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Similarities and Differences
Both MRI and CT provide cross-sectional, digital images of body parts. A digital image is one that depicts visibly a portion of the anatomy based on numerical computerized representation of certain qualities of tissue.
In CT the numerical value of each volume of tissue (voxel) correlates with a physical property of that tissue, called 'x-ray attenuation', or 'tissue density'. The computer assigns a visual gray scale to these voxels. The greater the numerical value of a voxel, the more "brightness" it has. The array of these numbers is called a 'matrix'. The computer reproduces the matrix of voxels in a tissue section that has been "scanned", as a pictorial representation of that tissue section, in 'black and white". Shades of gray in the image therefore represent the x-ray attenuation of those different volume elements, in the section studied. They are rendered in their exact positions by computer analysis. The voxels are 3D, but are displayed as 2-D pixels (picture elements) in the scan image. More attenuation of a tissue (more density eg bone) in the scanned voxel results in a bright or whiter signal parametered pixel. Size of voxels and pixels can be pre-determined varying between 0.5 mm to 1.0 cm on CT and approaching this on MR imaging.
Both MR and CT machines produce cross-sectional images in a gray scale.
CT "sprays" x-rays through selected tissue sections one at a time, and the resultant diminution (attenuation) of the x-rays coming out of the section are received by very small detectors opposite the x-ray source. Both source and receivers are contained in a large ring structure. A moving table carrying the patient is advanced, section by section, through the ring. It stops briefly at each section to have the ring rotate and spray collecting x-ray information. Preset guidelines for thickness of slices and movements determine what image is acquired. Visualizing the density differences between brain gray and white matter, for example, revolutionized neurosurgical diagnosis. Internal brain structure previously had to be inferred from arteriography or air studies (pneumo-encephalography). Plain x-ray films, while very useful to an extent, mostly are insensitive to differences within each of the gross "tissue' types: (bone, soft tissue, water) fat, air. The computer 3-D construction allows the brain to be seen inside the skull without overlying bone obscuring the view.
Magnetic Resonance (MR) is similar to CT, in its image display, being cross-sectional and grey scaled. However, each represents different physical properties in its tissue voxel. In MR, the digital values represented in its tissue voxels were quite different from that of CT. In MR, the signal recorded represents the intensity of a radio frequency signal, recorded coming back from a tissue voxel after it was "stimulated". In MR terms, this process is called excitation and relaxation respectively. Tissue lying within this supercooled magnet tunnel, first acquires a polarity of its hydrogen protons aligning themselves on average with the axis of the MR magnetic field. The super magnet (usually at 1.5 Tesla power) causes tissue protons which previously had a random 'polarity' to on average align with the magnetic field of the MR tunnel. Special complex coils within the bore deliver brief radio frequency pulse to the tissue, at a 90' angle versus baseline, usually around 5-25 msec in duration; then, immediately switched off for varying amounts of time before recording the energy given back as the stimulated protons relax back to their baseline axis relative to the bore of the magnet. The energy released in this process is called an echo. The intensity of the echo will vary with time, a key difference from x-ray and CT. The MR operator can therefore sample returning signal immediately, or at varied times, on this decay curve. Time between excitations affects the echo signal, as does other selectable parameters of simulation and receiving signals. For example, varying the time between excitations, (ranging from 20500 ms) can reduce or enhance signal echo from a given tissue type. Overall, two stimulated/echo basic sequences yield the two most commonly used MR results. TL- and T2 weighted scan reflect time to relaxation of the excited tissue voles. In brain tissue, for example, TI yields an image in which fat and x-ray dye both look brighter than surrounding tissue. T2, however, yields an image based more on water proton excitation. In 12 MR of brain, fat is dark and CSF is white. Brain tissue yields different gray scale intensity between the two. Different types of tissue respond differently on Tl-based versus 12-based scans. Learning these differences is not a part of your anatomy training expectations.
Plain X-Ray Film
These are the original and simple x-ray pictures on special photographic film reflecting x-ray transmittance/density of body parts positioned over an x-ray film cassette. The result is a "photographic' image of tissue density differences in 2-D. An "intensifying screen' contained within the film cassette will amplify the light image received by the film to make visual evaluation easier.
