Patient Image Optimization & Viewing the Medical Image PDF

Principles of Radiography 1 Patient-Image Optimization & Viewing the Medical Image Julie Kloft, MSRS, RT(R) Clinical Coordinator & Professor of Radiography  Patient-Image Optimization Technique Considerations 1. Patient/Subject Factors 2. Image-Quality Factors 3. Exposu...

Principles of Radiography 1 Patient-Image Optimization & Viewing the Medical Image Julie Kloft, MSRS, RT(R) Clinical Coordinator & Professor of Radiography  Patient-Image Optimization Technique Considerations 1. Patient/Subject Factors 2. Image-Quality Factors 3. Exposure Technique Factors  Patient/Subject Factors Subject Contrast Patient Thickness Tissue Mass Density Effective Atomic Number Object Shape Not positioning, but rather the technique to compensate for the patient’s size, shape, and composition. (Positioning relates to geometric factors.)  Patient/Subject Contrast Patient (or subject) contrast is the degree of differential absorption resulting from the differing absorption characteristics of the tissues in the body.  Subject Contrast Radiographic Contrast = IR Contrast x Subject Contrast (IR Contrast is expressed as the Average Gradient) Component of radiographic contrast determined by the size, shape, and x-ray attenuating characteristics of the subject being examined AND the energy of the beam.  Subject Contrast High Low  Body Types Hypersthenic: Large frame, overweight Sthenic: Average, strong, active Hyposthenic: Thin, but healthy appearance Asthenic: Small, frail, emaciated  Patient Thickness Assuming comparable body composition, a thick body section attenuates a greater number of x-rays than does a thin body The degree of subject contrast is directly proportional to the relative number of x-rays leaving those sections of the body. Attenuation Attenuation is the reduction in the total number of x-ray photons remaining in the beam after passing through a given thickness of material.  Attenuation is the result of x- rays interacting with matter and being absorbed or scattered. is determined by the amount and type of irradiated material. Patient Density Thicker, denser body parts absorb more radiation.  Tissue Mass Density Different patients may have equal body part thicknesses, but different mass densities. ≠  Effective Atomic Number When the effective Effective Atomic Number Substance atomic number of Human Tissue adjacent tissues Fat Soft Tissue 6.3 7.4 varies greatly, Lung 7.4 subject contrast is Bone 13.8 Contrast Material very high Air 7.6 Iodine 53 Barium 56 Other Concrete 17 Molybdenum 42 Tungsten 74 Lead 82  Pathology Certain diseases can increase or decrease tissue thickness or composition (change the effective atomic number or density).  Pathology Collecting information can begin even BEFORE you meet the patient, by reviewing the requisition. Any history or diagnosis provided should be reviewed, as it may give you clues about potential pathology. Examples:  Degenerative  Emphysema  Ascites  Hemothorax  Osteoporosis  Pathology Taking a good patient history helps prevent repeat radiographs! Example: You have a patient coming for a CXR because of a cough. If you find your patient has a history emphysema, you should adjust your technique. If you didn’t ask or get any additional information, other than a cough, you may have images like these and need to repeat.  Pathology In addition to talking to the patient, you should also observe the patient for any physical indications or outward signs of pathology. Radiopaque Radiopaque tissue Metal hardware ABSORBS x-rays and appears white on the Contrast radiograph. Bone Soft Tissue Radiolucent tissue attenuates few x-rays and Air appears black on the radiograph. Radiolucent Pathology & Technique As soon as you identify your patient, you should begin evaluating them (and their history) for indications that might affect your technique.  Questions to ask yourself when you meet your patient: 1. What are the physical characteristics of the part to be examined?  Internal deviations from normal are usually indicated by external deviations. 2. Can you identify indicators of pathology of the part being examined? 3. Is the patient going to be able to hold still for the procedure?  Pathology: Additive conditions (aka Constructive Pathology) If a disease causes the affected body tissue to increase in thickness, effective atomic number, and/or density, there will be greater attenuation of the x-ray beam. Hydrocephalus Additive pathology is inversely proportional to IR exposure.  Additive pathology means reduced IR exposure. Additive pathology is directly proportional to technique.  Additive pathology requires increased technical factors.  Pathology: Additive conditions Generally additive pathology requires increased kVp to penetrate thicker, more radiopaque parts.  +15% of the kVp will double the exposure to the IR, so increasing 5-15% of the kVp is sufficient for most additive pathology. Effusion identified on image with appropriate technique, but over- exposed radiograph does not demonstrate it.  Sample Additive Pathologies Chest Atelectasis Abdomen Extremities/Skull Bronciectasis Aortic Aneurysm Acromegaly Cardiomegaly Ascites Chronic Osteomyelitis Congestive Heart Failure Cirrhosis Hydrocephalus Empyema Calcified Stones Osteoblastic Metastases Pleural Effusion Multiple Sites Osteochondroma Pneumoconiosis Abscess Paget’s Disease Pneumonia Edema Sclerosis Pneumonectomy Tumors Pulmonary Edema Tuberculosis Pathology: Destructive conditions When a disease causes the affected body tissue to decrease in thickness, effective atomic number, and/or density, there will be less attenuation of the x-ray beam. Multiple Myeloma Destructive pathology is directly proportional to IR exposure.  