Radionuclide Imaging Techniques

Radionuclide imaging techniques are a subset of nuclear medicine that allow body functions to be observed. While imaging techniques such as X-rays, CT, or ultrasound scans can be powerful tools for viewing a patient’s internal anatomy, radionuclide techniques allow us to see how various bodily functions or processes take place.

A key difference between 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.

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. Seeing how medical tracers are treated and processed by organs within the patient’s body can be 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.

False-colour radionuclide image representation of a human brain (cross-section).

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 modification of the gamma camera technique; it allows for 3D representations of the medical tracer concentrations to be produced. One or several gamma cameras are used to produce 2D images ('projections') of the 3D distribution of the medical tracer within the patient’s body. By carefully controlling the position of the gamma camera(s), several projections are acquired from different angles around the patient. A SPECT scanner is used to do this, which typically consists of a motorised bed for the patient and rotating camera(s), as seen below.

Typically, 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 usually required for a high-quality reconstruction, meaning the total scan can take 30-40 minutes. Scanners that incorporate 2 or 3 gamma cameras can take images from several angles simultaneously, reducing the overall scan time.

Siemens SPECT Machine. The two cameras (highlighted) rotate around the motorised bed.

A computer then applies a tomographic reconstruction algorithm to the set of tracer concentration projections, which produces a 3D representation of the medical tracer concentrations within the body. The software can be used to view the entire 3D dataset, or thin 'slices' representing body cross-sections along any axis. 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 (voxel) resolution of around 5-10mm, but cannot provide real-time images due to the tomographic processing step involved.

A key advantage of SPECT over simpler techniques like a gamma camera scan is in imaging complex internal organs such as the brain, where 3D data allows the 'depth' of gamma activity to be understood. As gamma rays will be somewhat attenuated as they pass through the patient, the raw projection data would underestimate the gamma intensity coming from deep tissues compared to superficial ones.

An attenuation correction factor can be built into the tomographic reconstruction algorithm to account for this, which can be further improved by incorporating CT scan data to create a hybrid scan technique. The CT data provides a 3D map of patient anatomy, which can be used to estimate gamma attenuation caused by different tissues and correct for this in the SPECT data. This type of hybrid technology is also useful for locating the medical tracer concentrations with respect to other anatomical features, as the shape and position of some tissues can vary among patients.

SPECT scan medical tracers

As a SPECT scan is essentially a modification of a gamma camera scintigraphy scan, the same medical tracers can often be used. The most commonly-used radioisotope is Technetium-99m, which is combined with other compounds to form medical tracers that target certain 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.

While SPECT and Gamma camera scans can use the same gamma-emitting medical tracers, PET scans require different, positron-emitting tracers. Gamma radiation is produced as emitted positrons interact with electrons in the body. The two antiparticles annihilate each other and produce a pair of 0.51MeV gamma photons. These travel at the speed of light c in diametrically opposite directions from the point of annihilation, which ensures momentum is conserved. The gamma photons are produced by the positron-electron interaction, not from the positron source. On average, the positron travels 1mm through tissue before it is annihilated.

A 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 as the photons travel at the speed of light. Each gamma detector is similar to a gamma camera, consisting 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.

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.

PET scan medical tracers

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.