Detector Exposure & Noise
What is quantum mottle?
A speckled, grainy appearance on an image because too few x-ray photons reached the detector.
What is a detector element or DEL?
The physical component within an image receptor that collects image signal
Increasing this display setting shows a broader range of gray shades and lowers displayed contrast.
Window width
A thick body part places the anatomy farther from the receptor. Which distance increases: SID, SOD, or OID?
OID
A 5-cm object appears 6 cm long on an image. What is its magnification factor?
1.2
A radiographer wants to improve visibility of a low-contrast structure by limiting unwanted radiation reaching the detector while maintaining adequate exposure. What should the radiographer reduce?
Scatter radiation
Keeping field of view constant while increasing matrix size makes pixels smaller. What generally happens to spatial resolution?
It increases
A system uses 12 bits to assign grayscale values. How many values are possible?
4,096 gray values
Moving an object farther from the image receptor (increasing OID) generally increases this change in its image size.
Magnification
SID is 150 cm and OID is 25 cm. What is the magnification factor?
1.2
What does increasing mAs generally raise, if the kVp and patient positioning are kept constant, to get more useful image information relative to noise?
Signal-to-noise ratio
A 40-mm field of view is divided into a 2,000 × 2,000 matrix. What is the pixel size in millimeters?
0.02 mm
A technologist changes the window settings after exposure. Does the change alter the number of photons originally captured by the detector?
No
To reduce focal spot blur, should a radiographer choose a smaller or larger focal spot?
Smaller focal spot
An image measures 8.4 cm and its magnification factor is 1.40. What is the object’s actual length?
6 cm
Being able to distinguish a subtle, soft-tissue lesion against the surrounding grays of soft tissue is an example of?
Contrast resolution
What is aliasing?
Missing or distorted detail due to an imaging system not taking enough samples of data.
A detector redesign leaves less of each element available to collect signal. Which measure of its active collecting area has decreased?
Fill factor
A finger is angled away from the receptor while the central ray remains perpendicular to it. What shape distortion is likely?
Foreshortening
SID is 120 cm and the magnification factor is 1.25. What is the OID?
24 cm
If two detectors receive the same exposure, and one converts the incoming x-rays into a diagnostic image more efficiently than the other, what does this represent?
Detective Quantum Efficiency
A detector samples once every 0.20 mm. What is its Nyquist frequency?
2.5 lp/mm
This measure describes how well an imaging system preserves contrast as anatomical detail becomes progressively finer.
Modulation Transfer Function
What is focal spot blur?
The unsharpness (or blur) around the edges of the image caused by the size of the focal spot
OID is 18 cm. What SID produces a magnification factor of 1.20?
108 cm