NUCLEAR MEDICINE IMAGING Prof Jasmina Vujic Department of Nuclear Engineering U C Berkeley Pictures from www.ge.com
Nuclear Medicine, X-Rays, CT, and MRI • Nuclear medicine began approximately 50 years ago and has evolved into a major medical specialty for both diagnosis and therapy of serious disease. More than 3,900 hospital-based nuclear medicine departments in the United States perform over 10 million nuclear medicine imaging and therapeutic procedures each year. Despite its integral role in patient care, nuclear medicine is still often confused with other imaging procedures, including general radiology, CT, and MRI. • Nuclear medicine studies document organ and function and structure, in contrast to conventional radiology, which creates images based upon anatomy. Many of the nuclear medicine studies can measure the degree of function present in an organ, often times eliminating the need for surgery. Moreover, nuclear medicine procedures often provide important information that allows the physician to detect and treat a disease early in its course when there may be more success. It is nuclear medicine that can best be used to study the function of a damaged heart or restriction of blood flow to parts of the brain. The liver, kidneys, thyroid gland, and many other organs are similarly imaged.
General Radiology • The image, or a x-ray film, is produced when a small amount of radiation passes through the body to expose sensitive film on the other side. The ability of x-rays to penetrate tissues and bones depends on the tissue's composition and mass. The difference between these two elements creates the images. • The chest x-ray is the most common radiological examination. Contrast agents, such as barium, can be swallowed to highlight the esophagus, stomach, and intestine and are used to help visualize an organ or film.
Computed Tomography • Computed tomography or CT, shows organs of interest at selected levels of the body. They are visual equivalent of bloodless slices of anatomy, with each scan being a single slice. CT examinations produce detailed organ studies by stacking individual image slices. CT can image the internal portion of organs and separate overlapping structures precisely. The scans are produced by having the source of the x-ray beam encircle or rotate around the patient. X-rays passing through the body are detected by an array of sensors. Information from the sensors is computer processed and then displayed as an image on a video screen.
Magnetic Resonance Imaging • Like CT, MRI produces images, which are the visual equivalent of a slice of anatomy. MRI, however, is also capable of producing those images in an infinite number of projections through the body. MRI use a large magnet that surrounds the patient, radio frequencies, and a computer to produce its images. • As the patient enters a MRI scanner, his body is surrounded by a magnetic field up to 8,000 times stronger than that of the earth. The scanner subjects nuclei of the body's atoms to a radio signal, temporarily knocking select ones out of alignment. • When the signal stops, the nuclei return to the aligned position, releasing their own faint radio frequencies from which the scanner and computer produce detailed images of the human anatomy. • Patients who cannot undergo a MRI examination include those people dependent upon cardiac pacemakers and those with metallic foreign bodies in the brain or around the eye.
What is Nuclear Imaging? The process involves injecting into the body a small amount of chemical substance tagged with a short lived radioactive tracer. Depending on the chemical substance used, the radiopharmaceutical concentrates in the part of the body being investigated and gives off gamma rays. A gamma camera then detects the source of the radiation to build a picture. These are called scans. Radioisotope Treatments or Therapy Radiotherapy using external beam treatment is used commonly for treatment of cancers (see Oncology). However the use of unsealed, liquid sources in the treatment of disease is important in a few, specialized situations. For example Iodine-131 is taken orally to treat overactive thyroid and cancer of the thyroid.
LungCancer WHAT CAN WE VISUALIZE ? Lymphoma Colon Cancer
Nuclear Imaging Scans • Brain Scans These investigate blood circulation and diseases of the brain such as infection, stroke or tumor. Technetium is injected into the blood so the image is that of blood patterns. • Thyroid Uptakes and Scans These are used to diagnose disorders of the thyroid gland. Iodine 131 is given orally , usually as sodium iodide solution. It is absorbed into the blood through the digestive system and collected in the thyroid. • Lung Scans These are used to detect blood clots in the lungs. Albumen, which is part of human plasma, can be coagulated, suspended in saline and tagged with technetium.
Nuclear Imaging Scans • Cardiac Scans These are used to study blood flow to the heart and can indicate conditions that could lead to a heart attack. Imaging of the heart can be synchronised with the patient's ECG allowing assessment of wall motion and cardiac function. • Bone Scans These are used to detect areas of bone growth, fractures, tumors, infection of the bone etc. A complex phosphate molecule is labeled with technetium. If cancer has produced secondary deposits in the bone, these show up as increased uptake or hot spots.
