Medical Diagnostic Technology: X-Ray, CT, MRI and Ultrasound

The correct answer is B, Wilhelm Conrad Roentgen. Working with a discharge tube at the University of Wurzburg in November 1895, Roentgen noticed that a screen coated with a fluorescent salt glowed even when the tube was covered, and he traced the effect to an unknown penetrating radiation that he called X-radiation. For this he received the first Nobel Prize in Physics, awarded in 1901.

Option A, Marie Curie, with Pierre Curie discovered polonium and radium and won Nobel Prizes in Physics in 1903 and in Chemistry in 1911, but her work was on radioactivity and not on X-rays. Option C, Henri Becquerel, discovered natural radioactivity in uranium salts in 1896, a year after Roentgen, and his discovery was in fact prompted by the news about X-rays. Option D, J. J. Thomson, discovered the electron in 1897. Keep the sequence in mind: X-rays in 1895, radioactivity in 1896, the electron in 1897.

The correct answer is C, magnetic resonance imaging. MRI works by placing the hydrogen nuclei of the body in a strong static magnetic field and disturbing them with radiofrequency pulses, which lie in the harmless low-energy part of the electromagnetic spectrum. No ionising radiation is involved, which is why MRI can be repeated as often as the physician wishes.

Option A, plain radiography, uses X-rays, which are ionising by definition because they carry enough energy to knock electrons out of atoms. Option B, computed tomography, uses the same X-rays from many angles and delivers a larger dose than a single film. Option D, positron emission tomography, injects a radioactive tracer whose positrons annihilate with electrons to release gamma photons, so it too is ionising. Ultrasound is the other common technique that avoids ionising radiation, which is why ultrasound and MRI are preferred in pregnancy.

The correct answer is B, computed tomography. The Hounsfield unit, named after Godfrey Hounsfield, expresses how strongly a small volume of tissue attenuates X-rays, on a scale fixed by definition at zero for water and minus one thousand for air. Fat falls below zero, soft tissue a little above it, and dense bone runs to a thousand and beyond, which lets a radiologist judge tissue type from the numbers alone.

Option A is wrong because MRI images are described by signal intensity and sequence weighting, not by a fixed absolute scale. Option C is wrong because ultrasound images are described by echogenicity, with terms such as hyperechoic and anechoic, and the useful measurement is the Doppler velocity of flow. Option D is wrong because electrocardiography records voltages against time in millivolts and produces a trace, not an image. Pair the names in revision: Hounsfield with CT, tesla with MRI, megahertz with ultrasound.

The correct answer is C, it will be halved. The cutoff or minimum wavelength of the continuous X-ray spectrum arises when an electron gives up its entire kinetic energy in a single collision with the target. Equating the work done by the accelerating voltage with the photon energy gives the relation that the minimum wavelength equals the product of Planck's constant and the speed of light divided by the electronic charge and the voltage. The wavelength is therefore inversely proportional to the applied voltage, so doubling the voltage halves it. This is the Duane-Hunt law.

Option A is wrong because the cutoff depends only on the voltage, which has changed. Option B inverts the relationship and is the commonest error. Option D would follow from an inverse square dependence, which does not apply here. Note also what does not matter: the separation between filament and target and the target material affect intensity and the characteristic lines, not the cutoff wavelength.

The correct answer is B, hydrogen. A single proton, the nucleus of ordinary hydrogen, has a spin and therefore a magnetic moment, and hydrogen is by far the most abundant such nucleus in the body because the body is largely water and fat. This abundance, combined with a strong magnetic response, is why routine MRI is in effect proton imaging.

Option A, carbon, is abundant in the body but its common isotope carbon-12 has no net nuclear spin and gives no signal; only the rare carbon-13 is magnetically active, and it is used in research spectroscopy. Option C, sodium, and option D, phosphorus, do have magnetically active nuclei, and sodium-23 and phosphorus-31 imaging and spectroscopy exist in specialised research work, but their concentration in tissue is far too low for ordinary clinical scanning. Remember the chain: hydrogen proton, strong static field, radiofrequency pulse, relaxation signal, image.

The correct answer is B, tesla. The tesla is the SI unit of magnetic flux density, and clinical MRI scanners commonly work at one and a half or three tesla, produced by a superconducting coil kept cold by liquid helium. For comparison, the earth's own magnetic field is of the order of a few tens of microtesla, which is why a scanner room must exclude loose ferromagnetic objects.

Option A, the sievert, measures the biologically weighted equivalent dose of ionising radiation, which is irrelevant to MRI because it uses none. Option C, the becquerel, measures the activity of a radioactive source as one disintegration per second. Option D, the gray, measures absorbed dose of ionising radiation as one joule per kilogram. The three radiation units belong to X-ray, CT and nuclear medicine; only the tesla belongs to MRI. The older unit of flux density, the gauss, equals one ten-thousandth of a tesla.

