Basic & Detailed Tutorial on Nuclear Medicine & Imaging for Biomedical Beginners

Medical imaging is a mainstay in the field of nuclear medicine. In nuclear medicine, radioactive elements (as isotopes) that are part of specific fluids are introduced into the body (usually by injection into the blood). As it circulates, a particular radioisotope tends to distribute throughout the body at points served by the blood flow and may even concentrate preferentially in certain organs (for example, radioactive iodine in the thyroid gland). As the isotope decays, it gives off radiation (most commonly, gamma rays) which can be intercepted by a gamma camera or other detector. Variations in radiation intensity and in spatial location at point sources in the body activate film or more usually a detector array that responds by mapping the radiation intensity in X-Y space to create an image. The radioisotopes in normal usage have relatively short half lifes, thus decaying rapidly, and minimizing the exposure to damaging radiation.

One of the earlier techniques that uses radioisotopes is scintigraphy. Radioactive components, typically of such elements as iodine, techniceum, and thallium, are inserted into the body. After dispersion, as the isotope decays it emits gamma rays that are picked up by a gamma camera detector placed against the body in the area of interest. The buildup of scintillation light spots on the detector forms the image, that singles out location and intensity of the emitted rays over time. Here is a portable gamma camera capable of producing scintigrams:

By selecting the proper radioisotope and getting it into the body so as to selectively concentrate in the bones, skeletal anomalies are readily imaged, as shown in this pair of whole body views:

One common use is in looking for anomalies in the thyroid gland. In this view of a cat, the scintigram pinpoints abnormal conditions (red/yellow) in this cat’s thyroid gland in which injected radioactive Iodine (I126) has selectively concentrated:

Two high-powered imaging instruments in nuclear medicine which use the tomographic approach, and range in the same general size category and cost, are the SPECT (Single Photon Emission Computed Tomography) and the PET (Positron Emission Tomography) Scanners. These instruments are especially suited to monitoring dynamic processes such as blood flow and cell metabolism. We turn first to the SPECT instrument which preceded in general use the later PET technology.

Both instruments use a Gamma camera to detect gamma ray photons emitted from the radioisotopes used in imaging the body. A Gamma Camera is pictured below, and beneath it is a diagram that suggests its general operation:

The signal – gamma ray photons – passes into the instrument through collimators and then strikes detectors made from thallium-activated sodium iodide crystals. The light spots created by the gamma rays are picked up by photomultipliers, amplified, and sent through decoding circuits that establish the X and Y positions of each spot. The signal is then reconstructed as an image. A full PET unit is depicted here:

The injected radioisotopes have different half lifes, depending on species, but all are in the multiple hour range. Normally used is Tc99 (Technicium), other radioisotopes include I123, and Xe133; all are gamma emitters. Each decay produces a single gamma ray photon. SPECT is most commonly applied to brain scans to determine abnormalities but it works on other organs such as the heart and with special handling can image bone anomalies. The next group of images shows some results from SPECT scans of the normal brain; first is a high quality head slice:

The image gray tones can be assigned colors in different ways to bring out certain features; note the terminology applied to different view directions – Transaxial; Sagitall; Coronal (what applies to the above image?):

Here is a sequence of individual image slices (transaxial) through different levels of the brain.

Now to examples of brain disorders that are revealed by anomalous patterns. The first shows the effects a stroke in a transaxial view; the second of depression, as seen in a sagittal orientation (differences in pattern from the normal state shown above are subtle and need a neurologist’s expertise to interpret); the third displays patterns found in a patient with Chronic Fatigue Syndrome (CFS; closely related to Fibromyalgia, an illness that has beset the writer [NMS] since 1970):

The specialized diagram below, developed from a SPECT scan, indicates changes in alchoholic disease before and after treatment

This last pair, side by side, consists of colorized tomographic reconstructions of the brain using numerous slices. A normal brain appears on the left; the brain of a heroin addict in advanced deterioration is on the right. (This is a dramatic depiction of the brain’s deterioration that should be an effective warning in the “war against drugs”.)

