Nuclear Medicine is a complex subject. The aim of this Entry is to cover its role in imaging and throughout, many imaging protocols are outlined. The protocols are examples and techniques may vary between departments, but the broad principles remain the same.
So what is nuclear medicine's role in imaging?
Medicine employs many forms of imaging, including ultrasound, Magnetic Resonance Imaging (MRI) and plain radiography (X-ray). All these are, however, focussed on determining the structure of things; the only functional information is obtained by observing changes in structure. By contrast, nuclear medicine delivers detailed functional information. There is, however, a trade-off for structural detail. Someone used to seeing plain X-rays would consider a nuclear medicine bone scan fuzzy and unclear, earning it the nickname 'unclear medicine'.
The process of imaging generally breaks down like this:
- A radioactive isotope (most often technetium 99m (99mTc)1is attached to a pharmaceutical.
- Which pharmaceutical is chosen is dependent on the organ or system being studied. This combination is called a radiopharmaceutical.
- The radiopharmaceutical is introduced into the patient, usually by intravenous (IV, 'within a vein') injection. The amount given is, in Britain, controlled by the Administration of Radioactive Substances Advisory Committee (ARSAC) and is measured in mega-Becquerels (MBq).2 These Dose Reference Levels (DRL) are what ARSAC considers gives the optimum balance of image quality and radiation dose. The actual dose may vary.
- Often a delay is necessary for the radiopharmaceutical to reach its target.
Images are taken using a gamma camera. This is a machine capable of detecting the gamma-rays given off by 99mTc and converting them into an image.
The bulk of the imaging performed by most nuclear medicine departments is bone imaging.
Bone imaging can be required for a variety of different reasons, which can include possible hairline fractures, Paget's disease, inflammation around false joints and, most commonly, the monitoring of bony cancer metastases (secondary growths from a primary cancerous tumour).
The process of a bone scan starts with an injection of 99mTc hydroxymethylene diphosphonate (HDP) (DRL: 600MBq). The body is constantly breaking down and replacing the bone. The pharmaceutical HDP is used by the body in this process. So the Tc attached to the HDP becomes attached to the bones. In a normal, healthy individual the distribution will be even with a slight increase around the joints. Problems, however, cause increased blood flow and/or increased rate of bone replacement. This, in turn, causes more tracer to be taken up at the site of a problem. Increased tracer uptake increases the gamma ray emission from that point. On the image, this appears as a brighter 'hot'3 spot.
In some situations, images may be taken immediately after injection. These images show the blood supply to the bones. This is done in known or suspected cases of infection or inflammation. Normal blood flow will be even.
Lung Imaging - Ventilation and Perfusion scan.
The most common reason to have a Lung Perfusion and Ventilation Scan is the possibility of pulmonary embolism (PE). This is a blood clot on the lung and is often related to the more famous deep vein thrombosis (DVT). This scan is often done as two parts, Ventilation and Perfusion. However, the perfusion images can provide some information on their own and can mean a ventilation study is not needed. The ventilation images can be acquired using many different radioactive isotopes including the nuclear medicine favourite, 99mTc but in the example below Krypton-81m (81mKr)4.
A typical ventilation/perfusion scan works like this: the patient is injected with 99mTc linked to macroaggregates (MAA)(DRL: 100MBq). The MAA lodges in pulmonary arterioles and so covers the perfused (supplied with blood) area of the lungs. The patient's lungs are then imaged from several different angles. If krypton is used then each position is imaged twice. Once with the krypton being breathed in and once without. The krypton, as a gas, travels through the same pathways as the normal air breathed and so enters the ventilated (supplied with air) areas of the lung.
A normal lung will look smooth on both ventilation and profusion images. PE will typically present as a defect in the perfusion image that is not present in the ventilation image. Anything other than these may still be PE but the probability is lowered. If this is the case, an alternative diagnosis may be sought by the consultant and further tests necessary.
A renogram is one of two major types of kidney scan. The renogram is a study of the throughput of the kidney, while the other, a renograph looks at the state of the functional tissue. In some places the names are interchangeable. This section looks into the 'throughput' renogram.
The renogram technique used as an example here is named after the pharmaceutical used. It is called a MAG3 Renogram.
This test may be requested by a doctor for many reasons. Some of the most common are Intra-Venous Urograph (IVU), an X-ray technique for showing the renal system, showing distorted tissue or possible obstructed/dilated urinary tract loin pain which may be kidney-related or to aid surgical planning if kidney removal is being considered.
