L3 Medical Imaging PDF

Course instructor: Dr.Ali Hayder Ali  How do x-rays create an image of internal body structures?  What are the 5 basic radiographic densities?  Primary purpose is to identify pathologic conditions.  Requires recognition of normal anatomy. X-rays: a form of electromagnetic energy ...

Course instructor: Dr.Ali Hayder Ali  How do x-rays create an image of internal body structures?  What are the 5 basic radiographic densities?  Primary purpose is to identify pathologic conditions.  Requires recognition of normal anatomy. X-rays: a form of electromagnetic energy Travel at the speed of light Electromagnetic spectrum: Gamma Rays- X-rays Visible light -Infrared light Microwaves-Radar Radio waves.  X-rays can:  Pass all the way through the body  Be deflected or scattered  Be absorbed  Where on this image have x-rays passed through the body to the greatest degree? Dependent upon:  Physical density  Atomic number  Thickness Determine the gray scale of the radiograph:  Absorb few x-rays = film black  many x-rays = film white Depends on the energy of the x-ray and the atomic number of the tissue:  Higher energy x-ray -more likely to pass through  Higher atomic number -more likely to absorb the x-ray  X-rays that pass through the body to the film render the film dark (black)  X-rays that are totally blocked do not reach the film and render the film light (white)  Air = low atomic # = x-rays get through = image is dark  Metal = high atomic # = x-rays blocked = image is light (white) Air Fat Soft tissue Bone Metal least opaque to most opaque most lucent to least lucent Black to White  The X-rays used in medical diagnosis are produced from a small area within the X-ray tube when an exposure is made. They diverge outwards from this area, travel in straight lines, and can be detected by a variety of devices used for medical imaging.  As the X-rays pass through the body, some will be absorbed by the organs and structures within the body whilst others will pass through to the equipment used to form the image.  Five principal densities are recognized on plain radiographs listed here in order of increasing density:  1. Air/gas: black, e.g. lungs, bowel and stomach  2. Fat: dark grey, e.g. subcutaneous tissue layer, retroperitoneal fat  3. Soft tissues/water: light grey, e.g. solid organs, heart, blood vessels, muscle and fluid- filled organs such as bladder  4. Bone: off-white  5. Contrast material/metal: bright white.  Air  Fat  Soft tissue/fluid  Mineral  Metal  X-rays pass through the body to varying degrees  Higher atomic number structures block x- rays better, example bone.  Lower atomic number structures allow x- rays to pass through, example: air in the lungs.  Passed through absorbed Any structure, normal or pathologic, should be analysed for:  Size  Shape and contour  Position  Density (You must know the 5 basic densities)  Interpretation of ultrasound images depends on the echogenicity: the brightness of the image depending on the degree of reflection of the ultrasoun waves.  Terms used include hyperechoic, isoehoic, hypoechoic, and anechoic.  The images are also described in terms of the plane on which the sonogram is viewed, which is usually longitudinal or transverse in relation to the structure scanned. A sample feet first and supine patient thorax positioning in a multichannel phase array coil is shown. A sample feet first and supine patient thorax positioning in a multichannel phase array coil is shown. Respiratory bellow positioning is shown. A sample feet first and supine patient liver positioning in a multichannel coil is shown.  Certain nuclei in the body will absorb and reemit radio waves of specific frequencies when those nuclei are under the influence of a magnetic field.  These reemitted radio signals contain information about the patient that is captured by a receiver or antenna.  The electrical signal from the antenna is transmitted through an "analog-to-digital" (A to D) converter and then to a computer, where an image of the patient is reconstructed mathematically.  A proton has a spin, and thus the electrical charge of the proton also moves. A moving electrical charge is an electrical current, and this is accompanied by a magnetic field. Thus, the proton has its own magnetic field and it can be seen as a little bar magnet  Normally protons are aligned in a random fashion. This, however, changes when they are exposed to a strong external magnetic field. Then they are aligned in only two ways, either parallel or antiparallel to the external magnetic field.  A spinning top, which is hit, performs a wobbling type of motion. Protons in a strong magnetic field also show this type of motion, which is called precession. Questions?