Principles of imaging
Principles of ultrasound, Doppler, X-ray, CT, MRI, radioisotopes and lasers
Every imaging method sends energy into the body and reads how tissues reflect, absorb or re-emit it. CPSP asks the basic physics behind each method, why one frequency, voltage or sequence is chosen over another, and how tissues appear as a result, especially in the pelvis and pregnancy.
One of the most tested topics in past papers
Ultrasound
- Ultrasound is sound above 20 kHz (human hearing is 20 Hz to 20 kHz); diagnostic probes use about 2–15 MHz.
- The probe's piezoelectric crystal vibrates when voltage is applied (transmit) and produces voltage when echoes strike it (receive).
- Echoes arise at boundaries of different acoustic impedance (density × speed of sound). Tissue–air reflects about 99.9%, so coupling gel is used and bowel gas blocks the view; a full bladder displaces gas and gives an acoustic window for transabdominal pelvic scans.
- Depth is calculated from echo time, assuming soft-tissue speed of 1540 m/s (depth = speed × time ÷ 2).
- Wavelength = speed ÷ frequency. Higher frequency means shorter wavelength and better resolution but less penetration, because attenuation rises with frequency.
| Use | Typical frequency |
|---|---|
| Transabdominal obstetric | 3–5 MHz (lower for obese women) |
| Transvaginal (probe close to target) | 5–9 MHz |
| Breast, thyroid, superficial | 10–15 MHz |
- M-mode (motion against time on one line) records embryonic heart rate at low acoustic output.
- Artefacts: acoustic shadowing behind stones, bone and calcification (strong reflection and absorption); posterior acoustic enhancement behind cysts (fluid attenuates little); reverberation (comet tail) between strong reflectors.
- Transvaginal scanning is the best test to localise the placenta in suspected placenta praevia.
Doppler
- Sound reflected from moving red cells changes frequency in proportion to their velocity and the cosine of the beam angle (Doppler shift).
- Colour Doppler: red = flow towards the probe, blue = away (nothing to do with oxygenation); mosaic = turbulence or aliasing.
- Pulsed Doppler samples a chosen depth but aliases when the shift exceeds half the pulse repetition frequency (Nyquist limit); fix by raising the scale or lowering the baseline. Continuous-wave Doppler (two crystals, as in hand-held fetal heart detectors) has no depth information and does not alias.
- Indices: pulsatility index = (S − D) ÷ mean; resistance index = (S − D) ÷ S. As ratios, the angle cancels out, so they are used for umbilical and uterine arteries. Absent or reversed umbilical end-diastolic flow means high placental resistance.
- Pulsed Doppler has the highest thermal output; use it sparingly in the first trimester.
X-rays
- Electrons from a heated cathode strike a tungsten anode, producing mainly bremsstrahlung (braking radiation); about 99% of the energy becomes heat.
- kV controls photon energy and penetration; mAs controls photon number (and dose). High kV lowers contrast.
- Photoelectric absorption (photon fully absorbed, probability proportional to Z³) creates contrast: calcium, iodine (Z 53), barium look white. Compton scatter dominates in soft tissue, fogs the image (reduced by grids) and is the main source of staff dose.
- Mammography uses low kV (25–30 kV) to maximise photoelectric soft-tissue contrast.
Computed tomography
- A rotating X-ray beam takes many projections; the computer calculates attenuation in each voxel (filtered back-projection or iterative reconstruction), removing superimposition.
- Hounsfield units: water 0, air −1000, fat about −100, soft tissue +20 to +60, fresh blood 40–70, bone up to +1000. Fat at −100 HU in an ovarian mass means a dermoid.
- Windowing: level is the centre HU, width the range shown.
- CT carries a much higher dose than plain films.
Magnetic resonance imaging
- Hydrogen protons align in a strong field (1.5–3 T) and precess at the Larmor frequency (about 42.6 MHz per tesla). A radiofrequency pulse at that frequency tips them (resonance); as they relax they emit signal. No ionising radiation.