Patients are asked to ensure they are well-hydrated but have urinated immediately prior to the test. The patient is positioned with their lower back to the gamma camera. Usually this is done with the patient lying down and the camera underneath then. 99mTc linked to mercaptocetyltriglycine (MAG3)is injected (DRL: 100MBq) and the imaging started as soon as tracer is visualised on the screen. At some point before or during the scan a diuretic may be given. This increases the rate at which the kidneys work. Images are acquired for long time, maybe 40 minutes or even longer.
The actual visual appearance of the images acquired is of little consequence. What is more important is the quantitative information obtainable. Firstly a divided function can be calculated. This is a measure of how much of the work each kidney is doing, in percentage form. A 'perfect' patient would have a 50-50 split. Any significant deviation from this can be indicative of poor function in one kidney. It should be noted though that a patient with two poorly functioning kidneys could still have a 50-50 split. Secondly time/activity graphs can be drawn. That is, a graph is created based on the amount of activity detected within a region of interest (ROI) drawn around the kidney as it progresses throughout the scan. A normal kidney will have a steady climb initially to a peak. The activity would then drop promptly to just above the background level by the end of the scan. A dilated drainage system would fall less promptly and usually not before diuretic administration. An obstructed or partially obstructed system would be indicated by no little drop in activity during the scan.
So as explained above there are two main types of kidney scan. This one looks at the state of the functional tissue. The renography technique outlined here is also named after its pharmaceutical. It is called a DMSA Renograph.
Some of the possible reasons for this scan are to aid surgical planning if a kidney removal is being considered, to distinguish between cyst/tumour and pseudo-tumour or to assess possible scarring following a Urinary Tract Infection (UTI). It may follow a Intra-IVU showing distorted tissue.
The patient is injected with 99mTc linked to dimercaptosuccinic acid (DMSA)(DRL: 80MBq). This pharmaceutical is taken up in the function tissue of the kidney. This uptake process takes some time to be effective and therefore, a three-hour delay is necessary.
Three images are acquired. The first is a posterior view, which is from the back. The next two are what are known as oblique views. They are taken with the gamma camera positioned at 45 degrees to the patient. These allow a more all-round view of the kidneys.
If a previous IVU shows distortion of the functional tissue then reduced uptake, indicated by 'cold' spots, could be indicative of a cyst or tumour. If there is no evidence tissue distortion then 'cold' spots indicate scarring. DMSA Renographs can also be used to produce a measure of divided function in a similar manner to MAG3 renograms (see above).
Multi-gated Cardiac Blood Poll Acquisition
This scan is, for obvious reasons, usually shortened to MUGA5. This is one type of heart scan. This procedure is usually invoked to present the doctor with a measure of Left Ventricular Ejection Fraction (LVEF) that is the amount of blood the left ventricle6 of the heart squeezes out with each beat.
The test begins with the injection of pyrophosphate (PYP) which attaches itself to the red blood cells. This takes approximately 15 minutes. After that the patient is laid on a bed and connected to a Electrocardiogram (ECG) monitor that reads the heart-rate.
The patient is then given a second injection, this time the radioactive tracer. The tracer for this test is, in fact, 99mTc-pyrophosphate. This compound is actually formed within the patient; the pyrophosphate (PYP) has already been administered and we need only add the 99mTc. The 99mTc is introduced in a saline (salt water) solution (DRL: 800MBq). The 99mTc combines with the PYP for the tracer. This means the image detected by the gamma camera will be the red blood cell density determined. This means we clearly see the heart as it is full of blood.
The heart monitor is connected to the camera. An average heart rate is determined. Any heartbeats falling outside and acceptable range (usually 20%) centred on that average would not be used for imaging. This is the 'gated' part of the title. Images are acquired for a large number of beats, in the order of 500. These images are then turned into a composite representing one beat.
The images are processed to measure the size of the left ventricle over one beat. The amount it contracts is calculated and displayed as a percentage. This is the Left Ventricular Ejection Fraction (LVEF). Normal LVEF is 55% or higher. These images also show if the ventricle is contacting evenly.
And many more...
The above protocols are only a taster of what nuclear medicine can do. One entry could not cover them all; to do so would take several textbooks. There are many techniques not covered here. Maybe some of you have been sent appointments for tests like these and think they sound scary - if so, hopefully this article has reduced some of your nervousness. On that note, it should be pointed out that if the department you visit follows different procedures, it doesn't make them wrong. There are lots of variations in technique that either make it easier for the patient without adversely affecting the information gained or enhance the information with little extra inconvenience or pain for the patient.
As the pharmaceuticals to target different cell types become available, we are able to image more and more. New cameras and computers allow us to do image more systems in more detail. Hopefully this Entry has given you something of an insight into the current technology.