detector spin around the patient in syncrony

Question 8

what is the typical CT tube current?

Answer 8

1000 mA

(regular general radiography is 200-800 mA)

Question 9

large or small focal spot on the CT xray tube anode?

Answer 9

large focal spot to handle all the power

Question 10

what filter is typically used in CT?

Answer 10

copper or aluminum (6mm) to filter the xray beam

Question 11

what is a bow tie filter?

Answer 11

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

Question 12

where are collimators positioned in CT scanners?

Answer 12

both the xray tube and detector to shape the xray beam

also reduces some scatter

Question 13

what is filtered back projection?

Answer 13

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.

Question 14

what is minimal slice thickness determined by?

Answer 14

detector element aperture

Question 15

what is a ray

Answer 15

a measure of total xray attenuation along a line from the focal point to a single detector

Question 16

what is a projection?

Answer 16

all rays at a given angle of the xray tube (a series of rays that pass through the patient)

Question 17

what kind of xrays are used with CT?

Answer 17

highly filtered, high kV (average energy 75 keV)

Question 18

what is the matrix size for CT? each pixel is?

Answer 18

matrix is 512 x 512

each pixel representing 4096 possible shades of gray (12 bits)

2^x
2¹² = 4096

Question 19

what is the relationship between pixel width and height to voxels?

Answer 19

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

Question 20

How do you calculate pixel size?

Answer 20

pixel size = FOV/matrix

Question 21

what is a “kernel”?

Answer 21

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

Question 22

How do you improved spatial resolution in CT?

Answer 22

Decrease pixel size

Pixel size =FOV/Matrix

Question 23

what happens to the images if you turn down mAs?

Answer 23

less mAs = more noise

(decreased SNR)

Question 24

what is pitch?

Answer 24

the distance the table moves in a single revolution

pitch = table movement/beam width

Question 25

pitch \> 1

Answer 25

table moved fastr than the beam and there is a gap between slices this gap means spatial resolution suffers

Question 26

pitch \<1 affect on spatial resolution and dose

Answer 26

table moves slow and slices overlapped spatial resolution improves but so does DOSE

Question 27

typical pitch of cardiac CT

Answer 27

0.2 (slices overlap, high dose)

Question 28

sometimes a pitch \< 1 is referred to as

Answer 28

"over scanning"

Question 29

whta i the formula to calculate HU

Answer 29

HU = 1000 x (attenuation of material - attenuation of water)/attenuation of water

Question 30

What is the relationship between HU and Xray attenuation?

Answer 30

When HU increases by 10 HU, xray attenuation increases 10%

Question 31

does photon energy (keV) mess with the HU?

Answer 31

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)

Question 32

how does filtration affect HU?

Answer 32

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

Question 33

what is a classic artifact that can happen due to filtration and beam hardening

Answer 33

"cupping artifact", where things appear less dense in the center and more dense along the periphery

Question 34

what type of scanning results in better spatial resolution and less partial volume artifact?

Answer 34

axial scanning

Question 35

whta is partial volume artifact?

Answer 35

[partial volume a](https://radiopaedia.org/articles/partial-volume-averaging-ct-artifact-1?lang=us)rtifact 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. [https://pubs.rsna.org/doi/pdf/10.1148/rg.246045065](https://pubs.rsna.org/doi/pdf/10.1148/rg.246045065)

Question 36

less stair step arifact is seen with

Answer 36

helical scanning (axial has worse stair step artifact)

Question 37

how does increased beam width affect time?

Answer 37

larger coverage with 1 turn

Question 38

how does increased beam width affect motion artifact?

Answer 38

reduces motion artifact

Question 39

how does increased beam width affect partial volume?

Answer 39

increases partial volume (more divergent beam)

Question 40

unlike conventional radiography with automated exposure control, what does increased kVp do to dose in CT?

Answer 40

increases radiation dose to the patient since there is NO automated exposure control in CT

Question 41

how does increasing kVp affect noise?

Answer 41

increasing kVp, image noise will decrease

Question 42

iodinated contrast will be more conspicuos at what kVp in CT?

Answer 42

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)

Question 43

what are the advantages of a "sharp" kernel

Answer 43

better spatial resolution

Question 44

tube currents used for CT fluoro

Answer 44

low tube currents (20-50 mA) to minimize radiation dose

Question 45

is prospective cardiac gating a helical or axial acquisition?

Answer 45

axial!!

Question 46

ideal HR for cardiac imaging

Answer 46

50-65 bpm

Question 47

which has a higher dose: retrospective or prospective cardiac CT?

Answer 47

retrospective. Scans the whole time and has a low pitch

Question 48

noise in CT changes by a facotr of

Answer 48

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

Question 49

factors that improve SNR in CT

Answer 49

* higher mA * longer rotation time * higher kVp * **larger slice thickness** * large pixel (pixel size = FOV/matrix size) * decreased pitch

Question 50

increased mA affect on quantum mottle

Answer 50

decreased quantum mottle

Question 51

in conventional radiography and CT, HVL of soft tissue in a human is assumed to be:

Answer 51

**3 cm**

Question 52

HVL difference between conventional radiography, CT, and mammo

Answer 52

conventional: 3cm CT: 3-4 cm mammo: 1cm HVL in mammo is less due to the higher energ beam quality

Question 53

is contrast resolution better in XR or CT?

Answer 53

CT!

Question 54

How does bismuth shielding of the orbits affect HU just deep to the shield?

Answer 54

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.

Question 55

Changing the mA of a head CT from 200 mA to 400 mA will have what affect on SNR?

Answer 55

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.**

Question 56

Changing the mA of a head CT from 200 mA to 400 mA will have what affect on absorbed dose?

Answer 56

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.

Question 57

Changing the mA of a head CT from 200 mA to 400 mA will have what affec ton heat deposition in the anode disk?

Answer 57

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.

Question 58

Dose modulation refers to what?

Answer 58

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.

Question 59

Answer 59

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.

Question 60

what is "z-axis variation"?

Answer 60

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](https://tech.snmjournals.org/content/35/4/213)

Question 61

difference between dose distribution in Body Ct vs head CT

Answer 61

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

Question 62

What is the difference between CTDIw and CTDIvol?

Answer 62

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)

Question 63

strategies to reduce dose to the breast

Answer 63

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

Question 64

bismuth shielding results in\_\_\_\_\_\_\_\_\_\_\_\_\_directly deep to the shiled

Answer 64

bismuth shielding results in falsely elevated HU directly deep to the shiled bismuth shielding has been done to shield orbits or breasts.

Question 65

Compared to a conventional radiography x-ray tube, a CT x-ray tube typically has:

Answer 65

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.

Question 66

The term "filtered" in filtered back projection refers to

Answer 66

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).

Question 67

Z-filtering can be used to maintain

Answer 67

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.

Question 68

relationship between noise and mA

Answer 68

inverse relationship as you increase mA --\> decreasing noise noise = 1/ sq rt(mA)

Question 69

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?

Answer 69

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).

Question 70

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?

Answer 70

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.

Question 71

in CT, how do you calculate an estimated effective dose in mSv?

Answer 71

DLP x a conversion factor

Question 72

a change in contrast flow rate from 6mL/s to 4 mL/s would result in what?

Answer 72

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.

Question 73

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?

Answer 73

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.