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ACR CT and Dose
Phantom Testing
66
Medical
Post-Graduate
07/13/2016

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Cards

Term
The ACR CT phantom has how many modules?
Definition
4
Term
What is the size of each module?
Definition
4 cm in depth and 20 cm in diameter
Term
The first module tests
Definition
Alignment, CT #, and slice thickness.
Term
The second module tests
Definition
Low contrast resolution
Term
The third module tests
Definition
Uniformity, noise and in-plane distance accuracy.
Term
The fourth module tests
Definition
High contrast resolution
Term
What is marked on the top of the phantom?
Definition
Foot - at module 1 and Head and module 4.
Term
The name of the ACR CT accreditation phantom is
Definition
the Gammex 464
Term
How does module 1 assess positioning and alignment?
Definition
There are four BB's at the phantom surface at 3, 6, 9 and 12 o'clock.
Term
What are the different materials and their CT#'s in Module 1.
Definition
Bone +955 HU, Air -1000 HU, Acrylic +120 HU, Polyethylene -95 HU, and Water 0 HU.
Term
How is slice thickness tested in Module 1?
Definition
There are two ramps consisting of fine wires that are located 0.5 mm apart in the z-direction.  Count the number of visible wires and multiple by 0.5 mm for the slice thickness.
Term
In module 2, how many groups of cylinders are there?  How many cylinders are in a group?
Definition
5 groups of 4
Term
What are the diameters of the different cylinders in Module 2.
Definition
2, 3, 4, 5, and 6 mm.
Term
In the low contrast module, what is the CT # of the cylinder's and how different are they from background.
Definition
The CT # is 90 HU and it is 6 HU different from background.
Term
What is at the top of module 2?  What is it used for?
Definition
It is a 25 mm cylinder used to asses contrast-to-noise ratio.
Term
What does Module 3 primarily consist of?
Definition
A uniform tissue equivalent material.
Term
What objects are in Module 3 and what are they used for?
Definition
There are two small BB's located 100 mm apart to assess in-plane distance measurements.
Term
What is the spacing of the bar patterns in module 4?
Definition
4, 5, 6, 7, 8, 9, 10, and 12 lp/cm
Term
Besides bar patterns, what else is in Module 4.
Definition
Four external BB's just like module 1.
Term
What scans are done on the phantom?
Definition
Adult head, adult abdomen, pediatric head, pediatric abdomen.
Term
What display field of view should be used with the ACR phantom.
Definition
As close to, but not smaller than 21 cm.
Term
Proper alignment occurs when
Definition
All 4 BB's are visible and the central wires in the slice thickness ramp are symmetrically located.
Term
What size ROI should you use for the CT number calibration?
Definition
≈ 200 mm2
Term
What is the CT # calibration criteria for water?
Definition
HU between -7 and +7
Term
Which protocols do you have to measure all the CT #'s for?
Definition
Just the adult abdomen protocol.
Term
How large of an ROI should you use for the CNR measurement?
Definition
≈ 100 mm2
Term
Where do you place the two ROI's for the CNR measurement?
Definition
One over the large (25 mm) cylinder and one between the large cylinder and the 6 mm cylinders.
Term
What formula is used for CNR?
Definition
CNR = (A-B)/σB
Term
What are the criteria for the CNR to pass?
Definition
adult head and abdomen - CNR > 1; pediatric abdomen - CNR > 0.4, pediatric head - CNR > 0.7
Term
How large is the ROI for the uniformity test?
Definition
≈ 400 mm2
Term
How is uniformity measured?
Definition
Place 5 ROI's; one in the center and four on the edges (top, bottom, left, and right), edge ROI's should be inward one ROI diameter.  The CT #'s must be withing ±5 HU of the center ROI.
Term

What are the four pass/fail values for CTDIvol?

What are the diagnostic reference levels?

Definition
Adult head - 80 mGy, Adult abdomen - 30 mGy, Pediatric head - 40 mGy, Pediatric abdomen - 20 Gy.  Note - the DRL's are 5 mGy lower.
Term
Where are the dosimetry phantoms to be placed.
Definition
Adult head - in the head holder.  The other 3 scans, the phantom goes on the table top.
Term
What type of scan must be used to acquire dose information?
Definition
A single axial scan.
Term
What is the requirement for high contrast resolution.
Definition
>= 6 lp/cm for Adult Abdomen studies
>= 8 lp/cm for High Resolution Chest studies.
Term
What is the requirement for low contrast resolution?
Definition
At least resolve the 6mm pins for Adult Abdomen & Head studies
 
Term
What document defines k-factors?
Definition
AAPM Report 96
Term
What are k-factors.  What are the units.
Definition

They are used to convert from DLP to effective dose.