- T1 (spin–lattice) and T2 (spin–spin) relaxation create contrast:
| Tissue | T1-weighted | T2-weighted |
|---|---|---|
| Water, CSF, urine, cysts | Dark | Bright |
| Fat | Bright (drops with fat suppression) | Bright |
| Subacute blood (methaemoglobin) | Bright (stays bright with fat suppression) | Variable |
| Haemosiderin, iron | Dark | Dark (blooms on gradient echo) |
- Endometrioma stays T1 bright with fat suppression; a dermoid loses signal.
- Gadolinium contrast shortens T1 (enhancing tissue turns bright).
- Hazards: the always-on static field pulls ferromagnetic objects; RF pulses cause heating; gradients cause noise. Pacemakers and some implants are contraindications.
Radioisotopes
- A tracer emits gamma rays detected by a gamma camera (collimator, sodium iodide crystal, photomultipliers).
- Technetium-99m: ideal: half-life 6 hours, pure gamma of 140 keV, no beta (low patient dose), from a molybdenum-99 generator.
- PET: fluorine-18 FDG emits positrons (paired 511 keV photons) and tracks glucose use.
Lasers
- Light Amplification by Stimulated Emission of Radiation: needs a population inversion; light is monochromatic, coherent and collimated, giving high power density when focused.
| Laser | Wavelength | Behaviour and use |
|---|---|---|
| CO2 | 10,600 nm (far infrared) | Strongly absorbed by water: shallow, precise vaporisation (CIN, vulval and vaginal lesions); cannot work through fluid |
| Nd:YAG | 1064 nm (near infrared) | Passes through water and clear fluid: deep coagulation; fetoscopic ablation of placental vessels in twin–twin transfusion |
| Argon | Visible blue-green | Absorbed by haemoglobin and pigment: retinal photocoagulation |
Remember Higher ultrasound frequency sees finer detail but not as deep; that is why the transvaginal probe gives the sharper picture.
Exam trap Red and blue on colour Doppler show direction relative to the probe, not arterial and venous blood.
Exam trap Bright on T1 is not always fat: blood stays bright when fat suppression is applied.
Test yourself on this topic
5 of the 81 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A gynaecologist uses a laser at colposcopy. The light produced is a single wavelength, coherent and tightly collimated. Which physical process generates this light?
Answer: Stimulated emission of radiation
LASER stands for Light Amplification by Stimulated Emission of Radiation. Energy pumped into the lasing medium raises many atoms to an excited state (population inversion). A photon passing an excited atom stimulates it to release an identical photon with the same wavelength, phase and direction, and mirrors at each end of the cavity amplify this. The result is monochromatic, coherent, collimated light that can be focused to a very high power density.
Laser light
- Stimulated emission of radiation
- Needs population inversion
- Monochromatic, coherent, collimated
- High power density when focused
Remember One photon stimulates an identical twin.
Exam trap Laser light is not ionising; its effects are thermal and photochemical.
Why the other options are wrong
- Spontaneous emission from a filament An ordinary lamp produces mixed, incoherent light by spontaneous emission.
- Thermionic emission from a cathode Thermionic emission releases electrons in X-ray tubes.
- Radioactive decay of the medium Lasers are not radioactive.
- Piezoelectric vibration of a crystal This is how ultrasound is produced.
An unenhanced CT shows a thin-walled adnexal cyst whose contents measure 5 Hounsfield units, which the radiologist describes as simple fluid. On what basis is the Hounsfield scale set?
Answer: Water is defined as 0 HU
The Hounsfield scale expresses the X-ray attenuation of each voxel relative to water. By definition water is 0 HU and air is −1000 HU; fat is negative (around −100), soft tissue and clotted blood are positive (about +30 to +80), and dense bone reaches +1000 or more. A cyst content of 0–20 HU indicates water-like fluid, whereas a negative value would suggest fat, as in a dermoid.
Hounsfield units
- Water: 0 HU (definition)
- Air: −1000 HU
- Fat: about −100 HU
- Bone: up to +1000 HU
Remember Water is the zero point.