The correct answer is B, the piezoelectric effect. Certain crystals develop a voltage across their faces when they are mechanically squeezed, and conversely change shape when a voltage is applied. An ultrasound probe uses this in both directions: an electrical pulse makes the crystal vibrate and send out a burst of ultrasound, and the returning echoes squeeze the crystal and generate the electrical signal from which the image is built.

Option A, the photoelectric effect, is the emission of electrons from a surface struck by light, explained by Einstein, and it matters in photocells and in how X-rays are absorbed, not in sound. Option C, the Zeeman effect, is the splitting of spectral lines in a magnetic field. Option D, the Raman effect, is the inelastic scattering of light that shifts its frequency, discovered by C. V. Raman. Note the related fact often asked alongside: ultrasound means sound above twenty kilohertz, the upper limit of human hearing.

The correct answer is A, the Doppler effect. When sound is reflected from a moving target, the frequency of the echo is shifted: higher when the blood is moving towards the probe and lower when it is moving away. The machine measures this shift and converts it into a velocity, which is how a Doppler study detects a narrowed artery, a clot or a leaking heart valve, and how foetal heart sounds are picked up.

Option B, the Compton effect, is the scattering of an X-ray or gamma photon by an electron with a loss of energy, important in radiation shielding. Option C, the Tyndall effect, is the scattering of light by colloidal particles, which makes a beam visible in a dusty room. Option D, the Faraday effect, is the rotation of the plane of polarisation of light in a magnetic field. Only the Doppler effect concerns a shift in frequency caused by relative motion, which is also why it is used in radar speed guns and in astronomy.

The correct answer is B, Godfrey Hounsfield and Allan Cormack. Hounsfield, an engineer at EMI in Britain, built the first clinical CT scanner, and Cormack, a physicist, had independently worked out the mathematics of reconstructing a cross-section from measurements taken along many directions. The prize came in 1979, and the attenuation unit of CT is named after Hounsfield.

Option A, Lauterbur and Mansfield, won the same prize in 2003, but for magnetic resonance imaging, which is the pairing candidates most often swap. Option C, Bloch and Purcell, shared the 1952 Nobel Prize in Physics for discovering nuclear magnetic resonance in bulk matter, the physics that MRI later used. Option D is wrong on both counts: Roentgen won the first Physics prize in 1901 for X-rays and Becquerel shared the 1903 Physics prize for radioactivity. Learn the four as a table of prize, year and technique.

The correct answer is C, a gadolinium chelate. Gadolinium is a rare earth element whose ion is strongly paramagnetic, so it shortens the relaxation time of nearby hydrogen nuclei and brightens the tissue it reaches. Because the free ion is toxic, it is given bound in a chelate, and it is used to show tumours, inflammation and breaches in the blood-brain barrier.

Option A, barium sulphate, is the X-ray contrast swallowed or given as an enema to outline the gastrointestinal tract; it is insoluble and so is not absorbed. Option B, an iodine compound, is the X-ray and CT contrast injected for angiography and for contrast-enhanced CT, iodine being chosen for its high atomic number. Option D, technetium-99m, is a gamma-emitting radiopharmaceutical used in nuclear medicine scans with a gamma camera, not in MRI. Match each agent to its technique: barium and iodine with X-rays, gadolinium with MRI, technetium with nuclear scans.

The correct answer is B, the Pre-conception and Pre-natal Diagnostic Techniques Act, 1994, commonly called the PCPNDT Act. It regulates the use of pre-natal diagnostic techniques, requires every ultrasound clinic to be registered and to keep records, and prohibits sex selection before or after conception as well as the communication of the sex of the foetus to anyone. It was substantially amended in 2003 to strengthen enforcement.

Option A, the Medical Termination of Pregnancy Act, 1971, lays down the conditions in which a pregnancy may lawfully be terminated and by whom; it is a different statute often confused with this one. Option C, the Clinical Establishments Act, 2010, provides for the registration and minimum standards of clinical establishments generally. Option D, the Drugs and Cosmetics Act, 1940, regulates the manufacture and sale of drugs and cosmetics. The exam point is that the machine is legal and the diagnosis is legal; disclosing the sex of the foetus is the offence.

The correct answer is B, fluorodeoxyglucose labelled with fluorine-18. This is a glucose analogue in which fluorine-18 replaces a hydroxyl group. Cells take it up in proportion to how fast they consume glucose but cannot metabolise it fully, so it accumulates; the fluorine-18 emits positrons, each of which annihilates with an electron to give two gamma photons flying apart in opposite directions, and the ring of detectors uses that pair to locate the source. Tumours, which are glucose hungry, light up.

Option A, cobalt-60, is a gamma source used in radiotherapy and in industrial irradiation, not as an injected tracer. Option C, iodine-131, is used in nuclear medicine, but for imaging and treating the thyroid, and it emits beta particles and gamma rays rather than positrons. Option D, barium sulphate, is not radioactive at all; it is an X-ray contrast medium for the gut. Remember the pairing: fluorine-18 with PET, technetium-99m with SPECT and the gamma camera.