Since SPECT and CT are both tomographic methods (as is PET and MRI), computer-controlled image processing can combine results from two methods, as is illustrated in this SPECT-CT 3-D representation showing a human’s rib cage, spine, heart, and left kidney:

Let’s now look at a more advanced instrument, the PET – these two Web sites offer details in the theory and practice of its use: Lawrence Berkeley Laboratory, and TRIUMF, the last a consortion of Canadian Universities engaged in radiation research. PET technology began to be applied in the 1950s but its more advanced capabilities did not “go on line” until the 1980s. Compared with SPECT, PET images have about a factor of four improvement in resolution; the instrument is notably more expensive (around $600000) and the cost of a PET scan typically is 3 times that of a SPECT scan.

Isotopes such a C11, N13, O15, and F18, part of liquid compounds such as glucose, are injected into the body and travel to various locations that include organs of interest. When these isotopes decay, they emit positrons that can then collide with electrons, producing gamma ray photons. In this nuclear reaction, two gamma rays result and are paired such as to move away from the nuclide in exactly opposite directions. Both gamma photons are sensed simultaneously by detectors (180°) apart. This double set of radiation photons improves detectability and resolution. This is a typical PET instrument:

What is interesting about this setup is the presence of a small cyclotron which bombards compounds containing the element(s) that will be used as tracers, thusly producing “fresh” radionuclides. These have half lifes that range from seconds to minutes so they must be inserted into the patient (who is in a ring with detectors in a chamber at the back side of the PET scanner) almost in real time as the tracer compound moves out of the cyclotron to the individual being diagnosed. PET scans are especially targeted to soft tissue examination, and are in use in neurology, cardiology, and tumor detection in various parts of the body. We start with three images that show a PET scan version of whole body imaging (compare with the scintigram above); in this case the progression of removal of malignant tissue by chemotherapy is being monitored.

In this pair of images PET reveals the progression of a cancerous area in the left breast of a patient.

This series of PET slices shows the distribution in the brain of anomalous conditions (right side) associated with epilepsy.

A PET scan can show patterns in the brain which aid the physician in diagnosing and treating Parkinson’s Disease:

This next group of images illustrates how sometimes an MRI will not clearly pinpoint an abnormality such as a lesion associated with Huntington’s disease which is displayed effectively by a PET scan

This last PET image pair highlights the results of an interesting research study by Dr. Marcus Raidle of Washington University (St. Louis). He took a PET scan of the brain of a volunteer that indicates on the top two areas where some rudimentary skill/knowledge functional activity has left its imprint. After that volunteer was trained over four months to modify this skill and develop new capabilities, the bottom PET image shows a shift to new areas where this ability has become stored in the brain.

We now leave tomography and nuclear medicine techniques to cover several more imaging methods using different approaches. One is just an application of thermal remote sensing – thermography, described on page 9-9 (the sensors in thermal cameras used in medical imaging operate respond to mid-infrared wavelengths between 2.8 and 5.5 µm); other imaging devices utilize wavelengths in the 8 – 12 µm range. It is also known as Medical Infrared Imaging. Thermograms of the body simply indicate variations in temperature, which can be diagnostic for certain illnesses and pathologic conditions in which there is local heat inflammation. Most thermograms are made of the body exterior, in which the temperatures are that of the skin region; interior variations such as from local infections or muscle strain generate higher temperatures that result in heat flow to the body surface by direct conduction and through vascular transport. Medical thermography is limited by generally low image resolution but as it becomes more increasingly used with refined technology, it now serves as an inexpensive first look tool to determine if anomalies are present that warrant further imaging by more sensitive methods. Thermography is most often used in mammography, as an early detection method to be followed by an x-ray mammogram if significant abnormality is revealed. Here is a thermogram of an advanced tumor in a female breast.

A thermogram can also depict abnormal heating indicating infection, shown here:

A condition known as diabetic peripheral neuropathy can be detected using thermography. In the image below, the right foot of the patient is noticeably cooler, suggesting reduced circulation related to nerve damage (the writer has this condition):

Here is a pair of thermograms showing the effectiveness of treatment for fibromyalgia (see earlier on this page):

And lastly, NASA’s JPL has developed a small portable thermal scanner operating in the 8-12 µm range used in space medical studies by astronauts but also suited as a diagnostic tool for detecting certain maladies that have strong heat signatures. In the example below is a 3-D image of a human brain in which two tumors have developed (shown in red)

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