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Radionuclide Imaging and Therapy

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Radionuclide Imaging and Therapy

Radionuclide imaging is a process that provides images (scans) of internal body structures, particularly regions where cancer cells are present. Radionuclide imaging is a process that provides images (scans) of internal body structures, particularly regions where cancer cells are present. To be able to scan these regions, a small amount of a radioactive chemical (radionuclide) is injected into a vein or must be swallowed by the patient. The common use of radionuclide scanning is to diagnose, stage, and monitor diseases. In this article, we will explore radionuclides and the techniques of radionuclide imaging and therapy.

What are radionuclides?

Radionuclides are radioactive forms of elements. Some are found in nature, while others are created by humans, either intentionally or as a consequence of nuclear processes. Every radionuclide emits radiation at a certain pace, which is quantified in terms of half-life, and each radionuclide emits radiation at a different rate.

Radionuclide Imaging and Therapy: Half-life

When it comes to radioactivity, the half-life is the amount of time it takes for one-half of the atomic nuclei in a radioactive sample to decay. By releasing particles and energy, nuclear species may spontaneously transform into other nuclear species, which is called decay. Alternatively, the half-life of an isotope is the average time it takes for the number of unstable nuclei to halve.

Radionuclide Imaging and Therapy, Decay of Carbon, StudySmarterDecay of carbon. Flickr.com

Radionuclide imaging Techniques

There are three techniques of radionuclide imaging used in today's medical physics. They are planar scintigraphy, also known as single-photon emission computed tomography (SPECT), positron emission tomography (PET), and hybrid techniques.

Planar scintigraphy or single-photon emission computed tomography

The detection of gamma-emitting radionuclides by planar imaging or single-photon emission computed tomography (SPECT) requires radiopharmaceuticals containing gamma-emitting radionuclides. Both methods rely on gamma cameras for detection, with collimated detectors registering emitted gamma rays.

A series of collimators direct gamma rays into an array of scintillation crystals, which transforms them into optical photons and detects them with photomultiplier tubes (PMT). A two-dimensional image (scintigram) of radioactivity distribution is created from these data. SPECT has the advantage of three-dimensional imaging (tomography) for better radioactivity distribution detection and physiological and functional data.

RadionuclidesHalf-lifeEmax ( keV )radiationProduction
99mTc6.0 h141generator
111In67.9 h245, 172 (0.5-25)(Auger electrons)Accelerator
123I13.3 h159Accelerator

Collimators describe the optimum energy as being between 140 and 160 keV. 99mTc, 111In, and 123I are the most frequent gamma-emitting radionuclides utilized in planar scintigraphy and SPECT. (The numbers 99m, 111, and 123 indicate the mass number, also known as the nucleon number, and can be calculated by the addition of protons and neutrons.)

In order to minimize unwanted irradiation, the radionuclide's half-life should be short enough to fade away as soon as possible after imaging.

Positron emission tomography

By detecting the regional concentration of the imaging agent, positron emission tomography (PET) gives a unique opportunity to monitor and quantify in vivo physiological molecular interactions in real time. It is the most precise and non-invasive technique available.

PET necessitates the use of a radiopharmaceutical that contains a positron-emitting radionuclide. In order to attain a lower energy state, the positron-emitting radionuclide requires an extra neutron. The stabilization is accomplished by spontaneous decay, which results in the production of a neutron as well as the emission of a positron and a neutrino.

Radionuclide Imaging and Therapy, Results of a brain PET scan, StudySmarterResult of a positron emission tomography of a human brain. flickr.com

The positron travels a given distance (positron range), which is determined by the density of the environment and the positron energy. When its kinetic energy drops, it makes contact with an electron, resulting in its annihilation and the creation of two 511 keV photons. After that, PMT registers the photon counts. Single photon events are rejected, allowing for precise measurement of the radioactivity content in the target region. The registered events are rebuilt into pictures that represent the radioactive source's spatial distribution throughout the body.

It is possible to do recurrent examinations during the same day because of an isotope with a short half-life of up to 68 minutes (68Ga).

