Radiology physics-- concepts to reinforce, facts to memorize

Question 1

The CR reader extracts the x-ray exposure pattern using different colors of light in what order?

Answer 1

Red (what is applied), Blue (what is release/read), White (what is applied to erase)

(“RBW”, Ruth Bader Winsberg)

Question 2

For the following X-ray-based Modalities, what are their standard used effective Focal Spot Sizes?

Standard Radiography / 
CT /
Fluoroscopy	/   
Mammography (standard)  /	
Mammography (magnification)  /

Answer 2

X-ray-based Modality / Effective Focal Spot Size

Standard Radiography / 1.2 mm
CT / 1.0 mm
Fluoroscopy	/  variable from 0.3 mm to 1.2 mm 
Mammography (standard)  /	0.3 mm
Mammography (magnification)  /  0.1 mm

Question 3

What is the typical Resolution Limit (lp/mm) for each of these modalities:

CT	
Fluoroscopy	
 DSA
 CR-based digital radiography	
 film radiography	
digital mammography	
film mammography	)

Answer 3

Modality	Resolution Limit (lp/mm)
CT	1
Fluoroscopy	1-4
 DSA	2
 CR-based digital radiography	5
 film radiography	7
digital mammography	7
film mammography	11 & 13 (perpendicular vs. parallel to focal spot, respectively- MQSA standards)

Question 4

What is the typical lead apron thickness? And how much x-ray radiation is typically absorbed by it?

Answer 4

A lead apron is typically 0.5mm with 99% absorption (1% transmission to the operator) for typical kVp

Question 5

Typical cancer detection rates:

for digital mammo…
for mammo+tomo…

Typical recall rates:

….. for digital mammo (also the ACR practice standard for your practice)

…… for mammo+tomo

Answer 5

Typical cancer detection rates:

4 per 1000 for digital mammo
5 per 1000 for mammo+tomo
Typical recall rates:

10% for digital mammo (also the ACR practice standard for your practice)
9% for mammo+tomo

(callback rates ~10%, with cancer rate detection ~ .4% for digital mammo; one point better each for mammo+tomo)

Question 6

how many mammograms required under supervision of an interpreting physician for initial certification and continuing certification?

Answer 6

Initial certification - 240 exams within any 6 months of last 2 years of residency

Continuing certification - 960 exams per 2 years
Details

Other requirements you should know:

Initial - 3 months of training in mammography

Continuing - 15 category-1 CME credits per 3 years

Question 7

MQSA-mandated mean glandular dose limit, single view:

____mGy with grid
____mGy without grid

Answer 7

MQSA-mandated mean glandular dose limit, single view:

3 mGy with grid
1 mGy without grid

Question 8

What is the standard dose of Gadolinium contrast for breast MRI?

Answer 8

0.1 mmol/kg
(followed by 10 mL saline flush.)

MEMORIZE

Question 9

How much compression (force) should a mammo compression unit be capable of?

Answer 9

111 - 200 Newtons (25-45 lbs). Most residents memorize this range or at least the lower end (111 Newtons; 25 lbs.)

(remember 25lbs, 111N….. note this is what machine should be capable of, not what is necessarily used in each case)

Question 10

What findings are present in ACR breast phantom, and what is minimum required visualized to pass?

Answer 10

• ACR phantom contains 6-5-5 (fibers -speck groups-masses)
• Passing score requires visualizing at least 4-3-3

Actually the above is for an “old” phantom/requirements; for a “new” is:
Old: (6,5,5), passing (4,3,3)
· New: (6,6,6), passing (2,3,2)

Question 11

What are the CT dose thresholds for risk-level to detus in pregnant CT imaged patients:

Answer 11

< 14 days post-conception:

100 mGy threshold for fetal death. “All-or-nothing” survival: no elevated deterministic or stochastic risk.

Later in gestation:

50 mGy fetal deterministic effects threshold (teratogenicity & mental retardation esp. 2-15 wks.)

25 mGy in-utero exposure (CT abd/pelvis to mother) increases risk of childhood cancer by ~1%

Question 12

Exam Type Optimal kVp:

CT angiography

DSA

Barium Enema

Answer 12

Exam Type Optimal kVp

CT angiography 100

DSA 70

Barium Enema 90-110

Question 13

Which generation CT scanner is in common use?

Answer 13

3rd generation is the most common type of CT scanner in radiology departments.

Question 14

Which one of these CT scanners is capable of the greatest radiation dose efficiency?
Choose only ONE best answer.

A
SDCT
B
512 slice MDCT
C
256 slice MDCT
D
128 slice MDCT
E
64 slice MDCT

Answer 14

‘A’
Explanation

SDCT is more radiation dose efficient than MDCT.

Memorize This

Geometric efficiency is better for SDCT than MDCT (bc no septa, and no penumbra/”spill over required”).
However, Within MDCT types, geometric efficiency goes up with # slices.

Question 15

Attenuation formula for ultrasound in soft tissue (dB):

Answer 15

Attenuation of sound (dB) = (0.5 dB/cm/MHz) x (full travel path in cm) x (TF in MHz)

Question 16

What is the Nyquist limit?

Answer 16

The Nyquist limit represents the maximum Doppler shift frequency that can be correctly measured without resulting in aliasing in color or pulsed wave ultrasound.

Physics:
The Nyquist limit always equals Pulse Repetition Frequency (PRF)/2

Question 17

Radiopharmaceutical effective half-life formula:

Answer 17

1/effective-half-life = 1/physical-half-life + 1/bio-half-life

Question 18

What is the decay constant (λ) of Tc-99m?

Answer 18

~0.1155 hr-1

(dont memorize the above number, but understand how to get it from the formula for decay constant….know the difference between decay constant (λ, lambda) and half life (t1/2)……:

formula:
lambda = 0.693/half-life

Memorize This:

Half life of Tc-99m is 6 hrs.
lambda = 0.693/half-life (approx: lambda =0.7/half-life)
(equivalent: half-life = 0.693/lambda)

NOTE formula structure and constant pops-up for another half-life-like formula, half-value layer (the constant there labeled with mu, rather than lamba, but also ~0.7):
HVL = 0.693/mu)

Question 19

The dose calibrator QC schedule…

Answer 19

The dose calibrator QC schedule follows the CLAG acronym:
              Constancy - Daily
              Linearity - Quarterly
              Accuracy - Annually
              Geometry - On repair

To memorize the maintenance times, try “Dairy Queen Always Open” (Daily, Quarterly, Annually, On-repair)

Question 20

How is SUV calculated? (what is the formula?)

Answer 20

SUV relates to ROI activity, Activitity admnistered, and body weight, as follows Memorize This:

Standardized Uptake Value (SUV) = (Mean ROI activity / activity administered) / (body weight g)

Question 21

What is the formula relating pixel size, field of view, and matrix?

Answer 21

Pixel Size = FOV/Matrix

Question 22

What is special about “Spoiled” GRE imaging (similar to FLASH fyi)?

Answer 22

Enable use of short TR’s (for benefit of faster imaging times, i.e. during single breath holds) bc use a “spoiler gradient” after each echo which destrots transvere magenetization.

Question 23

Relationship between mCi and Bq…

Answer 23

1mCi = 37 MBq (Mega = 10^6)

Question 24

Half-life formula from time-constant (tau) of a material…

Answer 24

Half-life ~ 0.7/tau

Question 25

Half-life formula from time-constant (tau) of a material…

Answer 25

Half-life = ~ 0.7/tau