Attenuation and CT Scanners

For a narrow monochromatic beam passing through a uniform absorber, equal thickness increments remove equal fractions of the remaining intensity. This leads to exponential attenuation. The linear coefficient has units of inverse length; its value depends on material and photon energy.

A half-value thickness reduces the transmitted intensity by a factor of two. Two such thicknesses leave one quarter, not zero. The mass attenuation coefficient divides the linear coefficient by density, making comparisons by absorber mass more convenient. A broad polychromatic clinical beam may not obey a single coefficient exactly because preferential absorption changes its spectrum.

In CT, a narrow collimated X-ray beam and detector array acquire transmission measurements as the source rotates around the patient. These measurements sample the attenuation along many different paths. A computer reconstructs the distribution of attenuation within a slice rather than merely displaying one shadow image.

For the idealised AQA treatment use a narrow monochromatic beam. Real scanners use energy spectra and correction procedures. Explain the roles of the rotating source, detectors and computer; details of detector construction are not required for this part of the option.

CT can distinguish internal structures that overlap in a plain radiograph and is rapid for many examinations, but it uses ionising radiation and can give a larger dose than a single radiograph. MRI often provides strong soft-tissue contrast without ionising photons but can take longer and has strong-field restrictions.

Ultrasound is portable and allows real-time imaging without ionising radiation, but gas, bone and depth can limit it. Compare spatial resolution, convenience, cost and safety in the stated application; high resolution alone does not determine the best technique.

Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.