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Imaging in medicine
Imaging in medicine

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6.4 Taking the image

Activity 14

Now watch this video clip of a patients lungs being imaged, called a VQ (ventilation quotient) scan. What are the two different types of acquisitions used called? What radioactive substance is used for each acquisition, and why?

Click to view clip about planar scans [2 minutes 21 seconds]

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Transcript: Planar scans

Narrator
This patient has been admitted to hospital with a suspected pulmonary embolism. To investigate this, a VQ scan is going to be carried out. This examines both the blood supply, or perfusion, to lungs and the ventilation of the lungs. Two radionuclide’s, which emit gamma radiation at different energies, are used in this procedure. The patient has already received an injection of technesium combined with MAA. The particles of MAA tend to lodge in the very small capillaries of the lungs.
This generator contains Rubidium-81, which decays to produce Krypton-81m. Air is passed through the generator and carries the Krypton to the patient’s lungs.
The half life of Krypton-81 is 13 seconds, so the gas expired poses little hazard to the staff.
(during capture)
As the radionuclides decay, the detected gamma rays gradually build to form a useful image. The image on the left shows the technesium decay, and therefore represents the perfusion in the lungs. The image on the right is formed by the gamma rays emitted by the Krypton and shows ventilation.
A full VQ examination is completed by acquiring images from several different angles. It is possible to carry out gamma camera imaging by recording data from a large number of angles. These can be processed to produce tomographic images. This is known as SPECT.
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Answer

The two different scans mentioned were a perfusion and ventilation scan, to look at the blood supply and air supply to the lungs, respectively. In order to do this simultaneously two different radionuclides that have different gamma ray energies are used. This is a valuable diagnostic test for a pulmonary embolism (a blood clot in the lungs).

To image the perfusion the patient is given an injection of technetium-99m (gamma energy 140 keV, half-life 6 hours) combined with macro-aggregated albumin (MAA). The ‘large’ particles of MAA tend to lodge in the very small capillaries of the lungs and therefore give a good indication of where there is, or is not, a good blood supply.

The ventilation can be imaged by asking the patient to breathe a mixture of air and krypton-81. This is produced from a generator containing the parent radionuclide, rubidium-81. As the patient is breathing the krypton-81, it shows which areas of the lungs the gas reaches, and therefore, which areas are ventilated.

Figure 17: VQ scan of the lungs using Tc-99m MAA (100 MBq) for normal lungs
Figure 18: VQ scan of the lungs using Tc-99m MAA (100 MBq) indicating defects

Another widely used procedure is a dynamic renogram. In this case the radionuclide imaged is one that is taken up and excreted by the kidneys. Successive images are collected over a period of time (e.g. every 15 seconds for 20 minutes) and can then be analysed to compare the function of the kidneys.

Figure 19: Gamma camera images showing normal kidneys (dynamic renogram)
Figure 20: Gamma camera images showing obstructed left kidney (dynamic renogram)

Activity 15

Now watch this final video clip on radionuclide images, on the single photon emission computed tomography (SPECT) imaging technique. The sequence shows cardiac (heart) images being obtained. How does SPECT produce slices through the body?

Click to view clip about SPECT [3 minutes 23 seconds]

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Transcript: SPECT

This patient is being prepared to undergo a SPECT procedure to look at myocardial perfusion that is blood supply to the heart muscle. It is usual to scan the patient to look at the perfusion when the heart has been stressed, and also when the heart is rested. There can be several days between these two image acquisitions.
Stressing of the heart is achieved in two ways. First by a pharmaceutical which has a similar effect on the heart to taking exercise, and also by squeezing a dumbbell. The heart needs to be monitored carefully during this period, so an ECG is recorded.
Once the heart has been stressed, the radiopharmaceutical is injected. In this case, the pharmaceutical being used is technetium labelled tetrafosmim.
In order to obtain tomographic images, two gamma cameras must be rotated around the patient, as close as possible to the heart. The patient’s arms must be held out of the way above his head. The cameras are now trained to move around the patient as closely as possible without touching him.
The images are reconstructed from the data, using the same back projection techniques that are used in CT scanning. They are shown as slices through the heart in different directions, with the white part of the image corresponding to high take up of the radiopharmaceutical and blue representing low take up.
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Answer

Data is collected from a large number of directions by rotating one or more camera heads around the body. Then, tomographic images can be reconstructed, by filtered back projection, using a similar process to CT. The speed of rotation is much slower than that used in CT – the whole process taking at least 10 minutes.

Images of a normally perfused heart muscle (myocardium), and a heart with an inferior and lateral wall defect are shown below.

Figure 21: SPECT images of normal myocardial perfusion. Acquired using 400 MBq of Tc99m Tetrafosmin. Three different views of the heart are shown. The top row of each view is obtained with the heart stressed, and the lower row with the heart at rest
Figure 22: SPECT images showing a reversible perfusion defect (present at stress but not at rest) in the inferior and lateral walls

Activity 16

Take a moment to consider what you think the advantages and disadvantages of radionuclide imaging would be compared with ultrasound, MRI, CT and X-rays.

Answer

The advantages of radionuclide scans are:

  • demonstration of functional information that often cannot be obtained in other ways;

  • wide variety of organs can be imaged;

  • tomographic and 3-D images available (SPECT).

The disadvantages are:

  • poor resolution;

  • radiation dose to the patient;

  • slow and labour intensive;

  • specialised radiopharmacy and scanners are not readily available at all hospitals.