MR scanning and the endoscope

Light down a fibre: the endoscope

The option puts two more non-ionising instruments alongside ultrasound, and the simpler one is the endoscope, which is total internal reflection put to work. Light entering the core of a thin glass fibre strikes the wall of that core at a steep glancing angle. Because the core has the higher refractive index and the surrounding cladding the lower, any ray meeting the wall beyond the critical angle is reflected back inside with nothing escaping, and it goes on reflecting all the way along however the fibre bends.

One endoscope carries two bundles of those fibres, and the difference between them is the point. The coherent bundle carries the image out: every fibre holds the same place in the cross-section at both ends, so each fibre delivers its own piece of the picture to the right place and the picture arrives intact. The second bundle only carries light in to illuminate what is being looked at, so it does not matter which fibre ends up where, and it is left incoherent, which is cheaper to make.

Being a physical instrument, the endoscope reaches only where a tube can be threaded, along the gut or the airway or through a small incision, which is exactly what it gains in exchange: the surgeon sees the tissue in real colour and can pass instruments down the same tube.

The MR scanner

The patient lies inside a strong magnetic field, a few teslas from a superconducting magnet. The body is mostly water, every water molecule carries two hydrogen nuclei, and each of those protons behaves as a tiny magnet. In the field they do not simply snap into line. They precess about the field direction, the way a leaning spinning top wheels around the vertical, at a frequency set by the strength of the field, and for the fields an MR magnet supplies that frequency sits in the radio range.

The scanner then supplies energy at exactly that frequency. A radio-frequency pulse at the precession frequency is absorbed by the precessing protons and tips them away from the field direction. When the pulse ends they relax back into line, giving the energy up again as a radio signal that coils around the patient pick up. Nothing here is nuclear decay; the nucleus is being used as a magnet, not broken up, and nobody receives a dose.

The relaxation time, the time the protons take to settle back, depends on the tissue their water sits in. Protons in fat relax at a different rate from protons in fluid or in a tumour, so the timing of the returning signal labels the tissue that sent it. That is the contrast mechanism, and it is why MR tells apart soft tissues that X-rays cross almost identically. One board note: AQA states that de-excitation relaxation times will not be examined, so for AQA this paragraph is the why behind the scan, not a required point.

Locating the signal is done with the field itself. The precession frequency depends on the field strength, so gradient coils make the field vary slightly from place to place across the patient, and position along a gradient then maps to frequency. One gradient alone cannot pin down a point, so real scanners switch gradients to encode the remaining directions in the signal's frequency and phase, and a computer reconstructs cross-sectional images in any plane from that encoded signal, with nothing rotating around the patient.

Weigh it as the exam does, both ways at once. In its favour: excellent soft-tissue contrast, the finest of the modalities compared here, sections in any plane, and no ionising radiation, so repeat scans add no radiation dose, though every scan still passes the safety checks on implants, heating and any contrast agent first.

Against it: the superconducting magnet makes it expensive to buy and to run; a scan is slow and loud, with the patient holding still for tens of minutes inside a narrow bore; and the strong field rules out patients with MR-unsafe pacemakers or ferromagnetic implants, which the field would disturb or drag. Many modern implants are MR Conditional and scannable under controlled protocols; the safety label, rather than the presence of a pacemaker, determines whether a scan can proceed.