Angiography
Angiography is an important source of vascular information. Rapid, serial plain films (or digital films) of the selected body part are obtained as the bolus of contrast material passes through the chosen body part. The x-ray dye is typically injected by a power injector with preset volume and time according to the purpose of the study. Usually, 3 vascular phases are obtained in an arteriogram: arterial, capillary and venous. Multiple projections or views may be needed. Little discomfort is experienced by the patient, especially when using the newer low osmolality contrast agents. Much information is derived from such studies. However, the potential for vascular complications and reaction to x-ray dye, can occasion some risks. This is the most invasive of diagnostic techniques. Like plain film x-rays, a small amount of ionization occurs in the tissue radiated.
Ultrasound Imaging
This modality employs mechanical vibrations occurring at frequencies too high to be heard by the human ear (normal ear hears frequencies up to 20,000 cycles/sec or 20 kilohertz). Ultrasound means frequencies higher than this (1-15 megahertz). It is produced and detected via transducers. These are hand-held devices with piezoelectric effects. This means that pressure generates an electric effect and conversely electricity applied to this material will cause a deformation of the surface of. the disk. A disk of PZE-material is vibrated electrically, resonating at its resonant frequency, depending on its size and properties. In addition, thinner disks yield higher frequencies. A beam of ultrasound is focused at a desired depth (often 10cm). This probe both stimulates and receives reflected ultrasound energy (echoes), by pulsing sound then receiving it back, creating a voltage as it again resonates the PZE disks (ranging from 1-2cm in diameter and about Imm thick). Ultrasound frequency is inversely proportional to the wavelength of the beam. Higher frequencies penetrate a shorter distance into tissue than lower ones, but create a sharper image. Conversely, longer wavelengths lose sharpness, but penetrate more deeply. Echoes are received when tissue interfaces are encountered by the ultrasound beam. These are compiled in a variety of ways to represent a zone of ultrasound echoes and are presented on a TV-QW screen and photographed. Alternatively, 'real time displays' can be generated if these electronic pictures are produced at 40 frames/sec.: real-time scanning only appears real-time to our eye.
Power output from the probe is measure in decibels. Soft tissue boundaries/interfaces within the body are detected by ultrasound. This can differentiate between solid and cystic masses, and demonstrate the outlines and contents of organ or lesions. If multiple echoes are generated within a structure, it is said to be 'echogenic'. If an area does not create echoes it is called "trans-sonic'. When well-defined borders are found for a mass, they are often benign. Malignancies often have poorly defined margins on ultrasound imaging.
Ultrasound- is ideal for detection of fluid leaving a characteristically blank area within the tissue. Also an area of fluid will show hyperintensity of its posterior wall. Adjacent solid tissue will appear of diminished intensity, due to attenuation of ultrasound by solid structures. There are applications for ultrasound imaging in all body areas. A special adaptation of ultrasound is Doppler technique. Doppler can detect and measure the flow of a fluid such as arterial or venous blood.
Nuclear Medicine: Radio-isotope Scanning = Scintigraphy
Most radionuclides are artifically made. All degenerate spontaneously, emitting radiations. Mostly used is gamma piston, generated by Tc99m, attached to a harmless compound, e.g. DTPA. I.V. injection of this combination leads to its deposition in various organs. Gamma protons emitted from a body part (e.g. brain) will occur and detectors in scanners held or moved over the part of interest will create a dot diagram of the protons, in a pattern that yields the structure or shape of the organ. Also, areas of greater or lesser intensity in an image can reflect tumor, stroke or mass, for example, in nuclear brain scanning; renal function in kidney scanning; and visualization of intrathoracic and intraabdominal organs. Bone scanning is very useful as metastatic or primary tumors; fractures and other pathology will show areas of greater or lesser activity, compared to normal adjacent bone. Metabolic function can be assessed by use of PET scanning (mostly used in brain) - makes use of positron annihilation of isotopes of oxygen or others intravenously; a PET scanner can reconstruct a 3 D metabolic map of the brain and areas of high metabolic activity (tumor) or lower activity (Schizophrenia) can be identified.
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