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Physics
Ch 14
| Question | Answer |
|---|---|
| What is image acquisition in radiography? | The process of capturing x‑rays that have passed through the patient and converting them into a radiographic image. |
| What does SID stand for? | Source‑to‑Image Distance. |
| What does SOD stand for? | Source‑to‑Object Distance. |
| What does OID stand for? | Object‑to‑Image receptor Distance. |
| What is magnification in radiography? | Increase in the size of the image compared to the object due to geometric factors. |
| What is the magnification factor formula? | MF = SID ÷ SOD. |
| How can magnification be reduced? | Increase SID or decrease OID (i.e. bring object closer to image receptor). |
| What is spatial resolution? | The ability to distinguish small objects close together on an image. |
| How does OID affect spatial resolution? | Greater OID increases magnification and blur — reducing spatial resolution. |
| How does SID affect spatial resolution? | Increasing SID improves spatial resolution (less magnification/blur). |
| What is distortion in radiography? | Misrepresentation of the shape or size of an object on the image. |
| What causes size distortion? | Improper SID, OID, or magnification factor. |
| What causes shape distortion? | Misalignment of x‑ray tube, object, or image receptor (angulation). |
| What is foreshortening? | Object appears shorter than its real length due to improper angulation. |
| What is elongation? | Object appears longer than its real length due to improper angulation. |
| What is focal‑spot blur? | Loss of image sharpness due to the finite size of the x‑ray source (focal spot). |
| How does focal spot size affect image detail? | Smaller focal spot improves detail; larger focal spot reduces detail but allows higher tube loading. |
| What is the inverse square law? | Intensity is inversely proportional to the square of the distance from the source (I ∝ 1/d²). |
| How does changing SID affect beam intensity? | Doubling SID reduces intensity to one‑quarter (unless mAs is adjusted). |
| Why must mAs be increased when SID increases? | To compensate for decreased beam intensity and maintain receptor exposure. |
| What is contrast? | Difference in density or brightness between adjacent areas on an image. |
| What is subject contrast? | Contrast produced by differences in absorption between tissues in the patient. |
| What factors affect subject contrast? | Tissue thickness, density, atomic number, beam energy (kVp), and scatter. |
| What is exposure latitude? | Range of exposures that will produce an acceptable image on the receptor. |
| What receptor property influences exposure latitude? | Detector latitude or dynamic range. |
| How does a wide latitude receptor benefit imaging? | Allows acceptable image even with exposure variation — reduces repeats. |
| What is a phantom used for in image acquisition QC? | Test object to evaluate image quality parameters such as spatial resolution, distortion, magnification, and uniformity. |
| What is magnification radiography? | Technique that uses increased OID to enlarge small anatomy for better visualization. |
| What is a drawback of magnification radiography? | Increased unsharpness, higher dose, and greater distortion. |
| What is the “object–image–receptor alignment” principle? | Ensuring object, detector, and central ray are properly aligned to minimize distortion. |
| What happens if the object is not parallel to the image receptor? | Shape distortion (elongation or foreshortening). |
| What is beam divergence? | X‑ray beam spreads out from source, causing variation in intensity across field. |
| How does beam divergence affect image geometry? | Edges of image may be magnified more than center, causing distortion. |
| What is the principal plane of sharpness? | Plane at which object detail is most sharply imaged — lies perpendicular to central ray. |
| What is depth of field in radiography? | Range of object distances over which the image remains acceptably sharp. |
| What is the effect of large OID on depth of field? | Reduces depth of field — only a narrow zone remains sharp. |
| Why is proper positioning crucial in image acquisition? | To minimize distortion, maximize sharpness, and ensure correct representation of anatomy. |
| What is the purpose of radiographic centering? | Align anatomy of interest with central ray for accurate projection and minimal distortion. |
| What is a repetition error? | Need to repeat image due to improper positioning, distortion, or misalignment. |
| What is quantum mottle? | Noise caused by insufficient number of photons reaching the receptor — appears grainy. |
| What factors increase quantum mottle? | Low mAs, high patient thickness (attenuation), or high SID (low intensity). |
| How can quantum mottle be reduced? | Increase mAs, reduce SID/OID, optimize beam energy, or use more sensitive receptor. |
| What is the effect of patient motion during exposure? | Motion blur, loss of spatial resolution, and potential need for repeat. |
| How can motion blur be minimized? | Use short exposure time, immobilization, and patient instructions. |
| What is the relationship between exposure time and motion blur? | Longer exposure time increases risk of blur; shorter reduces risk. |
| What is contrast loss due to scatter? | Scatter adds unwanted uniform density — reduces contrast between tissues. |
| How does beam restriction (collimation) affect image acquisition? | Reduces field size, lowers scatter, improves contrast, and reduces patient dose. |
| What is the purpose of collimation light field calibration? | To ensure x‑ray field matches light field — proper anatomy coverage and dose control. |
| What is the effect of misaligned light field on image acquisition? | Portion of anatomy may be cut off or unnecessary tissue irradiated. |
| What is a cassette‑detector alignment error? | When the receptor is not perpendicular or centered to the beam — causes distortion or cutoff. |
| What is grid use in image acquisition? | Grid placed between patient and receptor to absorb scatter and improve contrast. |
| What is grid alignment importance? | Misaligned grid causes cutoff or uneven exposure — affects image quality. |
| What is the effect of incorrect SID with a focused grid? | Grid cutoff and loss of receptor exposure due to grid lead strip orientation. |
| What is patient centering? | Aligning patient anatomy with the central ray and receptor for optimal image. |
| Why is projection selection important? | Different projections minimize superimposition, distortion, and provide diagnostic view. |
| What is extremity imaging benefit from reduced OID? | Less magnification and improved sharpness/detail. |
| What is soft‑tissue imaging challenge with high OID? | Increased magnification and possible loss of detail; may require adjusted technique. |
| What is magnification factor in magnification radiography? | Ratio of image size to object size — used to calculate true object dimensions on image. |
| How does magnification radiography alter dose requirements? | Requires increased mAs due to greater scatter and magnification (usually higher dose). |
| What is the main goal of image acquisition technique selection? | Optimize image quality (detail, contrast) while minimizing patient dose. |
| What is a proper workflow for positioning and exposure? | Patient alignment → centering → collimation → appropriate exposure settings → receptor selection → shield if needed. |
| What is artifact? | Unwanted density or pattern on the image not representing anatomy — may stem from positioning, motion, equipment, or processing. |
| Why is regular quality control (QC) important in image acquisition? | Ensures consistent geometry, alignment, and reliable detector performance — maintains diagnostic quality. |
| What is the effect of OID on distortions when using grid vs no grid? | With grid: misalignment amplifies distortion/cutoff; without grid: increased scatter can reduce contrast — balancing is needed. |
| What is the significance of system calibration (SID, centering, collimation) before each exposure? | Prevents repeated exposures due to mis‑positioning, dose wastage, and poor image quality. |
| What is the “as low as reasonably achievable” principle (ALARA) in relation to image acquisition? | Use lowest exposure settings and best technique for diagnostic image, to minimize patient radiation dose. |
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