Evaluate Digital Image Characteristics - ARRT Radiography Exam

1. Which change increases spatial resolution: smaller pixels or larger pixels?

   Smaller pixels increase spatial resolution. Reducing pixel size allows for finer sampling of spatial frequencies, enhancing the ability to resolve small objects.

2. Which sampling condition prevents aliasing: sampling at $\ge 2f$ or $<2f$?

   Sampling at $\ge 2f$ (Nyquist criterion). The Nyquist criterion ensures accurate reconstruction by sampling at least twice the highest frequency, preventing foldover artifacts.

3. What is aliasing in digital imaging?

   False pattern from undersampling high spatial frequencies. Aliasing occurs when spatial frequencies exceed the Nyquist limit, causing high-frequency signals to appear as lower-frequency artifacts.

4. Which change increases spatial resolution: larger matrix or smaller matrix (same FOV)?

   Larger matrix increases spatial resolution. Increasing matrix size for a fixed FOV reduces pixel size, improving the system's ability to capture higher spatial frequencies.

5. What is spatial resolution in digital radiography?

   Ability to display small objects as separate (recorded detail). It quantifies the system's capability to distinguish fine anatomical details by resolving closely spaced structures without blurring.

6. What is signal-to-noise ratio (SNR) in digital imaging?

   Ratio of useful signal magnitude to noise magnitude. SNR measures image quality by comparing the strength of the desired signal to background noise fluctuations.

7. What is dynamic range in digital radiography?

   Range of exposures producing useful image receptor response. Dynamic range defines the detector's linear response to varying X-ray intensities, enabling capture of a wide exposure latitude.

8. Which characteristic is primarily improved by greater bit depth: contrast resolution or spatial resolution?

   Contrast resolution. Greater bit depth increases the number of gray levels, enhancing the detection of subtle attenuation differences without affecting pixel size.

9. What is modulation transfer function (MTF) used to describe in digital imaging?

   Spatial resolution performance versus spatial frequency. MTF quantifies how effectively the imaging system preserves contrast as spatial frequency increases, indicating resolution efficiency.

10. How many gray levels are available with a 12-bit system?

    $2^{12}=4096$ gray levels. A 12-bit system provides 4096 discrete values, enabling finer gradations in image intensity for improved contrast.

11. What does wide dynamic range allow regarding exposure variation?

    Acceptable images over a broad range of exposures. A wide dynamic range provides exposure latitude, compensating for variations while maintaining diagnostic image quality.

12. Which change generally increases SNR: higher mAs or lower mAs (all else constant)?

    Higher mAs increases SNR (more photons, less quantum mottle). Higher mAs increases photon fluence, reducing quantum noise and improving the signal relative to noise fluctuations.

13. What is contrast resolution in digital imaging?

    Ability to display small differences in tissue attenuation. Contrast resolution enables visualization of subtle density differences, limited by the system's ability to detect small signal variations.

14. What is bit depth in digital imaging?

    Number of bits used to represent each pixel value. Bit depth determines the precision of pixel value quantization, allowing representation of subtle intensity differences.

15. State the formula for the number of gray levels from bit depth.

    $\text{gray levels}=2^{\text{bit depth}}$. The number of gray levels exponentially increases with bit depth, as each bit doubles the possible intensity values.

16. State the formula for SNR in terms of signal and noise.

    $\text{SNR}=\frac{\text{signal}}{\text{noise}}$. SNR is calculated as the ratio of average signal intensity to the standard deviation of noise, indicating image clarity.

17. State the formula for pixel size using field of view and matrix size.

    $\text{pixel size}=\frac{\text{FOV}}{\text{matrix}}$. Pixel size is derived by dividing the field of view by the matrix dimension, determining the spatial sampling interval.

18. For quantum-limited imaging, how does SNR change with the number of detected photons $N$?

    $\text{SNR}\propto \sqrt{N}$. In quantum-limited systems, SNR improves with the square root of photon count due to Poisson statistics reducing relative noise.

19. What is exposure creep in digital radiography?

    Gradual increase in technique due to wide dynamic range masking overexposure. Exposure creep results from technicians incrementally raising exposure factors, as digital systems tolerate overexposure without visible saturation.

20. How many gray levels are available with a 10-bit system?

    $2^{10}=1024$ gray levels. A 10-bit system yields 1024 intensity levels, sufficient for many imaging applications but less than higher bit depths.

21. Which change increases spatial resolution: smaller FOV or larger FOV (same matrix)?

    Smaller FOV increases spatial resolution. Decreasing FOV for a fixed matrix reduces pixel size, enabling better resolution of fine details within the imaged area.

22. State the formula for Nyquist frequency in terms of pixel size.

    $f_N=\frac{1}{2\times \text{pixel size}}$. The Nyquist frequency represents the maximum spatial frequency that can be accurately sampled without aliasing, half the reciprocal of pixel size.

23. What is the limiting spatial resolution primarily determined by in DR/CR detectors?

    Detector element size (pixel size or phosphor laser spot size). Smaller detector elements capture finer details, directly limiting the maximum resolvable spatial frequency in digital systems.

24. What is quantum mottle in digital radiography?

    Noise from insufficient photon numbers (quantum noise). Quantum mottle arises from statistical variations in low photon counts, manifesting as graininess in underexposed images.

25. If detected photons increase from $N$ to $4N$, how does SNR change (quantum-limited)?

    SNR doubles because $\sqrt{4N}=2\sqrt{N}$. Quadrupling photons increases SNR by a factor of two, as noise scales with the square root of photon count.

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