Radiography and Digital Imaging: Key Concepts and Formulas

How is pixel size calculated, and what is 12 cm ÷ 512?

Pixel size = FOV ÷ matrix. 12 cm ÷ 512 = 0.0234 cm = 0.234 mm.

How do matrix and FOV affect pixel size and spatial resolution?

At the same FOV: larger matrix → smaller pixels → better resolution. At the same matrix: larger FOV → larger pixels → worse resolution.

What is the effect of a smaller pixel or detector element (DEL)?

Better spatial resolution and recorded detail.

What does increasing bit depth improve?

More gray shades and better contrast resolution—not spatial resolution. Gray shades = 2^bit depth.

What is Nyquist frequency?

The highest spatial frequency that can be accurately recorded: 1 ÷ (2 × pixel pitch).

What are the grid conversion factors?

No grid = 1; 5:1 = 2; 6:1 = 3; 8:1 = 4; 10:1 or 12:1 = 5; 16:1 = 6.

What formula is used when changing grids?

New mAs = old mAs × new GCF ÷ old GCF.

Change 10 mAs from a 5:1 grid to a 16:1 grid.

10 × 6 ÷ 2 = 30 mAs.

Change 25 mAs from an 8:1 grid to a 5:1 grid.

25 × 2 ÷ 4 = 12.5 mAs; use approximately 13 mAs.

Change 10 mAs from a 16:1 grid to a 6:1 grid.

10 × 3 ÷ 6 = 5 mAs.

Change 12 mAs with no grid to an 8:1 grid.

12 × 4 ÷ 1 = 48 mAs.

What is the CR imaging plate made from?

A photostimulable phosphor of europium-activated barium fluorohalide.

What is the CR image-production pathway?

X-rays create trapped electrons/latent image → laser releases light → photomultiplier or photodetector converts it to an electrical signal → ADC digitizes it.

What is the direct-conversion DR pathway?

X-rays → electrical charge in amorphous selenium → TFT → ADC. No light-producing scintillator is used.

What is the indirect-conversion DR pathway?

X-rays → light in CsI or GOS scintillator → photodiode converts light to charge → TFT/ADC.

H16 — What is the key difference between direct and indirect DR?

Direct DR converts x-rays directly to charge; indirect DR converts x-rays to light and then charge.

H17 — What do high-pass and low-pass processing do?

High-pass enhances edges/detail but also noise. Low-pass smooths the image and reduces noise but sacrifices detail.

H18 — What does the LUT do?

Maps stored pixel values to displayed gray shades; it primarily controls displayed contrast.

H19 — What does rescaling do?

Standardizes displayed brightness using histogram information; it cannot repair poor acquisition or positioning.

H20 — What are the histogram and VOI?

The histogram represents image exposure values; the VOI selects the relevant anatomy. Poor collimation, centering, or field recognition can cause histogram errors.

H21 — What is linearity versus reproducibility?

Linearity = proportional output using different mA/time settings at equal mAs. Reproducibility = repeated exposures at the same settings produce the same output.

H22 — What are the magnification and SOD formulas?

MF = SID ÷ SOD. SOD = SID − OID.

H23 — What geometric changes improve recorded detail?

Increase SID, decrease OID, and use a smaller focal spot.

H24 — How is motion blur reduced?

Use the shortest practical exposure time; increase mA if needed to maintain the required mAs.

M01 — What does AEC do?

Terminates the exposure after sufficient exit radiation reaches the selected detector, maintaining receptor exposure.

M02 — What is the course rule for full silicone breast implants and AEC?

Turn off AEC and use a manual technique; use minimal appropriate compression.

M03 — What technique is preferred when a pediatric patient does not cover an AEC chamber?

Use a manual technique with lower pediatric kVp and mAs.

M04 — What special pediatric adjustment applies to children younger than six?

Decrease kVp by at least 15% from the adult technique and limit grid use when possible.

M05 — What happens if a dense prosthesis covers an active AEC chamber?

It blocks radiation from the chamber, prolonging exposure and causing overexposure. Select another chamber or use manual technique.

M06 — What is the approximate intensity across the anode-heel effect?

Cathode side ≈ 120%; center = 100%; anode side ≈ 75%.

M07 — How is the heel effect used clinically?

Place thicker anatomy beneath the cathode and thinner anatomy beneath the anode to produce more uniform exposure.

M08 — How are photon number, SNR, and quantum noise related?

More photons → higher SNR and less relative quantum noise. Too few photons produce quantum mottle.

M09 — What produce saturation and fog?

Excessive receptor exposure can cause saturation/clipping; background or stray radiation produces fog.

M10 — How do poor collimation and scatter affect the image?

Poor collimation increases scatter and reduces contrast. Tight collimation and grids reduce scatter.

M11 — What exposure adjustment does a fiberglass cast require?

Fiberglass itself generally requires no change; select technique using the actual measured thickness. Plaster casts require increased exposure.

M12 — What technique adjustment is generally needed for bronchitis?

Use the standard chest technique unless another condition or patient thickness requires adjustment.

M13 — How should technique change for a patient with a BMI around 35?

Increase both kVp and mAs as needed for the increased tissue thickness.

M14 — How should lumbar technique change for a thin, frail patient?

Reduce mAs while generally maintaining appropriate kVp for penetration.

M15 — What is the image-intensifier brightness-gain formula?

Brightness gain = minification gain × flux gain. Minification gain = (input-phosphor diameter ÷ output-phosphor diameter)².

M16 — What does a contrast-detail phantom test?

The system's ability to demonstrate low-contrast objects of different sizes—contrast resolution.

M17 — What does an SNR test evaluate?

The relationship between useful signal and noise; higher SNR means a cleaner image.

M18 — What is the purpose of a daily CR phantom image?

To verify consistent receptor/reader performance and identify developing artifacts.

L01 — What is the proper rule for anatomical markers?

Place the correct lead side marker inside the collimated field during exposure; a postprocessing annotation is not a proper substitute.

L02 — What does a compensating filter do?

Equalizes receptor exposure across anatomy with unequal thickness.

L03 — What usually causes horizontal banding on a DR image?

Failed or improperly calibrated detector elements or detector rows.

L04 — How should lead aprons be tested and protected?

Inspect radiographically or fluoroscopically at least annually; never fold them because folding causes cracks.

L05 — What does the focusing cup do?

The negatively charged focusing cup directs electrons from the cathode filament toward the anode focal spot.

L06 — What are the major transformer functions?

An iron-core step-up transformer increases voltage; a step-down transformer lowers voltage and raises filament current; the autotransformer selects voltage/kVp.

L07 — What are voltage and amperage?

Voltage is the electrical force that pushes electrons; amperage is the rate of electron flow.

L08 — What are oscillation and frequency?

Oscillation is cyclic positive-to-negative electrical variation; frequency is the number of cycles per second, measured in hertz.

L09 — What does monitor dynamic range determine?

The range of brightness and contrast levels the monitor can display.

L10 — What are typical image-intensifier and monitor resolutions?

Image intensifier ≈ 4-6 lp/mm; monitor ≈ 1-2 lp/mm. Standard monitors use about 525 lines and high-resolution monitors about 1,024.

L11 — What is the difference between lossless and lossy image compression?

Both reduce file size. Lossless preserves all data; lossy permanently discards data and excessive compression can destroy diagnostic detail.