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Physics
Ch 12
| Question | Answer |
|---|---|
| What is the purpose of an image receptor? | To capture the x-ray energy transmitted through the patient and convert it into a visible image. |
| What are the two main types of image receptors? | Film-screen and digital (CR/DR) systems. |
| What does CR stand for? | Computed Radiography. |
| What does DR stand for? | Digital Radiography. |
| What is a photostimulable phosphor? | Material in CR plates that stores energy from x-rays and releases it as light when stimulated by a laser. |
| How does a CR reader extract the image? | A laser scans the PSP plate, releasing light proportional to stored x-ray energy, which is then converted to digital data. |
| What is a DR flat-panel detector? | A digital receptor where x-rays are directly or indirectly converted to electrical signals. |
| What is indirect DR? | X-rays are converted to light by a scintillator, then to electrical signal by photodiodes. |
| What is direct DR? | X-rays are directly converted to electrical signal by photoconductor material (like amorphous selenium). |
| What is detector element (DEL) in DR? | The smallest sensing unit in a digital detector that captures x-ray energy. |
| What is spatial resolution? | The ability to distinguish small objects as separate entities on the image. |
| What factor primarily affects spatial resolution in digital systems? | Pixel size; smaller pixels → higher resolution. |
| What is contrast resolution? | Ability to distinguish small differences in intensity (gray levels) in the image. |
| What does dynamic range mean? | The range of exposure levels that the detector can accurately capture. |
| What is image latitude? | The range of exposures that produce an acceptable image without loss of contrast. |
| How does film-screen speed affect exposure? | Faster film reduces exposure but decreases image sharpness. |
| What is mottle in film-screen imaging? | Random variations in optical density causing graininess. |
| What is quantum noise in digital imaging? | Random fluctuations due to insufficient x-ray photons reaching the detector. |
| What is the Modulation Transfer Function (MTF)? | Measure of detector or system’s ability to reproduce object contrast at different spatial frequencies. |
| How is MTF related to resolution? | Higher MTF at higher frequencies indicates better spatial resolution. |
| What is Detective Quantum Efficiency (DQE)? | Measure of efficiency of a detector in converting x-ray signal to image signal relative to noise. |
| How does higher DQE affect patient dose? | Higher DQE allows same image quality at lower patient dose. |
| What is fill factor in DR detectors? | Percentage of DEL area that is sensitive to x-rays; higher fill factor → better efficiency. |
| What is the difference between indirect and direct conversion in DR? | Indirect uses scintillator + photodiode; direct uses photoconductor to convert x-rays directly to charge. |
| What is the typical spatial resolution of CR systems compared to DR? | CR has slightly lower resolution (~3–5 lp/mm) than DR (~5–10 lp/mm). |
| What is a latent image in CR? | Stored energy pattern in the photostimulable phosphor after exposure, before scanning. |
| What is image plate erasure? | Process of exposing the CR plate to bright light to remove residual image for reuse. |
| What is the purpose of a scintillator in indirect DR? | To convert x-ray photons into visible light for detection. |
| What is the effect of pixel pitch on image quality? | Smaller pixel pitch improves spatial resolution but may increase noise. |
| What is the purpose of an analog-to-digital converter (ADC)? | Converts the light or charge signal from the detector into digital numbers for processing. |
| What is digital subtraction radiography? | Technique to remove background structures to better visualize changes or contrast media. |
| What is the effect of detector efficiency on patient dose? | Higher efficiency (higher DQE) allows lower dose to achieve same image quality. |
| What is automatic exposure control (AEC)? | System that terminates exposure when the detector senses sufficient radiation has reached the receptor. |
| How does backscatter affect digital detectors? | It may add unwanted signal and reduce contrast; shielding or anti-scatter grids help. |
| What is bit depth in digital imaging? | Number of bits used to represent each pixel; determines number of gray levels. |
| How does higher bit depth affect contrast resolution? | Higher bit depth improves ability to distinguish subtle differences in intensity. |
| What is aliasing in digital imaging? | Artifact caused by undersampling high-frequency information, producing moiré patterns. |
| How can aliasing be prevented? | By using adequate sampling rate and anti-aliasing filters. |
| What are CR artifacts caused by plate handling? | Scratches, dust, dirt, or incomplete erasure leading to image defects. |
| What are common DR artifacts? | Dead pixels, drop-out, gain calibration errors, software processing errors. |
| What is lag in digital detectors? | Residual signal from previous exposure affecting current image. |
| What is spatial resolution in lp/mm? | Line pairs per millimeter — measure of system’s ability to resolve fine detail. |
| What is the effect of scatter on digital image contrast? | Reduces contrast; mitigated with grids, collimation, or air gap. |
| What is the difference between film-screen and digital detector response to exposure? | Film has nonlinear response; digital detectors have linear response over wide range. |
| What is exposure index (EI) in digital systems? | Indicator of radiation level received by the detector; helps assess proper technique. |
| What is window width in digital imaging? | Range of gray levels displayed; narrow window → higher contrast. |
| What is window level? | Center point of the gray scale displayed; adjusts image brightness. |
| What is image processing in digital radiography? | Adjusting pixel values for optimal display, contrast, and noise suppression. |
| What is edge enhancement? | Post-processing technique to improve visibility of boundaries and structures. |
| What is smoothing? | Post-processing technique to reduce noise, but may reduce detail. |
| What is dynamic range advantage of digital imaging over film? | Digital systems can capture useful image over wider range of exposures. |
| What is the effect of overexposure in digital imaging? | Image may appear acceptable due to wide latitude, but patient dose is unnecessarily high (exposure creep). |
| What is the effect of underexposure in digital imaging? | Increases image noise; may reduce diagnostic quality. |
| What is modulation of digital receptor signal? | Ability of detector to reproduce detail contrast at varying spatial frequencies (MTF). |
| What is a collimator’s role in image receptor protection? | Limits x-ray field to reduce scatter and unnecessary exposure. |
| What is the effect of focal spot size on receptor resolution? | Smaller focal spot improves sharpness and resolution; larger focal spot reduces resolution. |
| What is detective quantum efficiency (DQE) related to? | Ability of detector to produce high signal-to-noise ratio relative to radiation dose. |
| What is pixel fill factor? | Percentage of pixel area sensitive to x-rays; higher fill factor → higher efficiency. |
| What is a cassette in CR? | Protective housing for the photostimulable phosphor plate. |
| How is spatial resolution affected by image matrix size? | Larger matrix with same FOV improves resolution (smaller pixel size). |
| What is a line-pair phantom used for? | To measure spatial resolution and test detector performance. |
| What is an example of digital receptor noise reduction? | Averaging multiple readings, smoothing filters, or software noise reduction. |
| How does detector thickness affect image quality? | Thicker phosphor layers absorb more x-rays (higher sensitivity) but reduce resolution. |
| What is the primary advantage of DR over CR? | Faster image acquisition, higher DQE, and often better resolution with lower dose. |
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