Increased destructive pathology means increased IR exposure. Destructive pathology is inversely proportional to technique.  Destructive pathology requires decreased technical factors. Pathology: Destructive conditions Generally destructive pathology requires decreasing the mAs to compensate for the changes in the patient.  A decrease of 25-50% of the mAs is usually sufficient for most pathologic conditions. Osteoporosis is evident on a properly exposed radiograph. If over- exposed, it may not be evident. Sample Destructive Pathologies Extremities/Skull Active Osteomyelitis Abdomen Aseptic Necrosis Aerophagia Carcinoma Bowel Obstruction Degenerative Arthritis Chest Fibrosarcoma Emphysema Gout Pneumothorax Multiple Sites Hyperparathyroidism Anorexia Nervosa Multiple Myeloma Atrophy Osteolytic Metastases Emaciation Osteomalacia Osteoporosis  SOME Pathology=No Changes Many diseases do not significantly affect thickness or density and do not require any adjustment to the technique. Examples: Fractures (without gross swelling) Ulcers Diverticula  Things to Think About… Skeletal: 50% loss of bone before a change can be seen on radiograph Respiratory: requires long scale contrast (120 kV) to see lung processes and penetrate mediastinum GI Tract: 2 major considerations Bowel Obstruction = taut (air) decrease technique Ascites = feels like dough (fluid) increase technique Radiographic Quality The fidelity (clarity/exactness) with which the anatomical structure that is being examined is imaged  What makes a high-quality radiograph??? While there are specific factors that affect quality, there is no precise formula for labeling it. Image Quality By  Image-Quality Factors Spatial Resolution Contrast Resolution Distortion IR Response These qualities are ALL influenced by the patient characteristics!  Spatial Resolution The ability to image small objects with high subject contrast (such as bone and soft tissue)  Spatial Resolution (Last week!) The ability to image small objects with high subject contrast. (In DR, limited by pixel size!) Most people can see objects as small as 200 um! Contrast Resolution The ability to distinguish anatomical structures of similar subject contrast (liver/spleen)  (Last week!) Contrast Resolution The ability to distinguish between and to image similar tissues. CR & DR have better contrast resolution than film-screen because of wider dynamic range.  Image or Recorded Detail (generic/old phrase often used to describe the appearance of anatomic structures on image) The degree of sharpness of structural lines on a radiograph. Sharpness of image or recorded detail was an attempt to describe spatial resolution.  Visibility of Detail The ability to visualize recorded detail when contrast and optical density are optimized. Visibility of image detail was related to contrast resolution and image contrast.  Resolution The ability to image two separate objects and visually distinguish one from the other.  Distortion A misrepresentation of the true size or shape of an object on a finished radiographic image. Shape Distortion Actual Size Foreshortening: appears shorter than actual length E l o n g atio n: appears longer than actual length  Size Distortion Magnification can distort the image and make an item appear much larger/smaller than it actually is.  Reducing Distortion Distortion is reduced by positioning the patient body part parallel to the image receptor and having the Central Ray perpendicular to both.  Viewing the Medical Image The adoption of digital imaging and viewing images on a digital display device requires an understanding of photometry (the science of the response of the human eye to visible light).  Photometric Quantities The first attempt to quantify human vision was made in 1924 by the International Commission on Illumination (CIE), and included a definition of light intensity, the candle, the footcandle, and candle power.  Response of the Eye The CIE recognized the difference between photopic (bright-light vision using cones) and scotopic (dim-light vision using rods).  Bright-light vision is best at 555 nm.  Dim-light vision is best at 505 nm.  Photometric Units The basic photometric unit is the lumen. Luminous flux is the fundamental quantity of photometry and is expressed in lumens (lm). Luminous flux describes the total intensity of light from a source. 100 watts of energy consumed and 1490 lumens of light output.  Illuminance The intensity of light incident on a surface.  One lumen of luminous flux incident on one square foot is a footcandle (fc).  One lumen of luminous flux incident on one square meter is a lux (lx). 1 fc = 10.8 lx Location Illuminance Digital Reading Room 1 fc Twilight 5 fc Indoor Room 100- 200 fc Cloudy Day 1000 fc OR 3000 fc Sunny Day 10,000 fc  Luminance Intensity Describes a property of the source of light (viewbox or digital display). Luminance intensity is the luminous flux that is emitted into the entire viewing area and is measured in lumens per steradian or candela.  Luminance Measures the brightness of a source  Expressed in units of candela per square meter or nit  Cosine Law When a digital display device is viewed straight on, the luminous intensity is maximum. When viewed from the side, illumination and contrast are reduced. The reduced projected surface area follows the mathematical function called cosine.  Inverse Square Law Luminous intensity decreases in proportion to the inverse square of the distance from the source.  Hard Copy vs Soft Copy Until the 1990s, essentially all medical images were “hard copy,” and presented to the Radiologist via film on a viewbox.  