Six Ways to Get the Inside Picture a Quick Reference for Students of Radiology by Josh Stone

Projection radiography
you may call them radiographs or more formally Roentgenographs, because they're named after the discoverer of X-rays, Wilhelm Conrad Roentgen. These are often used for evaluation of bony structures and soft tissues. An X-Ray machine directs electromagnetic radiation upon a region in the body. The lower the density of the object, the more light passes through. Thus radiation tends to pass through skin, fat, muscle, and other tissues, but is absorbed by bones, tumors, and lungs affected by severe pneumonia. Radiation which has passed through a patient then exposes onto an X-ray film. Areas of film exposed to higher amounts of radiation will usually appear dark gray after development. The unexposed areas of film of course stay white.

Fluoroscopy
Fluoroscopy and angiography are special applications of X-ray imaging, in which a fluorescent screen or image intensifier tube is connected to a small television system, which allows real-time imaging of structures in motion. Radiocontrast agents are administered, which are often swallowed or injected into the body of the patient, that help delineate anatomy such as the blood vessels, the genitourinary system or the gastrointestinal tract. There is a radiocontrast agent for each specific type of evaluation. For example, barium in a suspension is administered into the gastrointestinal tract and the image is taken with fluoroscopy or radiography. Radiocontrast agents, which 'soak up' X-ray radiation, in conjunction with the real-time imaging allows demonstration of dynamic processes. Peristalsis in the digestive tract or blood flow in arteries and veins can easily be seen dynamically this way, for instance.

CT scanning
CT imaging uses X-rays in conjunction with computing algorithms to take an image of a variety of soft tissues in the body. CT is acquired in the axial plane, while coronal and sagittal images can be rendered by computer software reconstruction. It is of course only recently that the combined studies of computer imaging such as 3D ray-tracing and Computer Assisted Design have made this process possible. Yes, CT imaging owes a little debt to movies like "Tron"! Radiocontrast agents are often used with CT for enhanced delineation of the patient's anatomy. Intravenous contrast allows 3D reconstructions of arteries and veins, showing them as a network of branching tunnels in real-time space. While radiographs provide higher resolution for bone X-rays, CT can generate much more detailed images of the soft tissues. CT exposes the patient to more ionizing radiation than a radiograph, which is the main reason it isn't used any more oftan than it needs to be.

Ultrasound
Medical ultrasonography uses ultrasound, literal high-frequency sound waves, to visualize soft tissue structures in the body in real time. No ionizing radiation is involved, but the quality of the images obtained using ultrasound is highly dependent on the skill of the person performing the exam, who is known as the ultrasonographer. The use of ultrasound in medical imaging has developed mostly within the last thirty years. The first ultrasound images were static and two dimensional, but with modern-day ultrasonography 3D reconstructions can be observed in real-time. Because ultrasound does not utilize ionizing radiation like radiography, CT scans, and nuclear medicine imaging techniques, it is generally considered safer. For this reason, this imaging method plays a vital role in obstetrical imaging. Fetal development can be thoroughly evaluated, allowing early diagnosis of fetal anomalies or confirmation of a normal gestation.

MRI/NMR
MRI uses strong magnetic fields to align spinning hydrogen proton nuclei within body tissues, then uses a radio signal to disturb the axis of rotation of these nuclei. It then observes the radio frequency signal generated as the nuclei return to their baseline states. MRI scans give the highest quality soft tissue contrast of all the imaging modalities. With advances in scanning speed and spatial resolution and improvements in computer 3D algorithms and hardware, MRI has made greatleaps forward in the recent years. One distinct disadvantage is that the patient has to hold still for long periods of time in a noisy, cramped space while the imaging is performed. Recent improvements in magnet design like wider, shorter magnet bores and more open magnet designs, have brought some relief for claustrophobic patients, who previously had to be sedated - unfortunate if you are looking at the brain on the MRI, since the brain shows different activity when sedated. MRI has it's best benefit in imaging the brain, spine, and musculoskeletal system. The modality can be contraindicated for patients with pacemakers (watch out for those magnets!), certain types of cerebral aneurysmal clips or metallic hardware due to the strong magnetic fields. Areas of present advancement include functional imaging, cardiovascular MRI, as well as MR image guided therapy.

Nuclear medicine
Nuclear medicine imaging, our newest technology, involves the administration into the patient of substances labeled with radioactive tracers which have affinity for particular tissues. The heart, lungs, thyroid, liver, gallbladder, and bones are commonly evaluated for particular conditions using nuclear medicine techniques. While anatomical detail is limited in these kinds of images, nuclear medicine is useful in displaying physiological functions. For instance, processes such as the growth of a tumor can often be monitored, even when the tumor cannot be adequately visualized using any of the other methods. The principal imaging device is the gamma camera which detects the radiation emitted by the tracer in the body and displays it as an image on a computer monitor. Often the information is converted into a series of slices through the body like a loaf of bread. In the most modern devices, nuclear medicine images can be fused with a CT scan taken at the same time so that the physiological information can be super-imposed with the anatomical structures to improve diagnostic accuracy. PET scanning is another kind of nuclear medicine. The applications of nuclear medicine can include the scanning of bones, which traditionally has had a strong role in the staging of cancers. Molecular Imaging is the new and exciting frontier in this field.

They say that a picture is worth a thousand words, but the development of each of these technologies to show doctors what's happening inside the body in a non-invasive fashion has been worthwhile for saving many times a thousand lives!

About the Author
Freelance writer for over eleven years.
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