Medical Imaging Flashcards
Magnetic Resonance Imaging (MRI)
Procedure: The body is exposed to a high-energy magnetic field, which causes protons (small positive particles within atoms, such as hydrogen) in body fluids and tissues to arrange themselves in relation to the field. Then a pulse of radio waves “reads” these ion patterns, and a color-coded image is assembled on a video monitor. The result is a two- or three-dimensional blueprint of cellular chemistry.
Comments: Relatively safe, but can’t be used on patients with metal in their bodies. Shows fine details for soft tissues but not for bones. Most useful for differentiating between normal and abnormal tissues. Used to detect tumors and artery-clogging fatty plaques, reveal brain abnormalities, measure blood flow, and detect a variety of musculoskeletal, liver, and kidney disorders.
Coronary (Cardiac) Computed Tomography Angiography (CCTA)
Procedure: Computer-assisted radiography in which an iodine-containing contrast medium is injected into a vein and a beta-blocker is given to decrease heart rate. Then, numerous x-ray beams trace an arc around the heart and a scanner detects the x-ray beams and transmits them to a computer, which transforms the information into a three-dimensional image of the coronary blood vessels on a monitor. The image procured is called a CCTA scan and can be generated in less than 20 seconds.
Comments: Used primarily to determine whether there are any coronary artery blockages (for example, atherosclerotic plaque or calcium) that may require an intervention such as angioplasty or stent. The CCTA can be rotated, enlarged, and moved at any angle. Since the procedure can take thousands of images of the heart within the time of a single heartbeat, it provides a great amount of detail about the heart’s structure and function.
Comments: Used primarily to determine whether there are any coronary artery blockages (for example, atherosclero
Computed Tomography (CT) [formerly called computerized axial tomography (CAT) scanning]
Procedure: Computer-assisted radiography in which an x-ray beam traces an arc at multiple angles around a section of the body. The resulting transverse section of the body, called a CT scan, is shown on a video monitor.
Comments: Visualizes soft tissues and organs with much more detail than conventional radiographs. Differing tissue densities show up as various shades of gray. Multiple scans can be assembled to build three-dimensional views of structures (described next). Whole-body CT scanning is also used. Typically, such scans actually target the torso. Whole-body CT scanning appears to provide the most benefit in screening for lung cancers, coronary artery disease, and kidney cancers.
Ultrasound Scanning
Procedure: High-frequency sound waves produced by a handheld wand reflect off body tissues and are detected by the same instrument. The image, which may be still or moving, is called a sonogram and is shown on a video monitor.
Comments: Safe, noninvasive, painless, and uses no dyes. Most commonly used to visualize the fetus during pregnancy. Also used to observe the size, location, and actions of organs and blood flow through blood vessels (doppler ultrasound).
Positron Emission Tomography (PET)
Procedure: A substance that emits positrons (positively charged particles) is injected into the body, where it is taken up by tissues. The collision of positrons with negatively charged electrons in body tissues produces gamma rays (similar to x-rays) that are detected by gamma cameras positioned around the subject. A computer receives signals from the gamma cameras and constructs a PET scan image, displayed in color on a video monitor. The PET scan shows where the injected substance is being used in the body. In the PET scan image shown here, the black and blue colors indicate minimal activity; the red, orange, yellow, and white colors indicate areas of increasingly greater activity.
Comments: Used to study the physiology of body structures, such as metabolism in the brain or heart.
Radionuclide Scanning
Procedure: A radionuclide (radioactive substance) is introduced intravenously into the body and carried by the blood to the tissue to be imaged. Gamma rays emitted by the radionuclide are detected by a gamma camera outside the subject and fed into a computer. The computer constructs a radionuclide can and displays it in color on a video monitor. Areas of intense color take up a lot of the radionuclide and represent high tissue activity; areas of less intense color take up smaller amounts of the radionuclide and represent low tissue activity. Single-photon-emission computed tomography (SPECT) is a specialized type of radionuclide scanning that is especially useful for studying the brain, heart, lungs, and liver.
Comments: Used to study activity of a tissue or organ, such as searching for malignant tumors in body tissues or scars that may interfere with heart muscle function.
Endoscopy
Procedure: The visual examination of the inside of body organs or cavities using a lighted instrument with lenses called an endoscope. The image is viewed through an eyepiece on the endoscope or projected onto a monitor.
Comments: Examples of endoscopy include colonoscopy, laparoscopy, and arthroscopy. Colonoscopy is used to examine the interior of the colon, which is part of the large intestine. Laparoscopy is used to examine the organs within the abdominopelvic cavity. Arthroscopy is used to examine the interior of a joint, usually the knee.
Radiography
Procedure: A single barrage of x-rays passes through the body, producing an image of interior structures on x-ray-sensitive film. The resulting two-dimensional image is a radiograph commonly called an x-ray.
Comments: Radiographs are relatively inexpensive, quick, and simple to perform, and usually provide sufficient information for diagnosis. X-rays do not easily pass through dense structures so bones appear white. Hollow structures, such as the lungs, appear black. Structures of intermediate density, such as skin, fat, and muscle, appear as varying shades of gray. At low doses, x-rays are useful for examining soft tissues such as the breast (mammography) and for determining bone density (bone densitometry).
It is necessary to use a substance called a contrast medium to make hollow or fluid-filled structures visible in radiographs. X-rays make structures that contain contrast media appear white. The medium may be introduced by injection, orally, or rectally, depending on the structure to be imaged.
Contrast x-rays are used to image blood vessels (angiography), the urinary system (intravenous urography), and the gastrointestinal tract (barium contrast x-ray).
