Basic Physics of Nuclear Medicine/Print version

Emission Tomography

The form of imaging which we have been describing is called Planar Imaging. It produces a two-dimensional image of a three-dimensional object. As a result images contain no depth information and some details can be superimposed on top of each other and obscured or partially obscured as a result. Note that this is also a feature of conventional X-ray imaging.

The usual way of trying to overcome this limitation is to take at least two views of the patient, one from the front and one from the side for example. So in chest radiography a posterio-anterior (PA) and a lateral view can be taken. And in a nuclear medicine liver scan an antero-posterior (AP) and lateral scan are acquired.

This limitation of planar X-ray imaging was overcome by the development of the CAT Scanner about 1970 or thereabouts. CAT stands for Computerized Axial Tomography or Computer Assisted Tomography and today the term is often shortened to Computed Tomography or CT scanning (the term tomography comes from the Greek word tomos meaning slice). Irrespective of its exact name the technique allows images of slices through the body to be produced using a computer. It does this in essence by taking X-ray images at a number of angles around the patient. These slice images show the third dimension which is missing from planar images and thus eliminate the problem of superimposed details. Furthermore images of a number of successive slices through a region of the patient can be stacked on top of each other using the computer to produce a three-dimensional image. Clearly CT scanning is a very powerful imaging technique relative to planar imaging.

The equivalent nuclear medicine imaging technique is called Emission Computed Tomography. We will consider two implementations of this technique below.

(a) Single Photon Emission Computed Tomography (SPECT)

This SPECT technique uses a gamma camera to record images at a series of angles around the patient. These images are then subjected to a form of digital image processing called Image Reconstruction in order to compute images of slices through the patient.

The Filtered Back Projection reconstruction process is illustrated below. Let us assume for simplicity that the slice through the patient actually consists of a 2x2 voxel array with the radioactivity in each voxel given by A1...A4:

The first projection, P1, is imaged from the right and the second projection, P2, from the right oblique and so on. The projections are firstly added to each as shown below:

and the summed (or superimposed) projections are normalised to generate an estimate of the radioactivity in each voxel.

An alternative image reconstruction technique is called Iterative Reconstruction. This is a successive approximation technique as illustrated below:

Projection	Patient	Additive Iterative Reconstruction
P1	Actual projection, P1.	First estimate of image matrix.
P2	Actual projection, P2.	Estimate of projection, P2. Second estimate of image matrix.
P3	Actual projection, P3.	Estimate of projection, P3. Third estimate of image matrix.
P4	Actual projection, P4.	Estimate of projection, P4. Fourth estimate of image matrix.

The first estimate of the image matrix is made by distributing the first projection, P1, evenly through an empty pixel matrix. The second projection, P2, is compared to the same projection from the estimated matrix and the difference between actual and estimated projections is added to the estimated matrix. The process is repeated for all other projections.

A comparison of these image reconstruction techniques is shown below for a slice through a ventilation scan of a patient's lungs:

The gamma camera is typiclly rotated around the patient in order to acquire the images. Modern gamma cameras which are designed specifically for SPECT scanning can consist of two camera heads mounted parallel to each other with the patient in between. The time required to produce images is therefore reduced by a factor of about two. In addition some SPECT gamma cameras designed for brain scanning have three camera heads mounted in a triangular arrangement.

A wide variety of strategies can be used for the acquisition and processing of SPECT images.

(b) Positron Emission Tomography (PET)

You will remember from chapter 2 that positrons can be emitted from radioactive nuclei which have too many neutrons for stability. You will also remember that positrons do not last for very long in matter since they will quickly encounter an electron and a process called annihilation results. In the process the positron and electron vanish and their energy is converted into two gamma-rays which are emitted at roughly 180^o degrees to each other. The emission is often referred to as two back-to-back gamma-rays and they each have a discrete energy of 0.51 MeV.

So if we administer a positron-emitting radiopharmaceutical to a patient an emitted positrons can annihilate with a nearby electon and two gamma-rays will be emitted in opposite directions. These gamma-rays can be detected using a ring of radiation detectors encircling the patient and tomographic images can be generated using a computer system. The detectors are typically specialised scintillation devices which are optimised for detection of the 0.51 MeV gamma-rays. This ring of detectors, associated apparatus and computer system are called a PET Scanner:

The locations of positron decays within the patient are highlighted by the solid circles in the above diagram. In addition only a few detectors are shown in the diagram for reasons of clarity. Each detector around the ring is operated in coincidence with a bank of opposing detectors and the annihilation gamma-rays thus detected are used to build up a single profile.

It has also been found that gamma cameras fitted with thick crystals and special collimators can be used for PET scanning.

The radioisotopes used for PET scanning include ¹¹C, ¹³N, ¹⁵O and ¹⁸F. These isotopes are usually produced using an instrument called a cyclotron. In addition these isotopes have relatively short half lives. PET scanning therefore needs a cyclotron and associated radiopharmaceutical production facilities located close by. We will be considering cyclotrons in the next chapter of this wikibook.

Standardized Uptake Value (SUV) is a semi-quantitative index used in PET to express the uptake of a radiopharmaceutical in a region of interest of a patient's scan. Its typically calculated as the ratio of the radioactivity in the region to the injected dose, corrected for body weight. It should be noted that the SUV is influenced by several major sources of variability and it therefore should not be used as a quantitative measure.

A number of photographs of a PET scanner are shown below:

The detectors and associated electronic circuitry.	The scanner itself - the detectors are under the covering panel.
Another view of the detectors.	The image processing computer.