Practise analysing diagnostic ultrasound, X-ray imaging, CT and PET using their physical principles, calculations, image formation and clinical trade-offs.
Fluorine-18 is a radioactive nuclide that is used as a tracer in positron emission tomography (PET scanning). Fluorine-18 decays to a nuclide of oxygen ( O ) according to
State what is meant by a tracer.
Explain how the radioactive decay of fluorine-18 results in the emission from the body of the gamma-ray photons that are detected during a PET scan.
Explain how the detection of the gamma-ray photons is used to produce an image of the tissue being examined.
Positronium is highly unstable, and after a very short period of time it becomes gamma radiation.
Describe how gamma radiation is formed from the two particles in positronium.
State one medical application of the process described in (c)(i).
An electron, at rest, has mass and charge -q.
A positron is a particle that, at rest, has mass and charge +q.
A positron interacts with an electron. The electron and the positron may be considered to be at rest.
The outcome of this interaction is that the electron and the positron become two gamma-ray ( -ray) photons, each having the same energy.
Calculate, for one of the -ray photons:
the photon energy, in J
energy =
State and explain the direction, relative to each other, in which the -ray photons are emitted.
In computed tomography (CT scanning), it is necessary to take a series of many X-ray images. Outline briefly the principles of CT scanning.
A student creates a model for CT scanning.
A section is divided into four voxels, with pixel numbers A, B, C and D , as shown in Fig. 9.1.
Fig. 9.1
The section is viewed from four different directions and , as shown in Fig. 9.1.
The detector readings for each direction are noted and then summed. The result is shown in Fig. 9.2.
Fig. 9.2
The background count is 26 .
Determine the pixel numbers A, B, C and D as shown in Fig. 9.1.
A B
D
C