Diagnostic Imaging Methods
Post on 01-Feb-2016
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DESCRIPTIONintroduces you to the requisite skills and knowledge to enable you to be aware of the role of diagnostic imaging. You will also begin to develop a capacity to interpret radiographic imaging to support clinical diagnostic skills and identify different modalities of Radiographic imaging techniques.
DIAGNOSTIC IMAGING METHODS
Produced the 1st X ray film image of his wifes hands
Form of radiant energy similar to visible light
Has very short wavelength
Penetrates many substances that are opaque to light
Produced by bombarding a tungsten target with an electron beam with an x-ray tube.
Utilize a screen film system within a film cassette ( x-ray detector)
COMPUTED RADIOGRAPHY (CR)
Produces A DIGITAL RADIOGRAPHIC IMAGES
Substitutes a fixed electronic detector on charge-coupled device for the film screen cassette or
phosphor imaging plate.
Provides radiographic images of slices of the patient
Done by simultaneously moving both x-ray tube and x-ray detector around a pivot point center
to the patient
Real time radiographic visualization of moving anatomic structures
Useful in evaluating motion such as gastrointestinal peristalsis, movement of diaphragm during
respiration and cardiac action
PRINCIPLES OF INTERPRETATION
5 basic radio densities
Soft tissue density
Metal/contrast agent density
Air attenuates very little of the x ray beam-most are transmitted-> black on radiograph
Bone, metal, contrast agent attenuates a large proportion of x ray beam-> white on radiograph.
Fat and soft tissue intermediate amt. of x ray beam-> shades of gray on radiograph
Anatomic structures are seen when outlined by tissues.
Contrast agents- suspensions of iodine or barium
Disease states may obscure normally visualize structures by silhouetting their outline.
CROSS SECTIONAL IMAGING TECHNIQUES
CT, MR, AND ULTRASOUND
Produced 2 dimensional image
Resulting image is made up of pixels which represent a voxel of patient tissue
Displays each splice separately
No superimposed blurred structures seen.
Hounsfield unit (H) scale
o Lung tissue :-400 to -600 H
o Fat:- 60 to -100 H
o Water: value of 0 H
o Soft tissue:+40 - +80 H
o Bone:+400 to + 1000 H
Most CT units allow slice thickness between 0.5 to .10mm
Advantages of CT compared to MRI:
o Rapid scan acquisition
o Superior bone detail and demonstration of calcification.
TYPES OF CT SCAN:
Conventional CT (non helical)
- Obtain image data one slice at a time- one slice per breath hold.
- Requires at least 2-3x the total scanning time of helical CT.
Helical CT (spiral)
- Improved speed of image acquisition.
- Improved visualization of small lesions.
- Scans entire abdomen, pelvis in 2-3 breaths.
Multidetector helical CT
- Latest technical advance in CT imaging
- Obtains multiple slices per tube rotation
- 5-8x faster than single slice helical CT.
- DA: radiation dose, 3-5x higher than single slice CT.
Contrast administration in CT
- Intervenous iodine based contrast agents are administered in CT to:
o Enhance density differences between lesions surrounding the parenchyma.
o To demonstrate vascular anatomy and vessel patency.
PRINCIPLES OF CT INTERPRETATION
Images are oriented so that the observer is looking at the patient below.
Patients right side is oriented on the left side of the image.
Optimal bone detail is viewed at bone windows
Lungs are viewed at lung windows
Window width of 2,000 H, window level of 400-600H
Window width of 400 -500 H, window level of 20-40H.
MAGNETIC RESONANCE IMAGING (MRI)
Produces images by means of magnetic fields and radio waves
Analyzes multiple tissue characteristics :
T1 and T2 relaxation times of tissue
Blood flow within tissue
Provides the best tissue contrast.
Most tissues can be differentiated by differences in their T1 and T2 relaxation times.
Blood flow has a complex effect on MR signal that may decrease or increase signal intensity
within blood vessel.
