Department of Nuclear Medicine and PET/CT services

Below are the services offered by the Department of Nuclear Medicine and PET/CT.

Positron emission tomography (PET)

PET is a non-invasive nuclear medicine technique that allows the evaluation of metabolic processes and the disturbance of these processes by disease. It allows the identification of metabolically active cancer cells and provides excellent information on the staging of the disease, and the impact of treatment on the progress of the disease.

PET takes advantage of short-lived radionuclides such as Fluorine-18, Oxygen-15, Carbon-11 and Nitrogen-13 that can label biological compounds of interest to create radiopharmaceuticals. The most widely used radiopharmaceutical in clinical oncological PET is Fluorine-18 fluorodeoxyglucose (18FDG) which is an analogue of glucose.

A number of radiopharmaceuticals are being developed for clinical use that identify different aspects of tumour biology such as DNA turnover (proliferation) and new blood vessel formation (angiogenesis). These short-lived radionuclides are made using a cyclotron facility and attached to biological molecules in a radiochemistry laboratory.

As many are very short-lived (half-lives of less than 20 minutes), a cyclotron and radiochemistry facility should be provided on-site for most radiopharmaceuticals, although the half life of 18F (110 minutes) means that radiopharmaceuticals with this label (e.g. 18FDG) can travel short distances by road.

How PET works

A PET scanner records the bio-distribution of the radiopharmaceutical in vivo and, using computer reconstruction techniques similar to CT and MR imaging, images can be produced to identify sites of tumour tissue.

As changes in function (e.g. increase in glucose utilisation) occur in diseased cells long before identifiable changes in structure, PET is inherently a very sensitive technique compared to anatomical scanning methods such as CT and MRI.

Development of PET

PET has moved from being a modality used almost exclusively in a research capacity to one that is now frequently used as a clinical tool. In particular, its use in the field of oncology is already well established and is rapidly expanding.

Complete surgical excision remains the best way of treating and curing many cancers, but many patients are found to be inoperable at surgery, or relapse soon after because undetected spread has already occurred. The problem of operations being performed on inoperable cancers has a significant cost to the patient as well as being an enormous but futile expense. It is in the area of reducing unnecessary surgery that PET scanning is making one of its significant contributions.

Planar gamma camera imaging

Planar gamma camera imaging allows us to take pictures of an organ or disease-specific radioactive tracer (radiopharmaceutical) within a patient’s body. The radiopharmaceutical can be administered in a variety of ways, including intravenous injection, ingestion and inhalation.

Once the radiopharmaceutical has been administered patients will need to wait until it has been given time to accumulate in the area of interest. The amount of time will vary depending on the test being performed, but can range from a few minutes up to several days.

Single photon emission computed tomography (SPECT)

SPECT is a nuclear medicine technique that produces 3D images showing the distribution of a radiopharmaceutical within a patient’s body. SPECT takes advantage of a wide range of radiopharmaceuticals that are selected based on their ability to localise within a specific organ or disease. Many of these radiopharmaceuticals can be produced in-house by trained radiopharmacy staff, whilst other more complex tracers have to be purchased from commercial suppliers.

Unlike PET, SPECT radiopharmaceuticals typically have long half-lives and they enable monitoring of tissue changes over time, further strengthening the ability to narrow down the characteristics of a specific disease process.

Also, the high specificity of SPECT radiopharmaceuticals means that more than one agent, each emitting a particular energy level, can be injected to track related processes simultaneously.

Evolution of SPECT

To take advantage of both these attributes, an array of new SPECT radiopharmaceuticals are currently being developed to carry radionuclide therapeutic agents to the disease site. The goal is to be able to pinpoint both the disease process and its ongoing response to targeted treatments.

While SPECT has been available for many years, it has been underused because the distribution of the radiopharmaceutical is often so target-specific that no normal tissue is visualised making the 3D images extremely hard to interpret.

The evolution of SPECT/CT has meant that this physiological information can now be localised anatomically, and nuclear medicine’s potential to diagnose and treat disease has advanced significantly.

PET/CT and SPECT/CT

PET/CT and SPECT/CT systems are hybrid scanners with the ability to perform the PET or SPECT and a CT scan consecutively on a patient without the patient moving between scans.

Accurately merged anatomical (CT) and functional (PET/SPECT) data aids interpretation, providing superior diagnostic information over PET/SPECT alone.

How PET/CT and SPECT/CT work

Both the PET/SPECT and the CT parts of the equipment are incorporated into the same gantry. Common practice is to perform the CT scan first, which takes less than two minutes. Usually this is a low radiation dose CT without using bowel and intravenous contrast. The PET/SPECT scan is then done, which takes 20 to 45 minutes.

This provides both a PET or SPECT scan and a low-dose anatomical CT scan that can be interpreted separately or as merged images. The CT data is also used to correct the PET/SPECT scan data for the effects of attenuation of the gamma photons within the patient’s body.

The CT data provides a fast, high-quality, virtually noiseless attenuation map to enable accurate attenuation correction of the PET/SPECT data. The CT scan results in an additional radiation dose to the patient but protocols have been optimised so that this is kept to a minimum by using only low-dose CT for anatomical fusion rather than a full diagnostic CT scan.

Radioisotope therapy (RIT)

RIT utilises radiopharmaceuticals to deliver therapeutic levels of radiation dose to disease sites. The choice of radiopharmaceutical is dependent on the tumour type.

The radiopharmaceutical comprises a radioactive element, for example Yttrium 90, that emits high-energy particles capable of destroying tissue, and a drug, for example Octreotate, that carries the radioactive element generally via the bloodstream to the site of treatment. The therapeutic radiopharmaceutical is manufactured either in The Royal Marsden Radiopharmacy Department or off-site in a commercial radiopharmacy.

For some therapies patients are treated as outpatients and can leave the hospital as soon as the therapy has been administered. However, other patients will need to be admitted to a dedicated, appropriately shielded unit (e.g. Smithers Ward) as they need to remain in hospital until the amount of radiation inside them has reduced to a required level.

The amount of retained radioactivity in these patients is monitored by the Radiation Physics staff until such time that the levels allow them to be safely discharged – usually two to three days, although this is dependent on the treatment.

Picture archiving and communications (PACS)

On 1 April 2006, we commissioned a picture archiving and communications system (PACS) for radiology, nuclear medicine and PET/CT images. This allows clinicians and other healthcare professionals to view and compare digital images on screen, although it does not provide sufficient quality for diagnostic review of nuclear medicine and PET/CT data.

PACS enables images to be instantaneously sent and viewed at both hospital sites, increasing the speed and efficiency of diagnosis and treatment. Digital archiving of images also improves storage and reduces the risk of images being lost or misplaced.

Radiology information system (RIS)

A radiology information system (RIS) was installed in 2011. This system allows Radiology and Nuclear Medicine to introduce a more efficient process for requesting, booking, scheduling, information recording and reporting images.