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Physics
Ch 8
| Question | Answer |
|---|---|
| What does “x‑ray quantity” refer to? | The number of x-ray photons in the useful beam (intensity/amount). |
| What does “x‑ray quality” refer to? | The penetrating power (energy distribution) of the x-ray beam. |
| What unit often expresses x-ray quantity/intensity in air? | Gray in air (Gy_a) or µGy/mAs. |
| What variable primarily controls x-ray quantity? | mAs (milliamperage × time). |
| What variable primarily controls x-ray quality/beam energy? | kVp (kilovoltage peak). |
| What happens to x-ray quantity if mAs doubles and all else stays same? | Quantity doubles — more photons produced. |
| What happens to x-ray quantity if kVp increases but mAs stays same? | Quantity increases (and quality increases too). |
| What happens to x-ray quality when kVp is increased? | Beam becomes more penetrating — higher average photon energy. |
| What is the maximum photon energy in an emission spectrum determined by? | The kVp used (equal to maximum electron kinetic energy). |
| What is the effect of added filtration on an x-ray beam? | It removes low-energy photons, increasing average beam energy (quality) and reducing quantity. |
| Why is filtration used in x-ray beams? | To reduce patient skin dose by removing soft (low‑energy) photons. |
| What does “inherent filtration” refer to? | Filtration by the tube’s glass, oil, housing, and built-in window. |
| What does “added filtration” refer to? | Extra filters (e.g. aluminum, copper) placed in the beam path by design. |
| What is “total filtration”? | The sum of inherent + added filtration. |
| What is beam “hardening”? | Removal of low‑energy photons so average photon energy increases (beam becomes “harder”). |
| What is the effect of increasing filtration on patient dose? | Dose decreases, especially skin dose, because soft photons are removed. |
| What is the effect of increasing filtration on image quality? | Beam quality increases, scatter may increase, contrast may decrease slightly depending on technique. |
| What happens to the emission spectrum when filtration increases? | Low‑energy part of the spectrum is reduced (attenuated), shifting average energy higher. |
| What is “volume of exposure” or “mAs” effect on emission spectrum? | Increases the amplitude (number of photons) but does not shift energy distribution. |
| What is the “amplitude” of emission spectrum? | The overall height — representing photon quantity at each energy level. |
| What is the “endpoint” of emission spectrum? | The maximum photon energy — determined by kVp. |
| What is the “average photon energy” roughly compared to kVp? | Approximately one-third of the kVp. |
| What is the difference between quantity and quality in x-ray beams? | Quantity = how many photons; Quality = how energetic/penetrating they are. |
| What is the relationship between kVp and photon energy? | Higher kVp → higher photon energies (greater quality). |
| What is the relationship between mAs and number of photons? | Directly proportional — more mAs → more photons. |
| How does increasing kVp affect patient dose if mAs stays constant? | Dose and penetration increase; more photons reach deeper structures. |
| How does increasing mAs affect patient dose if kVp stays constant? | Dose increases (number of photons increases), but photon energy remains same. |
| What is the impact of high photon energy on tissue penetration? | Greater penetration — less absorption in superficial tissues, more through thick/denser tissues. |
| What is the impact of low photon energy on image contrast? | Higher absorption in tissues → greater contrast between different densities. |
| Why might you choose lower kVp for a high contrast image? | Lower energy photons increase absorption differences (photoelectric effect), improving contrast. |
| What is a disadvantage of low‑energy x-rays for patient dose? | More absorption in tissues → higher skin dose. |
| What is “beam hardening” benefit in imaging? | Better penetration, reduced superficial dose, more uniform exposure across thick anatomy. |
| What effect does beam hardening have on contrast? | It may reduce contrast because fewer low‑energy photons remain to be absorbed. |
| What is the term for the thickness of material needed to reduce beam intensity by half? | Half‑Value Layer (HVL). |
| What does HVL indicate? | Beam quality / penetrating ability of the x-ray beam. |
| What happens to HVL when kVp increases? | HVL increases (beam becomes more penetrating). |
| What happens to HVL when filtration increases? | HVL increases (beam is hardened). |
| What is the effect of increasing SID (source-to-image distance) on x-ray intensity? | Intensity decreases (fewer photons reach receptor due to divergence). |
| What law describes change in intensity with distance? | Inverse square law. |
