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Medical imaging is the process by which physicians evaluate an area of the subject's body that is not externally visible (this may be termed clinical imaging), and could therefore involve any medical discipline, and scientists researching into new methods of creating these images. The thrust of this article is directed to medical imaging in the radiology department of a modern hospital. The medical specialist directing Medical Imaging is a Radiologist, and the medical aspects are known as Radiology. The technical aspects of Medical Imaging are known as Radiography and are carried out by the Radiographer or Radiologic Technologist.

Medical imaging may be clinically motivated, seeking to diagnose and examine disease in specific human patients (see pathology). Alternatively, clinical imaging or biological imaging may be used by researchers in order to understand processes in living organisms. Many of the techniques developed for medical imaging also have scientific and industrial applications.

Medical imaging often involves the solution of mathematical inverse problems. This means that cause (the properties of living tissue) is inferred from effect (the observed signal). In the case of ultrasonography the probe consists of ultrasonic pressure waves and echoes inside the tissue show the internal structure. In the case of projection radiography, the probe is X-ray radiation which is absorbed at different rates in different tissue types such as bone, muscle and fat.

Origins

In its most primitive form, imaging can refer to the physician simply feeling an area of the body in order to visualize the condition of internal organs. This was used historically to diagnose aortic aneurysms, fractures, enlarged internal organs, and many other conditions. It remains an important step today in making initial assessments of potential problems, although additional steps are often used to confirm a diagnosis. The primary drawbacks of this approach are that the interpretation may be quite subjective and that recording the 'image' is difficult.

Modern imaging technology

Projection Radiography

Radiographs, more commonly known as x-rays, are often used to determine the type and extent of a fracture as well as for detecting pathological changes in the lungs. With the use of radio-opaque contrast media, such as barium, they can also be used to visualize the structure of the stomach and intestines - this can help diagnose ulcers or certain types of colon cancer.

Fluoroscopy

Fluoroscopy produces real-time images of internal structures of the body in a similar fashion to Radiography, but employs a constant input of x rays. Contrast media, such as barium, iodine, and air are used to visualize internal organs as they work. Fluoroscopy is also used in image-guided procedures when constant feedback during a procedure is required.

Tomography

Tomography is the method of showing a single plane, or slice, of an object. There are several forms of Tomography:

Linear Tomography: This was the most basic form of Tomography. The X-ray tube moved from point "A" to point "B" above the patient, while the cassette holder (or "bucky") moves simultaneously under the patient from point "B" to point "A." The fulcrum, or pivot point, is set to the area of interest. In this manner, the points above and below the focal plane are blurred out, just as the background is blurred when panning a camera during exposure. No longer carried out, replaced by Computed Tomography.

Poly Tomography: This was a complex form of Tomography. With this technique, a number of geometrical movements were programmed, such as hypocycloidic, circular, figure 8, and elliptical. Philips Medical Systems* produced one such device called the 'Polytome.' No longer carried out, replaced by Computed Tomography.

Zonography: This is a variant of Linear Tomography, where a limited arc of movement is used. It is still used in some centres for visyalising the kidney during an Intravenous Urogram (IVU)

Orthopantomography (OPT): The only common tomographic examination in use. This makes use of a complex movement to allow the radiographic examination of the mandible, as if it were a flat bone. It is often refered to as an "OPG", but this is incorrect, as it is a trademark of a specific companies equipment

Computed Tomography (CAT or CT)

(Main article: Computed tomography): A CT scan, also known as a CAT scan (Computed Axial Tomography scan), is a Helical Tomography (latest generation), which traditionally produces a 2D image of the structures in a thin section of the body. It uses X-rays. It has a greater ionizing radiation dose burden than projection radiography, repeated scans should be limited.

Magnetic resonance imaging

An MRI uses powerful magnets to excite hydrogen nuclei in water molecules in human tissue, producing a detectable signal. Like a CT scan, an MRI traditionally creates a 2D image of a thin "slice" of the body. The difference between a CT image and an MRI image is in the details. X-rays must be blocked by some form of dense tissue to create an image, therefore the image quality when looking at soft tissues will be poor. An MRI can ONLY "see" hydrogen based objects, so bone, which is calcium based, will be a void in the image, and will not affect soft tissue views. This makes it excellent for peering into joints. As an MRI does not use ionizing radiation, it is the preferred imaging method for children and pregnant women.

Medical Imaging MRI, or "NMR" as it was originally known, has only been in use since the 1980's. Effects from long term, or repeated exposure, to the intense magnetic field is not well documented.

