veterinary_radiation_safety.ppt
TRANSCRIPT
Veterinary Radiation Safety
X-Ray (Diagnostic)
Nuclear Medicine
Other
Changes in Diagnosis & Treatment Options
Things to look for
• Radiation safety of veterinary personnel during – handling of animals– Dealing with excretia and bodily fluids
• Types of radiation generating machines and radionuclides utilized
• Calibration equipment
• Detection equipment
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.
Explanation - X-ray Production
• Accelerated electrons bombard the anode
• X-rays emerge with a scattering angle profile
• Beam collimation is inserted to reduce angle of divergence
X-Ray Beam Spectrum - 100 kVp
A. Hypothetical total Bremsstrahlung beam
B. - Spectrum from tungsten target without filtration
C. Spectrum with filtration equivalent to 2.4 mm Al (inherent + added)
Explanation of X-ray Terms• mAs (milli-amp second)
– governs the quantity (e.g. intensity) of X-rays produced. – directly proportional to patient dose. Double mAs, double dose.
• kVp (kilovolt peak)– governs “quality” of the X-ray beam – Relates to energy of the beam– influences image quality. – effects image contrast (ability to distinguish regions). – higher kVp radiographs show greater density and longer scale of
contrast. • For radiographs, setting the kVp as high as possible, without a loss
of contrast, will give the lowest patient dose because a greater fraction will penetrate through the body to the imaging medium.
Nuclear Medicine
• Also referred to as scintigraphy, is a sensitive diagnostic procedure.
• It often can detect abnormalities before they become apparent on other imaging studies.
• To perform a nuclear medicine procedure, a small quantity of a radioactive tracer is administered to the animal.
Nuclear Medicine
• The most common radioisotope used is Technetium-99m (99mTc)
• Technetium-99m has a short half-life (6 hours) and 94% of it will decay within 24 hours.
• A gamma camera is used to record the distribution of the radiotracer within the body.
• The radiotracer can be attached to a variety of biologically active chemicals to localize in certain areas of the body. Above is an example of a bone scan in a normal dog. The study was performed by injecting 99mTc-MDP. The 99mTc-MDP will localize in bone proportional to the metabolic activity of the bone.
Scintagraphy
• One of the most common uses of bone scintigraphy is to detect bone metastasis
• Below is an example of a dog with multiple sites of bone metastases seen as multiple areas of high intensity uptake.
Feline hyperthyroidism
• Recently been recognized as the most common endocrine disorder of the cat.
• Elevated circulating levels of the thyroid hormones thyroxine (T4) and triiodothyronine (T3) that occur in hyperthyroidism result in a multisystemic disease.
Feline hyperthyroidism
• Radiation safety precautions require that cats remain hospitalized following their 131I therapy until they have eliminated a majority of the radioactive iodine from their bodies.
• This typically requires a hospitalization of 3 to 7 days. Radioactive iodine therapy is considered the optimum treatment for feline hyperthyroidism.
• Involves a single nonstressful procedure that is without associated morbidity or mortality.
Feline hyperthyroidism
• Significant side effects have not been observed. Unlike surgery, anesthesia is not necessary.
• A single dose of radioactive iodine will result in a return to persistent normal thyroid function in a majority (>95%) of cats with hyperthyroidism.
• Since the cats's thyroid function returns to normal following 131I therapy, no ongoing thyroid medications are needed following this form of treatment.
CT
• A computed tomography (CT) scan uses X-rays to produce detailed pictures of structures inside the body. A CT scan is also called a computerized axial tomography (CAT) scan.
• A CT scanner directs a series of X-ray pulses through the body. Each X-ray pulse lasts only a fraction of a second and represents a “slice” of the organ or area being studied. The slices or pictures are recorded on a computer and can be saved for further study or printed out as photographs
Interventional Radiography
• Patient EDE– typical fluoroscopy doses: tens to thousands of
millirem– typical interventional fatal cancer risk ~ 0.001
• Dose to Radiologist– tens of millirad to head or extremities per procedure
The Interventional Fluoroscopic Suite
• C-arm fluoroscopic unit
• Arrows point to the X-ray tube beneath the table and the Image Intensifier above the table
Interventional Fluoroscopic Suite
• Note the low level of the X-ray tube beneath the table and the close proximity of the Image Intensifier above the table.
• Monitors as seen by the clinical team are in the background.
Interventional Fluoroscopic Suite
• A 23 cm. Phantom is positioned beneath the Image Intensifier.
• An ion chamber is located at the base of the phantom to measure Entrance Skin Dose
PHANTOM
RADIATION ONCOLOGY
RADIATION ONCOLOGY- Bracytherapy
Implant of nasal tumor, lateral view.
Iridium-192 treatment of canine nasal tumor, lateral
view.
.
Implant of nasal tumor, dorsoventral view.Iridium-192 treatment of
canine nasal tumor, dorsoventral view.
Large Animal Operations
• The equine and food animal sectors of the veterinary profession rarely have the luxury of transporting the patient to the x-ray machine, so they must take the machine to the patient.
• Fortunately, technology has made the modern veterinary portable x-ray machine safer than it ever has been, but it is far from risk-free.
• In some respects, portable x-ray machines may be more hazardous than the fixed versions, even though they are usually less powerful.
Large Animal Diagnostic Room
Large Animal Diagnostic Equipment
Equine Bone Scans
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.
• Doses are not low!
Typical dynamic image of a heart
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.
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 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.
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
Detail of the pin-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
Things to Look for
• Radiation safety of veterinary personnel during – handling of animals– Dealing with excretia and bodily fluids
• Types of radiation generating machines and radionuclides utilized
• Calibration equipment• Detection equipment• Dosimetry• Other?