Medical imaging is the technique and process used to create images of the human body (or parts and function thereof) for clinical
purposes (medical procedures seeking to reveal, diagnose
or examine disease)
or medical science (including the study of normal anatomy and physiology).
Although imaging of removed organs and tissues can be performed for medical reasons,
such procedures are not usually referred to as medical imaging, but rather are
a part of pathology.
As a discipline and in its widest sense, it is part of biological imaging and incorporates radiology (in the wider sense), nuclear
medicine, investigative radiological sciences, endoscopy,
medical photography and microscopy (e.g. for human pathological investigations).
Measurement and recording techniques which are not primarily
designed to produce images, such as electroencephalography (EEG),magnetoencephalography (MEG), electrocardiography (EKG) and others, but which produce
data susceptible to be represented as maps (i.e. containing positional
information), can be seen as forms of medical imaging.
noninvasive is a term based on the fact that following medical imaging
modalities do not penetrate the skin physically. But on the electromagnetic and
radiation level, they are quite invasive. From the high energy photons in X-Ray
Computed Tomography, to the 2+ Tesla coils of an MRI device, these modalities
alter the physical and chemical environment of the body in order to obtain data
common types of imaging
Two forms of radiographic images are
in use in medical imaging; projection radiography and fluoroscopy, with the
latter being useful for catheter guidance. These 2D techniques are still in
wide use despite the advance of 3D tomography due to the low cost, high
resolution, and depending on application, lower radiation dosages. This imaging
modality utilizes a wide beam of x rays for image acquisition and is the first
imaging technique available in modern medicine.
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, at a lower dose rate.
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. An image
receptor is required to convert the radiation into an image after it has passed
through the area of interest. Early on this was a fluorescing screen, which
gave way to an Image Amplifier (IA) which was a large vacuum tube that had the
receiving end coated with cesium iodide,
and a mirror at the opposite end. Eventually the mirror was replaced with a TV
resonance imaging instrument (MRI scanner), or "nuclear magnetic
resonance (NMR) imaging" scanner as it was originally known, uses powerful
magnets to polarise and excite hydrogen nuclei (single proton) in
water molecules in human tissue, producing a detectable signal which is
spatially encoded, resulting in images of the body. The MRI machine emits an RF
(radio frequency) pulse that specifically binds only to hydrogen. The system
sends the pulse to the area of the body to be examined. The pulse makes the protons
in that area absorb the energy needed to make them spin in a different
direction. This is the “resonance” part of MRI. The RF pulse makes them (only
the one or two extra unmatched protons per million) spin at a specific
frequency, in a specific direction. The particular frequency of resonance is
called the Larmour frequency and is calculated based on the particular tissue
being imaged and the strength of the main magnetic field. MRI uses three electromagnetic fields: a very strong (on
the order of units of teslas)
static magnetic field to polarize the hydrogen nuclei, called the static field;
a weaker time-varying (on the order of 1 kHz) field(s) for spatial
encoding, called the gradient field(s); and a weak radio-frequency (RF)
field for manipulation of the hydrogen nuclei to produce measurable signals,
collected through an RF antenna..
Tomography is the method of imaging a single
plane, or slice, of an object resulting in a tomogram.
several forms of tomography:
§ Linear tomography: This is 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 and 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.' This unit was still in use into the 1990s, as its resulting images
for small or difficult physiology, such as the inner ear, was still difficult
to image with CTs at that time. As the resolution of CTs got better, this
procedure was taken over by the CT.
Zonography: This is a variant of
linear tomography, where a limited arc of movement is used. It is still used in
some centres for visualising the kidney during an intravenous urogram (IVU).
§ Orthopantomography (OPT or OPG): 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 referred to as a "Panorex", but this is incorrect, as it is a
trademark of a specific company.
§ Computed Tomography (CT), or Computed Axial
Tomography (CAT: A CT scan, also known as a CAT 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 must be limited to avoid health effects. CT is
based on the same principles as X-Ray projections but in this case, the patient
is enclosed in a surrounding ring of detectors assigned with 500-1000
scintillation detectors (fourth-generation
scanner geometry). Previously in older generation scanners, the X-Ray beam was
paired by a translating source and detect
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