The Hounsfield unit, a scale on which water is zero and air is minus one thousand, is used in which imaging technique?

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 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.