Shriya Subramanian

Abstract

Nuclear medicine is a medical specialty that makes use of radiation in two primary areas of medicine: the diagnosis and the treatment of disease. There are various techniques used for diagnosis such as Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET), which are collectively known as nuclear imaging. The methods of treatment are known as radiotherapy, which is most commonly used for cancer treatment. This review will focus on these areas of nuclear medicine.
Keywords: Nuclear medicine, radiopharmaceutical, radiotherapy, nuclear imaging, gamma camera

Introduction

Nuclear medicine is a type of non-invasive imaging method which involves the injection or inhalation of radioactive tracers to visualize various organs^[1]. The basis of this technique is the administration of a chemical agent labeled with smaller molecules of radioactive tracers or radionuclides. These radioactive tracers are distributed according to clearance kinetics and allow for the assessment of particular organ or tissue functions in vivo, making it a more broad tool for diagnostic purposes. This type of technique is also highly beneficial in therapy procedures such as cancer treatment or thyroid abnormality by the administration of higher doses of radiations to specific targeted organs. The other radiological examinations, such as computed tomography (CT), computerised axial tomography (CAT), magnetic resonance imaging (MRI), and others, only assess anatomy, i.e., how the organ looks^[2]. In contrast, nuclear medicine determines how the organ functions^[3].

Diagnosis

• Radiopharmaceuticals

Radioisotopes have unstable nuclei that possess an excessive amount of internal energy, which causes them to decay to form more stable nuclei and emits energy spontaneously. This process is known as radioactivity. These unstable nuclei are called radionuclides, and the emissions are called radiation. Diagnostic techniques usually involve attaching a radionuclide, known as a tracer, to a biologically active molecule, forming a drug called a radiopharmaceutical.
Radiopharmaceuticals form the core of nuclear medicine. They are administered into the body either orally or via injection. With the help of the tracer in the radiopharmaceutical, the progress of the drug can be traced as it emits gamma rays from inside of the body. The emitted gamma rays are then imaged by conventional scintigraphy imaging using a gamma camera as an external detector that provides 2D.
The basis of nuclear imaging is gamma scintigraphy. Due to charged current interactions among the ionizing radiation (emitted gamma rays), a pulse of electromagnetic radiations takes place briefly in a visible light range, which is detected by suitable detectors such as gamma cameras and is then processed by computers to form two or three images. Additionally, PET, CT, and SPECT have been developed, which are now regarded as a standard modality in nuclear medicine imaging as they can produce 3D computerised images^[4].

• Positron Emission Tomography

As mentioned earlier, Positron Emission Tomography provides three-dimensional images of the concentrations of tracer in the body. In PET, the radionuclide consists of a positron-emitting tracer specifically. After injection, the tracer is accumulated in parts of the body where there is higher chemical activity^[5]. A certain radionuclide is used for that certain target tissue. These particular areas show up on the PET scan as bright spots. PET scans are performed for a number of different reasons, such as the detection of cancer, brain disorders, and heart issues^[6].
Cancer cells have a higher metabolic rate than noncancerous cells, which means that they have higher chemical activity. This allows for the detection of the spread of cancer, a cancer recurrence, or for checking whether the treatment is effective or not^[7].
To test for brain and central nervous system disorders, which include epilepsy, Parkinson’s disease and depression, compounds like glucose is added to the tracer because glucose is required for brain functionality. The PET scan can detect this radioactive glucose and at which areas the brain is using the glucose the fastest^[8].
PET scans can also help identify heart problems. As tracer will accumulate in greater amounts in healthy heart tissue compared to tissue, which is unhealthy or has decreased blood flow, PET scans are able to detect these areas of reduced blood flow in the heart^[9].
PET/CT, a combination of PET and CT (computerised tomography) scans, is a technique that combines the pictures from a CT and a PET scan. A CT scan makes use of x-ray equipment to produce images taken from various angles. Image fusion is done where a computer combines both scans to create a more detailed 3D image^[10]. PETCT allows for the diagnosis of a wider variety of diseases as it provides more abundant information.

• Single Photon Emission Computed Tomography

Single Photon Emission Computed Tomography (SPECT) is a technique similar to PET. SPECT scans produce three-dimensional images like PET scans. The primary difference between the methods is the type of radiopharmaceutical used. The tracer used for SPECT scans emit gamma radiation; hence, the gamma camera measures gamma rays^[11].

Treatment

• Radionuclide Therapy

Nuclear medicine therapy utilizes radiopharmaceuticals that target cancerous cells or specific tumors such as lymphomas and thyroid.^[12] The treatment involves irradiating the area of growth. The objective of the treatment is to either control or completely eliminate the tumor/cancerous cells. There are numerous methods of radionuclide therapy; the two types are external and internal.

• External Radionuclide Therapy

In external radionuclide therapy methods, an external source of radiation is used and is aimed at the site of a tumor or cancerous cells. One of the more commonly performed procedure is the linear accelerator (LINAC).^[13] A linear accelerator is a machine that uses microwave technology to accelerate electrons and then enables these electrons to collide with a heavy metal target.^[14] This process produces high-energy x-rays that leave the machine as a beam in shape specific to the tumor that it is being directed at.^[15]

• Internal Radionuclide Therapy

Internal radionuclide therapy works on the basis of radioactive material administration into the body. This way it reaches cells throughout the body by being carried in the bloodstream. The radioactive material can be administered in two different forms, either in solid form as an implant or in liquid form. The implant is put close to the area of the tumor and remains there for a given time period. The implant is a gamma or beta emitter. If a radioactive liquid is used instead, it is given orally or via injection. The radioactive liquid used is specific to the disorder or type of cancer. For example, a radioactive isotope of iodine, iodine-131 is used for thyroid cancer.

Conclusion

To conclude, nuclear medicine makes use of radiation in two main areas of medicine: the diagnosis and the treatment of disease. It is a non-invasive imaging method which involves the injection or inhalation of radioactive tracers to visualize various organs. There are various diagnosis techniques including PET, CT and SPECT. Radionuclide therapy is used to treat diseases with the help of nuclear medicine.

Bibliography

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8. What is external radiotherapy? n.d Accessed May 25, 2020. https://www.cancerresearchuk.org/about-cancer/cancer-in-general/treatment/radiotherapy/external/about.

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About the Author

Shriya Subramanian is a high school senior at The Indian Public School. Having immersed herself in psychology for many years, she plans to study psychiatry at university and pursue it as a career. A few of her many hobbies include photography, painting and writing.