Radionuclide Imaging and Therapy: Hybrid techniques

The addition and integration of computed tomography (CT) to SPECT and PET for the acquisition of morphologic information while the patient is in the same position resulted in further development and improvement of the nuclear imaging technique. This is critical for pinpointing the exact location of the lesions, particularly in the abdominal region.

Imaging processes are shortened with the use of a CT attenuation map. PET-CT is a hybrid imaging technique that combines the sensitivity of PET with the temporal and spatial resolution of CT. PET and SPECT quantification accuracy is also improved by CT attenuation and scatter correction.

Radionuclide Imaging and Therapy, A PET-CT Scan, StudySmarterA PET-CT scan. flickr.com

Radiotherapy

Ionizing radiation is extremely sensitive to rapidly proliferating cells, which is utilized in the application of radiation to regulate or destroy fast-dividing cancer cells. External and internal radiotherapy are both possible. The radiation source can be sealed and implanted ( brachytherapy ) or supplied intravenously for in vivo molecular interaction during internal radiotherapy.

The role of radiotherapy is getting bigger every day in nuclear medicine. Targeted radiotherapy of small tumors, micrometastases, and single cancer cells can all benefit from radionuclides that produce Auger electrons in the subcellular range. Radionuclides differ in terms of the type of radiation they emit, as well as their radiobiological efficacy and range of action, allowing for tumor type selection.

Radionuclide Imaging and Therapy, A radiotherapy session, StudySmarterA radiotherapy session. flickr.com

Radiotherapy has either found an application or shown potential in solving lymphoma, breast, prostate, colon, thyroid, lung, and brain cancer types, as well as in bone pain palliation.

Radionuclide Imaging and Therapy - Key takeaways

  • The half-life of a radioactive sample is the time it takes for one-half of the atomic nuclei to decay. The term decay explains that nuclear species can spontaneously transition into other nuclear species by releasing particles and energy.
  • The radioactive versions of elements are known as radionuclides. Some are found in nature, while others have been created by humans, either purposely or as a result of nuclear processes.
  • The three techniques used in today's medical physics for radionuclide imaging are single-photon emission computed tomography (SPECT), positron emission tomography (PET), and hybrid techniques
  • Radionuclide scanning is used to diagnose, stage, and monitor diseases, such as cancer, trauma, and infection.
  • The radionuclide's half-life should be short enough to fade away as soon as possible after imaging to reduce unwanted irradiation.

Frequently Asked Questions about Radionuclide Imaging and Therapy

The value of a radionuclide's half-life is important because it determines if it can be used in radionuclide imaging. If it is short enough, it allows for repeated examinations during the same day.

 The use of strong doses of radiation to destroy cancer cells and reduce tumors is known as radiotherapy.

Targeted radiotherapy of small tumors, micrometastases, and single cancer cells can all benefit from radionuclides that produce  Auger electrons in the subcellular range. Radionuclides differ in terms of the type of radiation they emit, as well as their radiobiological efficacy and range of action, allowing for tumor type selection. Also, it has either found an application or shown potential in solving lymphoma, breast, prostate, colon, thyroid, lung, and brain cancer types, as well as in bone pain palliation. 

Final Radionuclide Imaging and Therapy Quiz

Question

Can radionuclides be created by humans?

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Answer

Yes, they can.

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Question

What is the given name to a radionuclide's pace in emitting radiation? 


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Answer

Half-life.

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Question

The half-life is the amount of time it takes for one-half of the atomic nuclei in a radioactive sample to do what?

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Answer

Decay.

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Question

 Which radionuclide imaging technique is the most precise and noninvasive technique available?


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Answer

Positron emission tomography (PET).

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Question

 Can nuclear species spontaneously transform into other nuclear species by releasing particles and energy?

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Answer

Yes, they can.

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Question

 To minimize unwanted irradiation, should the half-life be short or long?


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Answer

 It should be short.