Because the MR signal is very weak, prolonged imaging time is often required for optimal
ADVANTAGES OF MRI DISADVANTAGES OF MRI
Outstanding soft tissue contrast resolution Limited in its ability to demonstrate dense bone detail or calcifications
Provides images in any anatomic plate Involves long imaging of many pulse sequences
Absence of ionizing radiation Possesses limited spatial resolution compared with CT
CONTRAST ADMINISTRATION IN MRI:
- A heavy metal ion with paramagnetic effect that shortens T1 and T2 relaxation times of
hydrogen nuclei within its local magnetic field.
- Essential in providing high quality MR angiographic studies by enhancing the signal
differences between blood vessel and surrounding tissues.
- If very high tissue concentrations, such as the renal collecting system, T2 shortening
causes a significantly loss of signal intensity best seen on T2WIs.
MR is contraindicated in patients who have electrically, magnetically or mechanically activated
Cardiac pacemakers, insulin pumps, cochlear implants, neurostimulators, bone-
growth stimulators, and implantable drug infusion pumps.
Intracardiac pacing wires or Swan-Ganz catheters
Ferromagnetic implants such as cerebral aneurysm clips, vascular clips, and skin
Bullets, shrapnel, and metallic fragments may move and cause additional injury
or become projectiles in the magnetic field.
Metal workers and patients with heinous story of penetrating eye injuries
should be screened with radiographs of the orbit to detect intraocular metallic
foreign bodies that may dislodge, tear the retina and cause blindness.
SAFE FOR MRI
No ferromagnetic vascular clips and staples and orthopedic devices.
Prosthetic heart valves with metal components and stainless steel greenfield
Pregnant patients can be scanned, provided the study is medically indicated.
Principles of Interpretation:
Soft tissue contrast is obtained through imaging sequences that accentuate differences in T1
and T2 tissue relaxation times.
Water is the major source of the MR signal in tissues other than fat.
Mineral rich structures, such as bone and calculi, and collagenous tissue such as ligaments,
tendons, fibrocartilage and tissue fibrosis are low in water contents and lack mobile protons to
produce an MR signal.
Low in signal intensity on all MR sequence
Long T1 (low signal) and long T2 (high signal)
Found mainly as extracellular fluid, also as intracellular free water
Organs with extracellular fluid (free water)
Kidneys (Urine); ovaries and thyroid (fluid-filled follicles); spleen and penis (stagnant
blood); and prostate, testes and seminal vesicles (fluid and tubules)
Edema (increase in extracellular fluid) - prolongs T1 and T2 relaxation times.
Most neoplastic tissues have increase in extracellular fluid as well as an increase in the
proportion of intracellular free water bright signal intensity on T2WIs.
Addition of protein and free water shortens T1 relaxation time bright.
T2 relaxation is also shortened, but the T1 shortening effect is dominant even on T2WIs-
remain bright on T2WIs.
Synovial fluid, complicated cysts, abscesses, many pathologic fluid collections, and
necrotic areas within tumors.
Soft tissues that have a predominance of intracellular bound water have shorter t1 and
t2 times than do tissues with large amounts of extracellular water.
Include the liver, pancreas, adrenal glands and muscle- intermediate signal
intensities on both t1wis and t2wis
Intracellular protein synthesis shortens t1 more.
Muscle (less active protein synthesis) is lower in signal intensity on t1wis
than are organs with more active protein synthesis
Benign tumors with a predominance of normal cells, such as focal nodular
hyperplasia in the liver, tend to remain isointense with their surrounding normal
parenchyma on all imaging sequences.
Hyaline cartilage has a predominance of extracellular water but extensively
bound to mucopolysaccharide matrix
Signal resembles cellular soft tissues intermediate on most imaging
Protons in fat are bound to hydrophobic intermediate sized molecules and exchange
energy efficiently within their chemical environment.
T1 relaxation time is short resulting in a bright signal.
T2 is shorter than t2 of water lower signal intensity for fat, relative to water.
On images with lesser degrees of t2 weighting, t1 effect predominates and fat
appears isointense or slightly hyper intense compared with water.
Specialized fat saturation imaging sequences may be used to reduce the signal
intensity of fat and enhance the visibility of edema and pathologic processes within
STIR sequences suppress signals from al