Dense structures, like bone or metal hip implants, appear white in X-rays.
Radiography, or the medical use of electromagnetic energy, began in 1895 with the discovery of the Roentgen ray, or X-ray. Researchers found that these rays could pass through soft tissues and leave an image of the denser structures on a photographic plate. Since then, this technology has developed into several kinds of diagnostic tools, differentiated by the way the image is detected, viewed and captured.
Projection Radiography
Of all the modern types of radiography, projection radiography most closely matches the original practice of using X-rays. In projection radiography, X-rays pass through the subject and are captured on the other side by either photographic film or a sensor hooked up to a screen. As the X-rays pass through the subject, they either scatter or become absorbed. Less substantial tissues allow more X-rays to pass through, turning the film black. Dense materials, like bone, absorb the electromagnetic energy, so the film shows varying shades of white or gray. When computers and monitor screens are used, electronic sensors detect the amount of energy passing through the tissues and convert that into images in black, white and shades of gray.
Radiography With Contrast
Procedures like imaging the digestive or urinary tracts, performing balloon angioplasty or placing stents use contrast material that block X-rays. For example, the urinary bladder and kidneys only show up well in X-rays when they hold a liquid that blocks X-rays. For circulatory and urinary system procedures, the contrast liquid generally contains iodine. For digestive system work, contrast materials contain barium sulfate mixed with other ingredients, which can be given orally or rectally. The contrast agents outline whatever structure contains them.
Fluoroscopy
In fluoroscopy, the patient stands between an X-ray source and a phosphor-coated screen. Contrast agents may or may not be used. The fluoroscopic image forms when the X-rays passing through the subject activate the phosphor. Fluoroscopy screens have an electronic brightening feature that uses technology from military night-vision equipment. This makes it possible to view the screen without darkening the room. Doctors often use fluoroscopy to look at the diaphragm, heart and chest masses.
Dual Energy X-Ray Absorptiometry
Dual energy X-ray absorptiometry, also called DEXA or DXA, detects bone mineral density and helps diagnose osteoporosis. Bone density is evaluated by passing X-rays through tissue at two different energy levels. One energy level is absorbed mostly by soft tissue, and the other level is absorbed mostly by dense tissue. Computer software compares results from the soft tissue energy and the dense tissue energy to calculate bone density. Doctors use two types of DXA machines--large ones for scanning the torso and smaller portable ones for scanning appendages.
Computed Tomography
Computed tomography, better known as CT scanning, employs a specialized scanning machine that moves the patient through X-rays at a steady speed. Computerized interfaces detect X-rays passing through the patient's body at several different points. This gives a sequence of images that look like multiple slices through the imaged area of the patient's body. A radiographer can combine these pictures to make 3-D images of scanned structures. Spiral CTs record images in a spiral pathway, giving a continuous band rather than slices. CT scans can also take place with or without contrast agents.
Tags: pass through, passing through, through subject, X-rays pass, X-rays pass through, absorbed mostly