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Question

 Which technique gives a unique chance to monitor and quantify in vivo physiological molecular interactions in real time?


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Answer

Positron emission tomography (PET).

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Question

What is the technique that uses radiation to regulate or destroy fast-dividing cancer cells?


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Answer

 Radiotherapy.

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What do we call it when nuclear species spontaneously transform into other nuclear species by releasing particles and energy?

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Answer

Decay.

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Question

SPECT and planar scintigraphy use gamma cameras for detection, with collimated detectors registering emitted gamma rays. True or false?


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Answer

 True.

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PET necessitates the use of a radiopharmaceutical that contains what?


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Answer

A positron-emitting radionuclide

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Are gamma cameras used in SPECT?


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Answer

Yes, they are.

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 What detects optical photons?


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Answer

Photomultiplier tubes.

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In hybrid techniques, what is used combined with SPECT and PET?

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Answer

Computed tomography (CT).

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Does SPECT have the advantage of three-dimensional imaging? 


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Answer

 Yes, it does.

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What is improved with the usage of hybrid techniques?


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Answer

Quantification accuracy.

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 Is tumor type selection possible in radiotherapy?


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Answer

Yes, it is.

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Question

In which particular region do hybrid techniques help pinpoint the exact location of lesions?


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Answer

In the abdominal region.

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Collimators describe the optimum energy as being between which values?


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Answer

Between 140 and 160 keV.

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Does radiotherapy have an application on bone pain palliation?

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Answer

Yes, it does.

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Question

What is the main function of the gamma camera?

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Answer

To detect gamma photons traveling into the camera parallel to its axis and convert them into an electrical signal that can be further processed for visualisation.

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Question

List the key components of a gamma camera?

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Answer

  • Collimator
  • Scintillator
  • Light guide
  • PMT array


Additional components are:


  • Radiopharmaceutical or medical tracer
  • Processing computer

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Question

Describe the function of the collimator?

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Answer

The collimator is required to permit only electrons travelling parallel to the camera axis to pass through it.

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Describe the structure of the collimator?

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The collimator consists of an array of long, thin lead tubes. This means that only a photon that passes perfectly along the tube will pass through, while those that travel off-axis and hit the side of the tube will be absorbed. This is often a honeycomb-shaped grid.

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Describe the function of the scintillator?

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The scintillator absorbs a single gamma photon and emits thousands of visible light photons. This is required as gamma photons will often pass straight through detection hardware, whereas visible photons are more easily measured.

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What is the most common scintillator material?

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Answer

Sodium pertechnetate.

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What is the function of the photomultiplier tubes?

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Answer

Photomultiplier tubes absorb a visible light photon and produce an electrical pulse signal. The signals from the array of PMTs are processed by a computer to calculate the intensities of gamma radiation beneath the camera.

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Question

On average, how many secondary electrons are produced from an electron-dynode impact within a PMT?

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Answer

4

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Question

How do radionuclide imaging techniques using a gamma camera differ from traditional techniques such as CT or MRI scans?

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Answer

Radionuclide imaging techniques visualise the concentrations of a medical tracer compound within the patient’s body, rather than the anatomy itself. This allows body function to be viewed, compared to traditional techniques that directly image the patient’s anatomy.

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Question

What is a radiopharmaceutical?

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Answer

A radiopharmaceutical, or medical tracer, is a compound formed of a radioisotope combined with a molecule transported by the body, such as glucose. The concentrations of medical tracers inside the body can be viewed with a gamma camera.

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Question

What is the most common gamma-source radioisotope used in radionuclide imaging?

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Answer

Technetium-99m.

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What is the half-life of Technetium-99m?

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Answer

6 minutes.

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Why is it important for the radioisotope used in a medical tracer to have a relatively short half-life?

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Answer

A short half-life ensures the source is highly active, meaning less is required, and that the substance decays quickly after the procedure, reducing the duration of exposure for the patient.

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Question

What is the probability a gamma photon will interact with the scintillator?

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Answer

10%.

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

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Answer

5-10mm.

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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.

Show question

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