Diagnostic Imaging Methods

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introduces 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.

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  • DIAGNOSTIC IMAGING METHODS

    HISTORY

    1895-Wilhem Roentegen

    Produced the 1st X ray film image of his wifes hands

    X RAY

    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.

    FILM RADIOGRAPHY

    Utilize a screen film system within a film cassette ( x-ray detector)

    COMPUTED RADIOGRAPHY (CR)

    Filmless system

    No processing

    Produces A DIGITAL RADIOGRAPHIC IMAGES

    DIGITAL RADIOGRAPHY

    Filmless system

    Substitutes a fixed electronic detector on charge-coupled device for the film screen cassette or

    phosphor imaging plate.

  • CONVENTIONAL TOMOGRAPHY

    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

    Improved detail

    FLUOROSCOPY

    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

    Air density

    Fat density

    Soft tissue density

    Bone 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

    COMPUTED TOMOGRAPHY

    Displays each splice separately

    No superimposed blurred structures seen.

    Hounsfield unit (H) scale

    o Air:1,000H

    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

    Soft tissues

    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 :

    Hydrogen

    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

    changes.

    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:

    Gadolinium chelates

    - 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.

  • SAFTETY CONSIDERATIONS:

    MR is contraindicated in patients who have electrically, magnetically or mechanically activated

    implants.

    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

    staples.

    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

    filters

    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

    Free Water

    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.

  • Proteinaceous fluids

    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

    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

    sequences.

    Fat

    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

    fat.

    STIR sequences suppress signals from al