*Quiz No. 6

What does Nuclear Medicine involve?

In nuclear medicine, we use gamma photons emitted from within the body to provide physiological and metabolic information. In radiography, we expose patients externally with x-rays to provide anatomical information.

What are the types of nuclear medicine imaging modalities?

1. Planar scintigraphy (2D projection images)
2. SPECT (single photon emission computed tomography)
3. PET (positron emission tomography)

*Describe radiopharmaceuticals and their desirable properties.

Radiopharmaceuticals consist of an agent, which concentrates in a particular organ, and a gamma-emitting radionuclide. They must have the following properties:

• Emit mono-energetic gamma photos (no α or β as these will be absorbed in tissue and will increase patient dose)
• Emit photons in the range of 70-300keV, which the PMTs are sensitive to and provide enough energy to escape the patient but still be detected but the gamma camera
• Have a half-life of minutes to several hours

Most common radionuclide: Technetium-99m

Describe the operation of a photomultiplier tube.

Gamma rays emitted from within patient interact with the scintillation crystal by photoelectric absorption or Compton scattering, resulting in the emission of visible light from excited electrons that are spontaneously de-excited. The burst of light or scintillation ejects an electrode from the photocathode in the PMT, which travels towards the first dynode held at 300V. The electron ejects 3-4 electrons, which are all then accelerated towards the second dynode held at 400V. This continues for the 10 dynodes before a voltage pulse is given off by the photomultiplier tube.

A pulse height analyser receives the voltage pulse height and the number of pulses, producing a pulse height spectrum (histogram). The photo peak of the spectrum indicates total energy absorption.

Why do mono-energetic photons produce a range of voltage pulse heights?

Mono-energetic photons may interact with the scintillation crystal by photoelectric absorption, meaning their total energy is lost in a single interaction. However, they can also act by Compton scattering, meaning only part of their energy is deposited onto the crystal.

*Describe the major components of a gamma (anger) camera?

Collimators consist of hundreds of small chambers separated by lead foil, which acts as a grid to only allow gamma photons heading in the appropriate direction to the scintillation crystal. The gamma photons then interact with the scintillation crystal, which emits a scintillation. Next step is the photomultiplier tube, which produces a voltage pulse which depends on the distance between the PMT and the light burst. The position-logic circuit then uses this information to determine where on the face of the scintillation crystal the event occurred. The combined voltage output of all tubes which represents the energy deposited but the gamma photon goes to the pulse height analyser. The z pulse then goes to the gating circuits to pass only the photo peak. The x and y signals of the photo peak pulses are sent to a computer which lights the corresponding pixel on a display.

Describe SPECT.

• Nuclear medicine counterpart to X-ray CT
• Lower resolution tomographic images to CT
• 1-3 gamma cameras used
• Causes more artifacts than in CT
• Attenuation in patients seems like a reduction in activity to gamma cameras, reducing image quality
• Low spatial resolution
• Can be used to distinguish between benign and malignant tumours, and also to quantify blood flow in the brain
• Sodium iodide used as scintillation crystal

Describe PET.

• Uses a positron-emitting radiopharmaceutical
• Uses high energy (511keV) annihilation gamma photons
• Positron produced by β+ decay loses energy until it annihilates with an electron, forming 2 x 511keV gamma photons
• The gamma photons speed off in opposite directions
• Coincidence detection by opposite detectors on PET machine reveals line of flight
• Region of intersection yields tomographic image
• Bismuth germinate used as scintillation crystal - more sensitive to 511keV gamma photons

What does Radiation Therapy involve?

Radiation therapy involves the use of high-energy ionising radiation in the treatment of cancer. The goal is to kill or at least shrink a cancer by damaging its DNA, making it impossible for the cell to grow and divide. There are three main divisions within radiation therapy:

• External beam radiotherapy (tele therapy)
• Brachytherapy or sealed source radiotherapy
• Unsealed source radiotherapy

External beam radiotherapy

Historically, x-ray tubes were originally used but were not penetrating enough. So, Cobalt-60 nuclei were used to emit 1.173 and 1.332 MeV gamma rays which were sufficiently penetrating to reach a tumour. However, they have a short half life (5.26 years) and must be replaced regularly.

And so came linear accelerators or linacs. Linacs can produce x-ray or electron beams with energies in the range of 4 to 25MeV. Typically, two x-ray beam energies and several electron beam energies are available. The function of a linac is to accelerate electrons and direct them to a tungsten target, where a spectrum of bremsstrahlung is produced. A waveguide uses microwaves to produce an electric field to accelerate the electrons to near the speed of light, thus increasing their kinetic energy.

Rectangular beams are used to direct the x-rays to the target tissue. The gantry is then rotated on a horizontal axis to point the radiation towards the isocentre.

Basically, Compton scattering occurs and the tissue is ionised at the intersection of the beams. Thus, tumour cells are damaged whilst healthy tissue is spared. 

However, if we remove the tungsten target, we can use electrons alone to treat superficial tumours.

Brachytherapy

This involves the use of radioactive sources in small capsules, seeds or wires placed in direct contact with the tumour. A high dose is therefore delivered locally but falls rapidly in adjacent tissue due to the inverse square law. Seeds are left in place for several days.

Brachytherapy is most commonly used to treat cervical and prostate cancers, as well as breast cancers after the surgical removal of a lump.

Unsealed Source Radiotherapy

This involves the use of radioactive sources that are taken by mouth or injected into the body, e.g. iodine-131 for thyroid cancer treatment.