Eduqas Physics for A Level Year 2: Student Bk

213 Option B: Medical physics (g) Therapeutic uses of ultrasound It has been mentioned that ultrasound is attenuated in tissues. This means that there is some energy transfer and that there are potential biological and therefore medical effects. These are insignificant at the intensities used for imaging but high power ultrasonic beams can be used in medical treatment. Here is a selection: • For warming muscles and joints prior to manipulation by physiotherapists, or to promote healing by the warming effect. • For descaling teeth by dental hygienists. • Breaking up kidney stones and gall stones (‘lithotripsy’). • Tumour ablation using high intensity focused ultrasound (HIFU). • Assisting liposuction. • Tooth and bone regeneration using low intensity pulsed ultrasound. B.3 Magnetic resonance imaging (MRI) 2 This technique, which produces the most exquisitely detailed images of the internal tissues of the body, employs advanced physics and some very expensive kit. It involves lying still and being passed through an intense non-uniform magnetic field (Fig. B19) and having radio waves fired at you. However, as far as is known, the procedure carries very low risk (unless you have a heart pacemaker or forget to take your jewellery off!). So what’s going on? (a) Magnetic nuclei All nuclei spin. For nuclei such as 1 1 H , with an odd number of protons or neutrons (see Study point), this results in the nucleus having a magnetic moment. In the absence of a magnetic field, the direction of the spin of hydrogen atoms in a material (such as water) is random. In a magnetic field, the nuclei line up approximately with the magnetic field lines. Actually, they can be pictured like a gyroscope, with the direction of the rotation precessing around the field lines (Fig. B21). There are two energy states available to the nuclei: parallel (low energy) and anti-parallel (high energy) to the magnetic field. Fig. B18 MRI head scan Fig. B19 MRI scan preparation Study point Remember that protons and neutrons are composite particles, consisting of quarks that are charged. So a spinning nucleon consists of circulating charges and so it behaves like a small magnet. Fig. B20 Spinning nucleus N spin 2 MRI scans used to be called NMR (nuclear magnetic resonance) scans. Rumour has it that the name was changed because ‘an NMR’ sounds like ‘an enema,’ which is quite a different medical procedure!