Emission tomography provides a means of displaying the complete 3D in vivo distribution of activity, rather than a planar projection, identical to the relationship between CT and planar X-ray imaging. Emission tomography is the general name that covers two closely related tomographic techniques, single photon emission computed tomography (SPECT) and positron emission tomography (PET). As the names suggest, these relate to the form of emission for the radionuclide used; single photon emission (as for radionuclides commonly used with the Anger gamma camera) or positron emission. Recall that, in the case of positron emission, the emitted charged positron travels a short distance in tissue, dissipating energy, before annihilating with an electron to produce two 511-keV photons travelling in opposite directions. It is these photons that are detected rather than the positron; so 'dual photon' emission rather than single photon emission! In this introductory section the basic principles of SPECT and PET are directly compared and the methods of tomographic reconstruction, which are essentially the same for both modalities, are discussed.
Both SPECT and PET were developed in the late 1960s and early 1970s, around the same time as the introduction of CT and MRI. SPECT found widespread use at an early stage, as it can be performed with only minor modification of the standard gamma camera as an option to planar imaging; acquisition simply involves rotation of the camera around the patient (most commonly with two detectors, rather than one). Images are acquired for a finite time (e.g. 30–40 s) at equally spaced angles around the patient, simultaneously acquiring data that can be used to reconstruct multiple tomographic axial images (not unlike modern multidetector CT systems). PET involves specifically designed instrumentation dedicated to tomographic imaging; as a more expensive, evolving technology, its use historically was in clinical research, but now it has an important role in clinical diagnosis and patient management. In the case of PET, multiple rings of detectors fully surround the patient so no rotation is necessary and fast dynamic acquisition is possible. The system checks pairs of detectors within a short time window for the 'simultaneous' detection of a pair of emitted annihilation photons: so-called coincidence detection. The event defines a line of response (LOR) between the two detectors that identifies the origin of the annihilation without the need for the physical collimation that is necessary in SPECT. As a result the PET system is much more sensitive than SPECT (typically around 100 times), and in modern clinical PET cameras resolution is roughly twice as good as SPECT (limited in SPECT due to the choice of collimation that balances resolution against sensitivity). Although appearing quite different from SPECT acquisition, the acquired PET data can be reorganised so as to represent a set of parallel projections, effectively the same as in SPECT; similar tomographic reconstruction can therefore be performed for both systems.
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