Radionuclide imaging techniques are a powerful tool for medical diagnosis. They are a type of nuclear medicine that allow us to observe how the body functions and processes take place. Unlike X-rays, CT, or ultrasound scans, which provide direct representations of the patient’s body, radionuclide imaging techniques show the concentrations of a radiopharmaceutical medical tracer within a patient’s body.

Radionuclide techniques have been described as 'an x-ray in reverse'. While X-rays detect radiation that passes through the body after being produced outside the patient, radionuclide techniques detect radiation that is produced within the patient. This allows us to see how medical tracers are treated and processed by organs within the patient’s body, providing a powerful diagnostic tool for assessing bodily function.

Radionuclide imaging techniques

These techniques rely on the detection of gamma photons emitted from a radiopharmaceutical medical tracer compound that is injected into the patient's body or ingested by the patient. Gamma radiation is used as it is the least ionising type of nuclear decay radiation. This also allows it to pass from the point of emission, through the patient’s tissues to the detector without being obstructed.

2D scans, known as scintigraphy, can be produced using a gamma camera, which is a special kind of camera used to visualise the impact positions of gamma photons on a scintillator layer within the device. This data is then used to compute the concentrations of the medical tracer in the region beneath the camera.

‍

Gamma cameras are relatively simple, can be portable, and provide useful data for diagnosis. SPECT and PET scans are more complex adaptations of the gamma camera technique, capable of producing 3D images with more sophisticated equipment.

SPECT scans

Single-Photon Emission Computed Tomography (SPECT) is a medical imaging technique that produces 3D representations of medical tracer concentrations within the body. It is a modification the gamma camera technique that uses or several gamma cameras to produce 2D images of the tracer's 3D distribution within the patient's body. The gamma cameras are positioned at different angles around the patient to capture several projections that are later reconstructed into a 3D image by a computer.

The SPECT scanner consists of a motorized bed for the patient and rotating camera(s). The projections are acquired at 3-6 degree intervals, with each capture requiring an exposure time of around 15-20 seconds. A full 360-degree set of projections is required for a high-quality reconstruction, taking approximately 30-40 minutes. Scanners with 2 or 3 gamma cameras can take images from several angles simultaneously, reducing the overall scan time.

The reconstructed image is typically lower resolution than the original projections and is susceptible to image noise, or blurring from any movement of the patient during the scan. SPECT scans have a 3D pixel resolution of around 5-10mm, but cannot provide real-time images due to the tomographic processing step involved.

SPECT is useful for imaging complex internal organs such as the brain, where 3D data allows the 'depth' of gamma activity to be understood. Attenuation correction factors can be built into the tomographic reconstruction algorithm to account for the gamma attenuation caused by different tissues. The CT data provides a 3D map of patient anatomy, which can be used to estimate gamma attenuation and correct for this in the SPECT data.

SPECT scans use medical tracers such as Technetium-99m, which is combined with other compounds to form medical tracers that target specific tissues or organs within the body. Other radioisotopes used for SPECT scans include Iodine-123 for neurological tumour scans and Indium-111 for white blood cell scans.

PET scans

A Positron Emission Tomography (PET) scan operates similarly to a CAT scan by using a ring of detectors to produce a 3D image of the patient. Unlike a SPECT scan, the detectors remain stationary for the duration of a PET scan.

PET scans require different, positron-emitting tracers, which produce gamma radiation as emitted positrons interact with electrons in the body. The two antiparticles annihilate each other and produce a pair of 0.51MeV gamma photons, which travel at the speed of light c in diametrically opposite directions from the point of annihilation. On average, the positron travels 1mm through tissue before it is annihilated.

The ring of gamma detectors monitors the arrival locations and precise timings of each pair of gamma photons, which allows the point of annihilation to be calculated. Each gamma detector consists of a photomultiplier tube and sodium iodide scintillator. The gamma detectors will automatically discard any photon detection without a second detection within a certain time window, as this would mean the two photons were not produced together at an antiparticle annihilation. This filtering means PET scans are largely unaffected by phenomena such as scattering, as valid gamma detections are guaranteed to originate from an antiparticle annihilation – which will have occurred within 1mm of a medical tracer molecule.