Ultrasound

Medical ultrasonography uses high frequency sound waves of between 2.0 to 10.0 megahertz that are reflected by tissue to varying degrees to produce a 2D image, traditionally on a TV monitor. This is often used to visualize the fetus in pregnant women. Other important uses include imaging the abdominal organs, heart, male genitalia and the veins of the leg. While it may provide less anatomical information than techniques such as CT or MRI, it has several advantages which make it ideal as a first line test in numerous situations, in particular that it studies the function of moving structures in real-time. It is also very safe to use, as the patient is not exposed to radiation and the ultrasound does not appear to cause any adverse effects, although information on this is not well documented. It is also relatively cheap and quick to perform. Ultrasound scanners can be taken to critically ill patients in intensive care units saving the danger of moving the patient to the radiology department. The real time moving image obtained can be used to guide drainage and biopsy procedures. Doppler capabilities on modern scanners allow the blood flow in arteries and veins to be assessed.

Other imaging techniques

Other medical imaging techniques (often termed modalities) include:

Clinical Imaging, or Biological Imaging Techniques

Electron Microscopy

The electron microscope is a microscope that can magnify very small details with high resolving power due to the use of electrons as the source of illumination, magnifying at levels up to 2,000,000 times.

Electron microscopy is employed in anatomic pathology to identify organelles within the cells. Its usefulness has been greatly reduced by immunhistochemistry but it is still irreplaceable for the diagnosis of kidney disease, identification of immotile cilia syndrome and many other tasks.

Creation of three-dimensional images

Recently, techniques have been developed to enable CT, MRI and Ultrasound scanning software to produce 3D images for the physician. Traditionally CT and MRI scans produced 2D static output on film. To produce 3D images, many scans are made, then combined by computers to produce a 3D model, which can then be manipulated by the physician. 3D ultrasounds are produced using a somewhat similar technique.

With the ability to visualize important structures in great detail, 3D visualization methods are a valuable resource for the diagnosis and surgical treatment of many pathologies. It was a key resource (and also the cause of failure) for the famous, but ultimately unsuccessful attempt by Singaporean surgeons to separate Iranian twins Ladan and Laleh Bijani in 2003. The 3D equipment was used previously for similar operations with great success.

Other proposed or developed techniques include:

Some of these techniques are still at a research stage and not yet used in clinical routines.

Non-diagnostic imaging

Neuroimaging has also been used in experimental circumstances to allow people (especially disabled persons) to control outside devices, acting as a brain computer interface.

Medical Imaging Service

This is a specialized area of medical equipment service and repair, which is separate from the Biomedical field. Although a hospital with their own service group may include them in the Biomed department.

At one time, there were only two ways to receive training for this field. One was to learn it in the military, and the other was On-The-Job training (OJT) from the manufacturer. But since the 1980's several independent training centers have been started. One such school is RSTI *.

There are different means of employment in this occupation. Working for the manufacturer's field service department (OEM), working for a hospital (In-house), and working for an independent (Outside, or Independent provider). The best positions are with the OEM or hospital, as you can remain current through on-going training, and the two have good working relationships.

An independent is typically someone who has left an OEM, and started their own service business. There's nothing wrong with being independent, but getting training on new equipment is difficult or expensive, and the OEM is usually reluctant to work with you. Often the OEM will give the hospital, or clinic, a cut in price on equipment purchases if they retain some form of OEM service, thereby making it more difficult for the independent.

The OEM Service Engineer can expect to spend a lot of time driving from one site to another during the work day, and working non-standard hours. They will install, remove, diagnose, repair, calibrate, perform preventive maintenance, and interface equipment all while ensuring good customer relations. You may also be required to do yearly testing of the radiation sources for compliance.

The In-house person will work in the hospital, or with larger medical care organizations, travel between the hospitals, to perform preventative maintenance, repairs, and calibration. You may also be required to do yearly testing of the radiation sources for compliance. The OEM or independent will provide installation of purchased equipment.

The Independent may install refurbished equipment, or remove equipment. They will repair, calibrate, and perform preventative maintenance. Because many of the tasks associated with imaging service require expensive, specialized equipment, there is a financial limit to what the independent can do. Typical equipment used routinely are a Storage Oscilloscope and multimeter (if servicing old vacuum tube equipment, a VOM would help). Additional equipment: Keithley Dosimeter, mAs meter, Biddle contact tachometer, Light to radiation template, etc.

See also

External links

Image processing Medical imaging Nuclear medicine Radiology

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This article is licensed under the GNU Free Documentation License. It uses material from the "Medical imaging".

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