Radiology physics-- concepts to reinforce, facts to memorize Flashcards
The CR reader extracts the x-ray exposure pattern using different colors of light in what order?
Red (what is applied), Blue (what is release/read), White (what is applied to erase)
(“RBW”, Ruth Bader Winsberg)
For the following X-ray-based Modalities, what are their standard used effective Focal Spot Sizes?
Standard Radiography / CT / Fluoroscopy / Mammography (standard) / Mammography (magnification) /
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
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 )
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)
What is the typical lead apron thickness? And how much x-ray radiation is typically absorbed by it?
A lead apron is typically 0.5mm with 99% absorption (1% transmission to the operator) for typical kVp
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
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)
how many mammograms required under supervision of an interpreting physician for initial certification and continuing certification?
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
MQSA-mandated mean glandular dose limit, single view:
____mGy with grid
____mGy without grid
MQSA-mandated mean glandular dose limit, single view:
3 mGy with grid
1 mGy without grid
What is the standard dose of Gadolinium contrast for breast MRI?
0.1 mmol/kg
(followed by 10 mL saline flush.)
MEMORIZE
How much compression (force) should a mammo compression unit be capable of?
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)
What findings are present in ACR breast phantom, and what is minimum required visualized to pass?
- 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)
What are the CT dose thresholds for risk-level to detus in pregnant CT imaged patients:
< 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%
Exam Type Optimal kVp:
CT angiography
DSA
Barium Enema
Exam Type Optimal kVp
CT angiography 100
DSA 70
Barium Enema 90-110
Which generation CT scanner is in common use?
3rd generation is the most common type of CT scanner in radiology departments.
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
‘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.
Attenuation formula for ultrasound in soft tissue (dB):
Attenuation of sound (dB) = (0.5 dB/cm/MHz) x (full travel path in cm) x (TF in MHz)
What is the Nyquist limit?
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
Radiopharmaceutical effective half-life formula:
1/effective-half-life = 1/physical-half-life + 1/bio-half-life
What is the decay constant (λ) of Tc-99m?
~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)
The dose calibrator QC schedule…
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)
How is SUV calculated? (what is the formula?)
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)
What is the formula relating pixel size, field of view, and matrix?
Pixel Size = FOV/Matrix
What is special about “Spoiled” GRE imaging (similar to FLASH fyi)?
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.
Relationship between mCi and Bq…
1mCi = 37 MBq (Mega = 10^6)
Half-life formula from time-constant (tau) of a material…
Half-life ~ 0.7/tau
Half-life formula from time-constant (tau) of a material…
Half-life = ~ 0.7/tau