Liver Sagittal, Coronal and Transaxial Slices. 3D Rendering
Radioisotopes Used in Nuclear Medicine • For imaging Technetium is used extensively, as it has a short physical half life of 6 hours. However, as the body is continually eliminating products the biological half life may be shorter. Thus the amount of radioactive exposure is limited. • Technetium is a gamma emitter. This is important as the rays need to penetrate the body so the camera can detect them. • Because it has such a short half life, it cannot be stored for very long because it will have decayed. It is generated by a molybdenum source (parent host) which has a much greater half life and the Tc extracted on the day it is required. The molybdenum is obtained from a nuclear reactor and imported. For treatment of therapy, beta emitters are often used because they are absorbed locally.
HOW IS TECHETIUM USED FOR A HEART SCAN • The technetium heart scan is a nuclear heart scan, which means that it involves the use of a radioactive isotope that targets the heart and a radionuclide detector that traces the absorption of the radioactive isotope. The isotope is injected into a vein and absorbed by healthy tissue at a known rate during a certain time period. The radionuclide detector, in this case a gamma scintillation camera, picks up the gamma rays emitted by the isotope. • The technetium heart scan uses technetium Tc-99m stannous pyrophosphate (usually called technetium), a mildly radioactive isotope which binds to calcium. After a heart attack, tiny calcium deposits appear on diseased heart valves and damaged heart tissue. These deposits appear within 12 hours of the heart attack. They are generally seen two to three days after the heart attack and are usually gone within one to two weeks. In some patients, they can be seen for several months. • The technetium heart scan is not dangerous. The technetium is completely gone from the body within a few days of the test. The scan itself exposures the patient to about the same amount of radiation as a chest x ray. The patient can resume normal activities immediately after the test.
HOW IS TECHETIUM USED FOR A HEART SCAN • After the technetium is injected into a blood vessel in the arm, it accumulates in heart tissue that has been damaged, leaving "hot spots" that can be detected by the scintillation camera. The technetium heart scan provides better image quality than commonly used radioactive agents such as thallium because it has a shorter half life and can thus be given in larger doses. • During the test, the patient lies motionless on the test table. Electrocardiogram electrodes are placed on the patient's body for continuous monitoring during the test. The test table is rotated so that different views of the heart can be scanned. The camera, which looks like an x-ray machine and is suspended above the table, moves back and forth over the patient. It displays a series of images of technetium's movement through the heart and records them on a computer for later analysis.
HOW IS TECHETIUM USED FOR A HEART SCAN • The test is usually performed at least 12 hours after a suspected heart attack, but it can also be done during triage of a patient who goes to a hospital emergency room with chest pain but does not appear to have had a heart attack. Recent clinical studies demonstrate that technetium heart scans are very accurate in detecting heart attacks while the patient is experiencing chest pain. They are far more accurate than electrocardiogram findings. • The technetium heart scan is usually performed in a hospital's nuclear medicine department but it can be done at the patient's bedside during a heart attack if the equipment is available. The scan is done two to three hours after the technetium is injected. Scans are usually done with the patient in several positions, with each scan taking 10 minutes. The entire test takes about 30 minutes to an hour. The scan is usually repeated over several weeks to determine if any further damage has been done to the heart. The test is also called technetium 99m pyrophosphate scintigraphy, hot-spot myocardial imaging, infarct avid imaging, or myocardial infarction scan.
General-Purpose Circular Detector High-Performance Circular Detector
The Gamma CameraWhat is about ? The modern gamma camera consists of:- multihole collimator - large area (e.g 5 cm ) NaI(Tl) (Sodium Iodide - Thallium activated) scintillation crystal - light guide for optical coupling array (commonly hexagonal) of photo-multiplier tubes - lead shield to minimize background radiation
A crucial component of the modern gamma camera is the collimator. The collimator selects the direction of incident photons. For instance a parallel hole collimator selects photons incident OS the normal. Other types of collimators include pinhole collimator often used in the imaging of small superficial organs and structures (e.g thyroid,skeletal joints) as it provides image magnification. Fan beam (diverging) and cone beam (converging) collimators are often used for whole body or medium sized organ imaging. Such collimators are useful because they increase the detection efficiency because of the increased solid angle of photon acceptance. The action of a parallel hole collimator
Features and parameters The following are the typical features of the scintialltion crystal used in modern gamma cameras • most gamma cameras use thallium-activated NaI (NaI(Tl)) • NaI(Tl) emits blue-green light at about 415 nm • the spectral output of such a scintillation crystal matches well the response of standard bialkali photomultipliers (e.g SbK2Cs ) • the linear attenuation coefficient of NaI(Tl) at 150 KeV is about 2.2 1/cm . Therefore about 90% of all photons are absorbed within about 10 mm • NaI(Tl) is hyrdoscopic and therefore requires hermetic encapsulation • NaI(Tl) has a high refractive index ( ~ 1.85 ) and thus a light guide is used to couple the scintillation crystal to the photomultiplier tube • the scintillation crystal and associated electronics are surrounded by a lead shield to minimize the detection of unwanted radiation • digital and/or analog methods are used for image capture
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