Units are mSv per mGy·cm

Term
How is effective dose calculated in CT?
Definition
E (mSv) = k (mSv / mGy·cm) * DLP (mGy·cm)
Term
K-factors are based on what?
Definition
Adult (all) and pediatrics of various ages, and then by region of the body (head, neck, chest, abdomen/pelvis)
Term
Effective dose represents radiation detriment averaged over ______ and _______.
Definition

gender and age.

(this seemed like an important point - effective dose is not age or gender specific)

Term

What does DLP stand for?

How is it calculated?

Definition

Dose Length Product

DLP (mGy·cm) = CTDIvol (mGy) × scan length (cm)

Term
What is DLP a good reflection of?
Definition
It reflects the total energy delivered for a given scan protocol.
Term
Equation for CTDIvol
Definition
CTDIvol = CTDIW / Pitch
Term
What does CTDIvol represent?
Definition
The average absorbed phantom dose over they x, y, and z directions at the center of the scan length.  It's averaged over the z-direction because it includes the effects of pitch.
Term
What does CTDIW represent.
Definition
The average absorbed dose over the x and y directions at the center of the scan where the scatter tails are negligible beyond the 100-mm integration limit.
Term
How is CTDIW calculated?
Definition
CTDIW = 1/3 CTDI100,center + 2/3 CTDI100,peripheral
Term
How is Pitch calculated?
Definition

Pitch = I / NT

I - table travel per rotation of the gantry

N - number of data slices 

T - slice thickness (mm)

Term
How is CTDI100 calculated?
Definition

CTDI100 = C × f × E × L / (NT)

f - 8.7 mGy/R

C - chamber factor (typically 1.0)

E - exposure measured with the pencil chamber

L - length of the chamber, 100 mm

N - number of data channels

T - thickness of a data channel (mm)

Term
Define CTDI.  What does it represent?
Definition

CTDI = Integrated Dose from ± infinity / NT

It represents the average absorbed dose along the z-axis from a series of continguous radiations.

Term
What are the requirements for XR-29 compliance.
Definition

1. DICOM Structured Dose Reporting

2. Pediatric and Adult reference protocols

3. CT Dose Check

a. Dose notifications (user defined)

b. Dose alerts (CTDIVOL > 1000 mGy)

4. Automatic Exposure Control

Term
What is another name for XR-29?
Definition
MITRA Smart Dose
Term
What is the purpose of CTDIvol?
Definition
To compare radiation output levels between different CT scanners and different protocols using a reference phantom.
Term
Why doesn't CTDIvol estimate patient dose.
Definition
Patient dose is a function of both patient size and scanner radiation output.  CTDIvol is only a measure of scanner output.
Term
What does the effective diameter of a patient represent?  What is the formula for it?
Definition

It is the diameter of a circle that has the same cross sectional area as the patient.

 

Deff = √(AP×LAT)

Term
What does SSDE stand for.  How does it work?
Definition

SSDE - Size Specific Dose Estimate

CTDIvol is multiplied by a factor based on patient size (effective diameter) to get a size specific dose estimate.

SSDE (mGy) = fsize × CTDIvol

fsize depends on 16 or 32 cm phantom

Term
Why is SSDE needed?
Definition
Because CTDIvol is not a measure of patient dose.
Term
What does SSDE not account for?
Definition
It only accounts for the actual patient dimension, it does not account for the actual patient attenuation which is what really determines absorbed dose.
Term
What is DW.  What does it represent?
Definition

Water equivalent diameter.

It's the diameter of a cylinder of water having the same attenuation as a patient.

Term
Size Specific Dose Estimates (SSDE) are defined in what report.
Definition
AAPM Task Group 204
Term
For the same CTDIvol how does the SSDE of a small patient compare to that of a large patient?
Definition
SSDE small patient > SSDE large patient
Term
How does kVp affect dose?
Definition

Lower kVp → lower dose

Higher kVp → higher dose

Term
For identical CT settings, a child (smaller cross sectional area) will get _______ dose than an adult (larger cross sectional area).
Definition
more
Term
What are the fundamentral principles of radiation protection during imaging?
Definition

1. justification - the exam must be indicated

2. utilization - the correct modality must be used

3. optimization - appropriate balance between dose and image quality

Term
What is the approximate HVL for tissue at 120 kVp?
Definition
4 cm
Term
SSDE apply to what types of scans?
Definition

Torso (chest, abdomen, and/or pelvis)

Don't use SSDE for head scans.

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