Radionuclide imaging

Technetium-99_m (^99mTc) is produced from the decay of molybdenum-99 (⁹⁹Mo). In hospitals,^99mTc is extracted from a molybdenum-technetium generator.

⁹⁹Mo is put into the generator at time t = 0 As⁹⁹Mo decays, the amount of^99mTc increases to a maximum at t = 24 hours. ^99mTc is extracted from the generator at regular intervals. At each extraction, the activity of^99mTc in the generator falls to zero and then increases again.

Figure 3 shows how the activities of⁹⁹Mo and^99mTc in the generator vary with t when extractions are made at t = 24 hours and t = 48 hours.

Show that about 6 × 10¹⁵ nuclei of⁹⁹Mo decay between t = 0 and t = 24 hours.

decay constant for⁹⁹Mo = 2.8 × 10⁻⁶ s⁻¹

90% of the⁹⁹Mo nuclei decay to form^99mTc nuclei. The other 10% decay to form different nuclei.

Estimate the number of^99mTc nuclei that are extracted from the generator at t = 24 hours.

Information about heart function can be obtained from a multi-gated acquisition (MUGA) scan or from a transthoracic echocardiogram (TTE) scan. Heart specialists use these scans to diagnose a range of heart conditions.

In a MUGA scan, a patient is injected with a compound containing^99mTc which binds to red blood cells. Using images of the red blood cells, a precise reading can be made of blood flow rate through the heart.

A TTE is an ultrasound scan of the heart. Images of the heart are seen on a monitor as a transducer is moved across the surface of the patient’s chest.

Compare the MUGA method of investigating heart function with the TTE method of investigating heart function.

In your answer you should:

• outline how an image of the red blood cells is obtained during a MUGA scan

• explain the advantages that a MUGA scan and a TTE scan have in common

• compare the strengths and weaknesses of a MUGA scan and a TTE scan.

Fluorine-18 has a biological half-life of 6.0 hours.

Explain what is meant by this statement.

In a PET scan, fluorine-18 is used as a tracer and is injected into the person being scanned.

The physical half-life of fluorine-18 is 110 minutes.

Calculate the percentage of fluorine-18 that remains in the person 4.0 hours after it is injected.

Name the particles emitted when a fluorine-18 nucleus decays.

Figure 1 shows the cross-section of a body inside a ring of detectors during a PET scan. The side of each square represents 5 cm.

One of the products from the fluorine-18 decay goes on to produce two new particles. These particles travel in opposite directions in the plane shown in Figure 1. The particles are then detected by the detectors labelled A and B. Detector A detects a particle 0.79 ns before detector B.

Determine the square in Figure 1 in which the particles were produced. You should identify the square with a letter and a number, eg B 5.

A hospital uses the radioactive isotope technetium-99m as a tracer. Technetium-99m is produced using a Molybdenum-Technetium generator on site at the hospital.

Explain why the value of the half-life of technetium-99m:

• makes it suitable for use as a tracer

• means that it must be produced in a generator on site.

Technetium-99m emits only gamma rays.

Explain why this makes technetium-99m suitable for use as a tracer.

A gamma camera can be used to form images when using a tracer. Figure 1 shows a photomultiplier tube from a gamma camera.

At the crystal scintillator, each photon of gamma radiation leads to the emission of one visible light photon.

Describe how the current produced by the photocathode is amplified in the photomultiplier tube.

Iodine-131 is a medical tracer that can be detected using a gamma camera.

Iodine-131 has a physical half-life of 8.0 days.

A patient is injected with iodine-131 that has an initial activity of 3.2 GBq. For this patient, the biological half-life is 66 days. For safety reasons, the patient cannot be discharged from hospital until the activity due to the iodine in the patient’s body drops to 1.1 GBq.

Determine whether the patient can be safely released from hospital after 10 days.

Figure 2 shows scanned images of three different human heads. Each image used one of the following scanning techniques:

• magnetic resonance (MR)

• ultrasound

• PET.

Figure 2

Identify the scanning technique used for each image. Go on to explain how the features of each image enabled you to identify the type of scan.

A: Scanning technique

B: Scanning technique

C: Scanning technique

on a horizontal table and is moved into the PET scanner. The scanner has many detectors positioned in a vertical circular pattern around the patient.

State what is meant by a radioisotope.

The radionuclide used in the PET scan has a physical half-life of 110 minutes. The radionuclide is excreted from the body with a biological half-life of 185 minutes.

Show that the effective half-life of the radionuclide in the body is about 70 minutes.

Discuss what might be a suitable length of time for the patient to relax between injecting the radionuclide and moving the patient into the PET scanner.

The decay of the radionuclide results in the emission of a positron. Two of the detectors, directly opposite to each other, are triggered as they each receive a gamma photon.

Explain the process in which the gamma photons are created.

Figure 1 shows the head of a patient that is 0.2 m across, placed centrally between two of the many detectors in a PET scanner.

To determine the position where the gamma photons are produced between the detectors, the scanner measures the short interval of time t between the triggering of the first detector and the triggering of the second detector.

Discuss, for the detector positions shown in Figure 1, the range of the values of that the scanner must measure to perform a PET scan on the head. Assume that the speed of the gamma photons in the head is 3 × 10⁸ m s⁻¹.