‍

Positron-electron annihilation location example Simplifying the 3D principle to 2 dimensions: Detector A is at 2D position (0m,0m) and detector B is at position (1m,0m).  Detector A records a photon impact at 0s, and detector B records an impact  1.467⋅10-9 s later. Assuming c=3⋅108 m/s, calculate the position of the antiparticle annihilation that produced these photons.SolutionThe distance travelled by a photon in 1.467⋅10-9 s:We know one of the photons travelled 0.44 m farther (d1) than the other (d2) before reaching the detectors. As the detectors are 1m apart, we know that:and that:Therefore,  This shows that the antiparticle annihilation took place at (0.28 m, 0 m).

Recording the calculated locations of antiparticle annihilation produces a 3D scan of the concentrations of the medical tracer inside the body, with the approx 1mm voxel resolution being better than SPECT techniques. As there is no computed tomography stage required, PET scans can also display tracer activity in real-time.

A medical tracer used for a PET scan is required to emit positrons and have a short half-life. A commonly used isotope is Fluorine-18. This is a positron emitter with a half-life of 110 minutes and decays into an oxygen-18 nucleus, a positron, and a neutrino. Due to its short half-life, Fluorine-18 has to be produced on-site using a particle accelerator. The radioisotope is combined with glucose to produce a medical tracer called fluorodeoxyglucose (FDG), which is treated very similarly to glucose by the body. This means FDG accumulates in tissues with high rates of respiration when used as a tracer.

Another commonly used tracer is carbon monoxide with a carbon-11 atom, which emits a positron as it decays with a half-life of around 20 minutes. Carbon monoxide readily attaches to haemoglobin molecules, meaning this tracer is useful for monitoring blood flow.

PET scanners provide 3D, real-time observation of body function that can be used to diagnose and plan effective treatment for heart, brain, and cancer treatments. A key disadvantage of PET scans is that they are relatively expensive, due to the facilities required to produce the medical tracers on-site. Because of this they are typically only found at large hospitals and are only used for complex health problems.

Below is a table of the three key radionuclide imaging techniques, and some examples of medical tracer compounds they can each use for different scan types.

Radionuclide Imaging Techniques - Key takeawaysA key difference between traditional anatomical and radionuclide imaging techniques is that anatomical images are a direct representation of the patient’s body, while radionuclide images show the concentrations of a radiopharmaceutical medical tracer within a patient’s body.A Gamma camera can be used to perform scintigraphy scans which show the real-time 2D concentrations of a medical tracer in the tissues beneath the camera. This is the quickest and cheapest type of radionuclide imaging, but is limited to 2D images with no information about the depth of tracer concentration. Single Photon Emission Computed Tomography (SPECT) is an adaptation of the scintigraphy technique which uses an automated gamma camera(s) to take projections of the tracer concentrations from 360 degrees around the patient, before using software to create a 3D image.SPECT techniques can be enhanced when combined with CT data to correct for gamma attenuation and provide reference anatomical data for locating tracer concentrations. Positron Emission Tomography (PET) is the most precise radionuclide imaging technique, as it provides a direct representation of the activity of positron-electron annihilations that occur in the medical tracer. A disadvantage is that it is very expensive, primarily due to the requirement for on-site radioisotope production.

‍

Radionuclide Imaging Techniques

What are three common radionuclide imaging techniques?

The three widely-used radionuclide imaging techniques are 2D scintigraphy scans using a gamma camera, 3D Single Photon Emission Computed Tomography (SPECT), and 3D Positron Emission Tomography (PET) scans. Hybrid techniques incorporating CT scan data into SPECT or PET are also used.

Which radionuclide imaging techniques can capture real-time images?

2D Gamma camera scans and PET scans can both capture images in real-time, which makes them useful for viewing dynamic processes in the body such as blood flow. The reason these techniques can provide real-time imagery is that they do not require a computational tomography step. While the real-time data is processed by a computer before being displayed, this can be done quickly. SPECT scans require the entire 360-degree set of scintigraphy projections to be captured before the computed tomography step takes place, making a real-time display impossible.

Which radionuclide imaging techniques are used for brain imaging?

All three commonly used radionuclide imaging techniques, 2D Gamma camera scintigraphy, SPECT and PET can be used to image brain function. However, as the brain is a complex organ that has regions deep inside the skull, 2D scintigraphy can only be used to image 'shallow' parts of the brain. To view deep brain structures, a 3D technique such as SPECT or PET is required.

‍