Radiation therapy for cancer and PET scans fall in the realm of nuclear medicine. Nuclear medicine uses radioactive substances to image the body and treat disease. Nuclear medicine looks at both the physiology (functioning) and the anatomy of the body in establishing diagnosis and treatment.
The techniques combine the use of computers, detectors, and radioactive substances. Techniques include Positron Emission Tomography (PET), Single Photon Emission Computed Tomography (SPECT), cardiovascular imaging, and bone scanning. These techniques can detect tumors, aneurysms (weak spots in blood vessel walls), bad blood flow to various tissues, blood cell disorders, dysfunctional organs, and other diseases and ailments.
Positron Emission Tomography (PET)
PET produces images of the body by detecting the radiation emitted from radioactive substances. These substances are injected into the body, and are usually tagged with a radioactive atom, such as Carbon-11, Fluorine-18, Oxygen-15, or Nitrogen-13, that has a short decay time.
These radioactive atoms are formed by bombarding normal chemicals with neutrons to create short-lived radioactive isotopes. PET detects the gamma rays given off at the site where a positron emitted from the radioactive substance collides with an electron in the tissue.
In a PET scan, the patient is injected with a radioactive substance and placed on a flat table that moves in increments through a donut-shaped housing, similar to a CAT scan. This housing contains the circular gamma ray detector array, which has a series of scintillation crystals, each connected to a photomultiplier tube. The crystals convert the gamma rays, emitted from the patient, to photons of light, and the photomultiplier tubes convert and amplify the photons to electrical signals. These electrical signals are then processed by the computer to generate images.
Again, like CAT scans, the table moves and the process is repeated, resulting in a series of thin slice images of the body. The images are assembled into a 3-D model. PET provides images of blood flow or other biochemical functions, depending upon the type of molecule that is radioactively tagged. PET scans can show images of glucose metabolism in the brain or rapid changes in activity in various areas of the body. There are few PET centers because they must be located near a particle accelerator device that produces the short-lived radioisotopes used in the technique.
Single Photon Emission Computed Tomography (SPECT)
SPECT is similar to PET, but the radioactive substances used in SPECT (Xenon-133, Technetium-99, Iodine-123) have longer decay times, and emit single instead of double gamma rays. SPECT can provide information about blood flow and the distribution of radioactive substances in the body. The images are less sensitive and detailed than PET images. However, SPECT is cheaper and do not have to be located near a particle accelerator.
Cardiovascular imaging techniques use radioactive substances to chart the flow of blood through the heart and blood vessels. One example of a cardiovascular imaging technique is a stress thallium test, in which the patient is injected with a radioactive thallium compound, exercised on a treadmill, and imaged with a gamma ray camera. After a period of rest, the study is repeated without the exercise. The images before and after exercising are compared to reveal changes in blood flow and are useful in detecting blocked arteries and other anomalies.
Bone scanning detects radiation from a radioactive substance (technetium-pp methyldiphosphate) that when injected into the body, collects in bone tissue. Bone tissue is good at accumulating phosphorus compounds. The substance accumulates in areas of high metabolic activity, and so the image shows “bright spots” of high activity and “dark spots” of low activity. Bone scanning is useful for detecting tumors, which generally have high metabolic activity.
Magnetic Resonance Imaging (MRI)
In 1977, the first MRI exam ever performed on a human being took place. It took almost five hours to produce one image, and the image quality was poor. The machine that performed the exam is now in the Smithsonian Museum. By the early 80s, there were a handful of MRI scanners. Now, in the new millennium, there are 1000s, with images produced in seconds, not hours.
The basic design used in most is a giant cube. The cube in a typical system might be 7 feet tall by 7 feet wide by 10 feet long. Newer models are getting smaller. There is a horizontal tube running through the magnet from front to back. This tube is known as the bore of the magnet. The patient slides into the bore on a special table. Once the body or body part to be scanned is in the exact center or isocenter of the magnetic field, the scan begins.
In conjunction with radio wave pulses of energy, the MRI scanner can pick out a very small point inside the patient’s body and determine tissue type. The MRI system goes through the patient’s body point by point, building up a 2-D or 3-D map of tissue types. It then integrates all of this information together to create 2-D images or 3-D models.
