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Radionuclide Imaging Techniques

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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.

Radionuclide Imaging Techniques Radionuclide Brain Image StudySmarter

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

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.

Radionuclide Imaging Techniques SPECT Machine StudySmarter

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

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.

Radionuclide Imaging Techniques PET Scanner Diagram StudySmarter

Diagram showing main components of a PET scanner. Ross MacDonald - StudySmarter Originals

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.

Solution

The 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.

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.

Scan

Medical tracer

Gamma Camera Scan (Scintigraphy)

Heart (coronary artery disease)

Thallium-201

Thyroid

Technetium-99m, Iodine-123

Whole-body (Gallium scan)

Gallium-67

Single Photon Emission Computed Tomography (SPECT)

Myocardial perfusion imaging (Heart)

Tc-99m-Tetrofosmin, Thallium-201 chloride

Functional brain imaging

Technetium (Tc-99m) exametazime

Most scintigraphy scans can also be performed in 3D using SPECT.

Positron Emission Tomography (PET)

Neuroimaging

Oxygen-15, fluorodeoxyglucose

Cardiac PET

Oxygen-15 water, Rubidium-82

Bone metabolism

Fluorine-18 Sodium Fluoride

Whole-body (Gallium Scan)

Gallium-68

Radionuclide Imaging Techniques - Key takeaways

  • A 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.

Frequently Asked Questions about 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.

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.

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.

Final Radionuclide Imaging Techniques Quiz

Question

Why have radionuclide imaging techniques been described as 'taking an x-ray in reverse'?

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Answer

X-rays detect radiation that have passed through the body after being produced outside the patient, while radionuclide techniques detect radiation that is produced by a radiopharmaceutical medical tracer within the patient. 

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Question

What type of radiation do radionuclide imaging techniques rely on?

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Answer

Gamma. All the techniques detect gamma photons with their sensor - but while 2D scintigraphy and SPECT use gamma-emitting medical tracers, PET uses a positron-emitting tracer that emits gamma upon their annihilation.

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Question

Which technique(s) can produce real-time images?

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Answer

2D Gamma camera scintigraphy

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Question

What are the benefits of a SPECT - CT scan hybrid imaging technique?

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Answer

The gamma emitted by the medical tracer will be attenuated by tissues in the patient's body, meaning the levels of tracer detected deeper inside the patient will be underestimated. CT data provides a map of the tissue attenuation and can be used to add an attenuation correction into the computed tomography step.

CT data can also be used to view reference anatomy, which can help to understand the locations of the concentrations of medical tracer.

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Question

What does SPECT stand for?

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Answer

Single Proton Emission Computed Tomography

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What does PET stand for?

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Answer

Photon Emitting Tomography

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Question

Why are 2D scans taken using a gamma camera known as scintigraphy?

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Answer

Scintigraphy refers to the scintillator component within a gamma camera. This layer absorbs a single gamma photon and emits thousands of visible light photons, making the impact much easier to reliably detect.

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Question

What angle interval are SPECT projections typically taken at?

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Answer

3-6 degrees. Most scans will require a full 360-degree set of projections.

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Question

What is a typical exposure time for each projection in a SPECT scan? How long might a full 360-degree scan take?

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Answer

15-20 seconds per projection exposure. Assuming the full 360 degrees are captured at 3-6 degree intervals, the total scan time could be 15-40 minutes.

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Question

Why do machines with 2 or 3 gamma cameras reduce the duration of a SPECT scan?

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Answer

Projections can be taken from several angles simultaneously.

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Question

What is a typical 3D pixel (voxel) resolution for a SPECT scan?

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Answer

5-10mm.

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Question

What is a typical 3D pixel (voxel) resolution for a PET scan?

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Answer

Around 1mm. This is the average distance a positron will travel after its emission before its annihilation.

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Question

What is the most commonly used medical tracer radioisotope for scintigraphy and SPECT scans?

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Answer

Technetium-99m. (Tc-99m)

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Question

Why do the gamma photons created at a positron-electron annihilation travel in diametrically opposite directions?

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Answer

The photons emitted move at the speed of light c, so they travel in diametrically opposite directions to produce a net momentum of 0, ensuring momentum is conserved.

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Question

Why are PET scans resistant to phenomena such as scattering?

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Answer

The gamma detectors in a PET scanner will automatically discard any detections that do not have a second corresponding detection within a time window. This ensures that all detected photons were produced in pairs at the antiparticle annihilation locations, filtering out photons affected by scattering.

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Question

What is a common radioisotope used in PET medcial tracers? Why does this make the process expensive?

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Answer

Fluorine-18. Due to its short half-life of 110 minutes, Fluorine-18 has to be produced on-site using a particle accelerator - raising its cost substantially.

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