| Write the inverse square law formula. | I1 / I2 = (D2² / D1²) |
| What happens to beam quantity at the image receptor if distance doubles and technique remains same? | Quantity drops to one-quarter (intensity falls by 1/4). |
| How can you compensate for increased SID while maintaining receptor exposure? | Increase mAs (or adjust kVp) to maintain quantity. |
| What is the effect of voltage waveform on x-ray emission? | Waveform affects consistency and quantity — smooth waveforms (three‑phase, high‑frequency) produce more output. 4 |
| What is the effect of generator type on x-ray output and quality? | More efficient generators (three‑phase, high‑frequency) yield more photons and more consistent quality. 5 |
| What happens to patient dose when generator efficiency improves? | Possible reduction in mAs needed → can reduce dose while maintaining quality. |
| What does the term “output intensity” refer to? | The exposure or dose delivered per mAs at a given distance and kVp. |
| How is output intensity typically measured? | In µGy/mAs at a reference SID (e.g. 100 cm). 6 |
| What happens to output intensity if you increase kVp while keeping mAs constant? | Output increases — more photons and higher photon energy reach receptor. |
| What happens to output intensity if filtration is added and technique remains same? | Output decreases (fewer photons reach receptor), but beam quality increases. |
| How does tissue atomic number affect absorption and image contrast? | Higher Z tissues absorb more → greater contrast (especially at lower energies). |
| What is the benefit of increasing kVp for thick body parts? | Better penetration to image deep anatomy and avoid underexposure. |
| What is a disadvantage of high kVp regarding image contrast? | Reduced contrast due to more Compton scatter and less differential absorption. |
| What role does collimation have in x-ray emission and patient dose? | Restricting beam size reduces scatter and patient dose by limiting exposed volume. |
| What is scatter radiation? | X-rays that change direction after interacting with matter. |
| How does scatter affect the useful beam and image quality? | It adds unwanted radiation, decreases contrast, and contributes to patient/occupational dose. |
| Why is proper beam restriction important for radiation protection? | Reduces patient dose and improves image quality by limiting scatter. |
| What happens to photon energy distribution when filtration is increased drastically? | Low-energy photons are removed, spectrum shifts toward higher energies — beam hardened. |
| What is the relationship between beam hardening and skin dose? | Skin dose decreases because soft photons are absorbed before reaching deep tissues. |
| What is the trade-off when increasing beam quality with filtration or high kVp? | Reduced contrast (less differential absorption), potentially more scatter. |
| Why might a technologist choose lower kVp with higher mAs for extremity imaging? | To maximize contrast (due to absorption differences) while maintaining sufficient photon quantity. |
| What is the meaning of “quality control of emission”? | Ensuring proper filtration, output consistency, and safe x-ray emission parameters. |
| What is a common unit for image receptor exposure in radiography? | µGy (microgray) or mGy (milligray). |
| What does “optical density” refer to in film radiography? (or receptor exposure) | The darkness level of the image, dependent on photon quantity and energy. |
| How does emission spectrum affect receptor exposure and image contrast? | Spectrum shape (energy distribution) affects penetration (image receptor exposure) and absorption (contrast). |
| What is the effect of using a high‑frequency generator vs single-phase on x-ray emission? | Higher efficiency, greater output, less ripple, more constant beam, better image quality. |
| What is “ripple” in x-ray generation? | Fluctuations in voltage output during exposure affecting beam consistency (less ripple = smoother beam). |
| How does high ripple (single‑phase) affect x-ray emission? | Lower efficiency, lower photon output, less penetration. |
| What two major factors should you adjust when changing SID to maintain exposure? | mAs and/or kVp. |
| What’s the purpose of balancing kVp, mAs, filtration, and SID during radiography? | To optimize image quality and minimize patient dose. |
| What does “useful beam” refer to? | The portion of the x-ray beam that exits the tube, passes through patient, and contributes to image formation. |
| What is “leakage radiation”? | Unintended radiation escaping outside the useful beam or tube housing (should be minimal). |
| Why is consistent monitoring of output important? | To ensure safe, predictable exposures and avoid over- or underexposure. |
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