MRI provides an unparalleled view inside the human body. The level of detail is extraordinary compared with any other imaging technique. MRI is the method of choice for the diagnosis of many types of injuries and conditions because of the incredible ability to tailor the exam to the particular medical question being asked. MRI systems can also image flowing blood in any part of the body.
The MRI machine applies an RF (radio frequency) pulse that is specific only to hydrogen. The system directs the pulse toward the area of the body being examined. The pulse causes the protons in that area to absorb the energy required, making them spin in a different direction. This is the “resonance” part of MRI.
The RF pulse forces the protons to spin at a particular frequency, in a particular direction. When the RF pulse is turned off, the hydrogen protons begin to slowly (relatively speaking) return to their natural alignment within the magnetic field and release their excess stored energy. When they do this, they give off a signal that the coil now picks up and sends to the computer system. What the system receives is mathematical data that is converted into a picture that can be put on film. This is the “imaging” part of MRI.
Most imaging techniques use injectable contrast, or dyes, for certain procedures. So does MRI. These agents work by blocking the X-ray photons from passing through the area where they are located and reaching the X-ray film. This results in differing levels of density on the X-ray film. The dyes have no physiologic impact on the tissue in the body.
MRI contrast works by altering the local magnetic field in the tissue being examined. Normal and abnormal tissue will respond differently to this slight alteration, giving differing signals. These varied signals are converted into images, allowing the visualization of many different types of tissue abnormalities and disease processes.
Before MRI and other imaging techniques, the only way to see inside the body was to cut it open. MRI is used for a variety of diagnoses, such as multiple sclerosis, tumors, infections in the brain, spine or joints, seeing torn ligaments, tendonitis, cysts, herniated discs, strokes, and much more. MRI systems do not use ionizing radiation or contrast materials that produce side effects.
MRIs can image in any plane. They have a very low incidence of side effects. Another major advantage of MRI is its ability to image in any plane or cross-section. The patient doesn’t have to move as is required in x-ray analysis. The magnets used in the MRI system control exactly where in the body images are to be taken.
Some people are too big to fit into an MRI scanner. Pacemakers prevent MRI analysis as well. MRI machines make a lot of noise and can be claustrophobic. Patients don’t have to move, but they do have to lie very still for long periods of time. The slightest movement can cause distorted images. Artificial joints and other metallic devices in the body can cause distorted images. The machines are very expensive and so are the exams.
Very small scanners for imaging specific body parts are being developed. Another development is functional brain mapping–scanning a person’s brain while performing a physical task. New research will image the ventilation dynamics of the lungs and produce new ways to image strokes.
Computerized Axial Tomography (CAT Scans)
Computerized axial tomography (CAT) scan machines produce X-rays. X-ray photons are basically the same thing as visible light photons, but have much more energy. This higher energy level allows X-ray beams to pass straight through most of the soft material in the human body.
A conventional X-ray image is basically a shadow where light is shined on one side of the body and film on the other side captures the silhouette of bones. Shadows provide an incomplete picture of an object’s shape. If a larger bone is directly between the X-ray machine and a smaller bone, the larger bone may cover the smaller bone on the film. In order to see the smaller bone, the body has to turn.
In a CAT scan machine, the X-ray beam moves all around the patient, scanning from hundreds of different angles. The computer takes all this information and puts together a 3-D image of the body. A CAT machine looks like a giant donut tipped on its side. The patient lies down on a platform, which slowly moves through the hole in the machine.
The X-ray tube is mounted on a movable ring around the edges of the hole. The ring supports an array of X-ray detectors directly opposite the X-ray tube. A motor turns the ring so that the X-ray tube and the X-ray detectors revolve around the body. Another kind of design is where the tube remains stationary and the X-ray beam is bounced off a revolving reflector.
Each full revolution scans a narrow, horizontal “slice” of the body. The control system moves the platform farther into the hole so the tube and detectors can scan the next slice. The machine records X-ray slices across the body in a spiral motion. The computer varies the intensity of the X-rays in order to scan each type of tissue with the optimum power.
After the patient passes through the machine, the computer combines all the information from each scan to form a detailed image of the body. Usually only part of the body is scanned. Doctors usually operate CAT scan machines from a separate room so they aren’t repeatedly exposed to radiation. Since they examine the body slice by slice, from all angles, CAT scans are much more comprehensive than conventional X-rays. CAT scans are used to diagnose and treat a wide variety of ailments, including head trauma, cancer and osteoporosis.