Image intensifier screens (fig below) allow the production of an image changing in real time, which can be advantageous, particularly when used with high contrast material like Ba which shows the passage of barium sulphate through the gut, for example. It does, however, subject the patient to long exposure times, hence high radiation dose. Since the objective is to minimize this, we look elsewhere for more sophisticated techniques.
Faster computing in the 1970’s yielded CAT scanning (Computerised Axial Tomography) Tomos is Greek for ‘slice’ and the technique involves imaging a slice through the body by rotating digital detectors and emitters, before moving on to the next slice, consequently reducing exposure time. It is to X-rays what digital cameras are to photography. A brain scan takes less than a second to image. (see p705) Resolution (remember Rayleigh) has much improved over the last three decades and kidney stones less than 2mm in diameter can be detected.
A disadvantage is that exceptional skill is required to diagnose with this technique.
Positron Emission Tomography. A source of positrons (beta + emitters) is injected. These annihilate with the electrons in the tissues of the patient producing a pair of identical photons traveling in opposite directions, each with an energy of 1.02/2 MeV = 0.511MeV. Detectors find the point of annihilation with a resolution of about 1mm, allowing abnormality at cellular level to be visualized . Best for metabolic studies with the biologically active molecule fludeoxyglucose (FDG), an analogue of glucose. The concentrations of tracer imaged will indicate tissue metabolic activity because of the local glucose uptake. Use of this tracer to explore the possibility of cancer metastasis (i.e., spreading to other sites) is the most common type of PET scan.
MRI Magnetic Resonance Imaging. better resolution, contrasts soft tissues, no patient dose, much more expensive.
Charged particles have a property called ‘spin’, aligning themselves either up or down in an external magnetic field. These two states have different energies. Energising a sample containing H for example with a radio frequency, a ‘spin-up’ proton absorbs resonant photons to a higher energy ‘spin down’ state, subsequently decaying back to spin-up with the emission of a characteristic photon of the same frequency, an event which detectors can record and display. The rate at which these transitions take place is the important part, since this is directly related to tissue type. The location mechanism depends on the application of uniform fields to act as a background to the resonant changes so only one plane has the correct value of B for absorption to occur. I think it unlikely that you will be asked for further detail.