Exam trap A negative HU in an ovarian mass means fat, not fluid.
Why the other options are wrong
- Air is defined as 0 HU Air is −1000 HU.
- Fat is defined as 0 HU Fat is about −100 HU.
- Muscle is defined as 0 HU Muscle is about +40 HU.
- Blood is defined as 0 HU Fresh clotted blood is about +60 to +80 HU.
For an abdominal radiograph in a heavily built woman, the radiographer raises the tube voltage from 70 kV to 85 kV. What change in the X-ray beam does this produce?
Answer: Higher photon energy and penetration
Tube voltage (kV) sets the maximum energy of the photons produced: higher kV gives a harder, more penetrating beam that can pass through a thicker patient. Tube current and exposure time (mAs) mainly control the number of photons. Higher-energy photons undergo relatively less photoelectric absorption and more Compton scatter, so contrast between soft tissues falls. That is why mammography uses low kV.
kV and mAs
- kV: photon energy, penetration
- mAs: photon number
- High kV: less contrast, more scatter
- Low kV for soft tissue (mammography)
Remember kV for quality, mAs for quantity.
Exam trap Raising kV does not simply add photons; that is the job of mAs.
Why the other options are wrong
- More photons at the same energy This is the effect of raising the mAs.
- Longer wavelength photons Higher energy means shorter wavelength.
- More photoelectric absorption Photoelectric absorption falls as photon energy rises.
- Lower maximum photon energy Raising kV raises, not lowers, the maximum photon energy.
In a woman with secondary infertility, a hysterosalpingogram outlines the uterine cavity and shows spill from both tubes. Why does the injected contrast medium appear white on the X-ray?
Answer: Iodine's high atomic number absorbs X-rays
Water-soluble HSG contrast media are organic molecules carrying iodine atoms. Iodine has an atomic number of 53 and a K-shell binding energy (33 keV) close to the energy of diagnostic photons, so it absorbs X-rays strongly by the photoelectric effect. Fewer photons reach the detector, and the contrast-filled cavity and tubes appear white against soft tissue. The same principle applies to barium in gastrointestinal studies.
Positive contrast
- Iodine: Z = 53
- K-edge 33 keV suits diagnostic beams
- Strong photoelectric absorption
- Appears white (radio-opaque)
Remember Iodine and barium stop X-rays.
Exam trap Contrast agents are not radioactive; they only absorb the beam.
Why the other options are wrong
- It reflects X-rays back to the tube X-rays are absorbed or scattered, not reflected back.
- It emits gamma rays after injection Contrast is not radioactive.
- Its low density lets X-rays pass Low density would make it appear dark, like gas.
- It fluoresces under the X-ray beam Fluorescence is not the basis of image contrast.
A midwife uses a hand-held fetal heart detector at 16 weeks. It gives an audible heartbeat but no image and no information about depth. Which technique does the device use?
Answer: Continuous-wave Doppler
A hand-held fetal Doppler has two crystals: one transmits continuously and the other receives continuously. Because transmission never stops, the device cannot time echoes, so it cannot tell how deep the moving reflector is, but it detects any movement in the overlap of the two beams and converts the frequency shift into sound. It cannot alias, making it simple and robust. Pulsed systems time each echo and so can sample flow at a chosen depth.
Continuous-wave Doppler
- Two crystals: one sends, one receives
- No depth information
- Does not alias
- Hand-held fetal heart detectors
Remember Always sending, never knowing depth.
Exam trap Lack of depth information is the trade-off for never aliasing.
Why the other options are wrong
- Pulsed-wave Doppler Pulsed Doppler gives depth-specific information and needs a display.
- Colour flow Doppler Colour Doppler is superimposed on a 2-D image.
- M-mode ultrasound M-mode is an imaging mode with a display.
- B-mode ultrasound B-mode produces a grey-scale image.
Where this sits in the CPSP syllabus
- Obstetrics & Gynaecology, Paper 2: Biophysics, Principles of ultrasound, Doppler, X-ray, CT, MRI, radioisotopes and lasers