CT and MR were the first to move to digital images. However, it was common for these images to be laser printed and viewed on a viewbox!  Liquid Crystal Display (LCD) Monitor Images are displayed pixel by pixel with an intense white backlight to illuminate each pixel. Most have 1 MP (1000 x 1000 pixels). High resolution = 8 MP (2160 x 3840 pixels). Image Luminance Only about 10% of the backlight is transmitted through a monochrome LCD monitor. About half of that is transmitted through a color monitor.  Aperture Ratio On an LCD monitor, the portion of the pixel face that is available to transmit light is called the Aperture Ratio.  The intrinsic noise of an LCD monitor is low, so there is better contrast resolution.  Aperture Ratio is to an LCD monitor what Fill Factor is to a digital image receptor.  Light-Emitting Diode Display (LED) Monitor LED monitors display images by turning on and adjusting the brightness of colored diodes. An entirely white image with black text will therefore require many more illuminated diodes – and far more power – than white text on a black background.  Light Emission Any material that emits light in response to an outside stimulus is called a phosphor and the resulting light is called luminescence.  Fluorescence is when visible light is emitted ONLY while the phosphor is stimulated.  Phosphorescence is when the phosphor continues to emit light after stimulation.  Electroluminescence is the production of light by the flow of electrons, such as within crystals.  Backlight  All video monitors are really LCDs with a more intense backlight powered by LED.  LED monitors have a longer life-expectancy by at least a factor of two over fluorescent backlit monitors.  LED monitors use less energy and give off less heat.  Ambient Light LCD monitors reduce the influence of ambient light on image contrast. Best viewing condition is in near total darkness.  Viewing Angle There is a serious loss of image contrast when the image on an LCD is viewed off- center. The principal disadvantage of an LCD monitor is the dependence of the viewing angle—contrast falls sharply as the viewing angle increases.  Ergonomics The viewing requirements of flat panel digital display devices has led to reconfigured ergonomically-designed digital workstations for the Radiographers and Radiologists. Ambient light levels must be reduced to near-darkness for best viewing!  Software Artifacts The digital imaging system will manipulate the output of the IR to correct for potential artifacts (dead pixels, dead rows/columns of pixels).  Digital images are obtained as raw data sets which are ready “for processing”  “For processing” images are then manipulated into “for presentation” images  Calibration Calibrating digital equipment allows the software to recognize dead pixels and “auto- fill” the areas. Offset and gain images are automatic calibration images to make the IR response more uniform. Gain images are generated every few months. Offset images are generated many times each day. Pre-Processing The ability to manipulate images BEFORE display. Primarily automated process of producing artifact-free digital images. Before the image is sent “for- processing,” dead pixel areas are repaired using interpolation techniques to assign digital values to each dead pixel, row, or column.  Pre-Processing: Flatfielding Flatfielding is performed pre- processing to offset the irregular variation of the anode-heel effect. Before flatfielding After flatfielding  Pre-Processing: Signal Interpolation Digital IRs and monitors have millions of pixels, so it is to be expected that some individual pixels will become defective (to respond differently or not at all). Signal Interpolation corrects the defective pixels by averaging the response of the surrounding pixels. Problem Solution Defective Pixel Interpolate adjacent pixel signals Image Lag Offset correction Line Noise Correct from dark reference zone.  Pre-Processing: Image Lag Each type of digital IR generates an electronic latent image that may not be made completely visible. Image lag is the image that is not completely visible and is corrected by applying offset voltage before the next image is acquired. Problem Solution Defective Pixel Interpolate adjacent pixel signals Image Lag Offset correction Line Noise Correct from dark reference zone.  Pre-Processing: Line Noise There are voltage variations in the buses that drive each pixel. Line noise is the linear artifacts that appear on the final image due to the voltage variations and is corrected via voltage correction from a row or column of pixels in a dark, unexposed area of the IR. Problem Solution Defective Pixel Interpolate adjacent pixel signals Image Lag Offset correction Line Noise Correct from dark reference zone.  Post-Processing Operator manipulation of the image after it is acquired. Process Results Annotation Label the image Window/Level Expand the digital grayscale Magnification Improve visualization and spatial resolution Image Flip Reorient the image presentation Image Inversion Make white-black and black-white. Subtraction (DSA) Improve image contrast Pixel Shift Re-register an image to correct for patient motion in series Region-of-Interest Determine average pixel value  Post-Processing: Image Annotation Annotation is the process of adding text to an image.  (Last week!) Dynamic Range A 16-bit Dynamic Range DR system has 65,536 shades of gray. The human eye can only discern about 30 shades. Window/Leveling allows us to fully visualize each region. The exact number of shades of gray is determined by bit depth. 12-bit dynamic range: 2bit depth = 212 = 4096 14-bit dynamic range: 2bit depth = 214 = 16,384 16-bit dynamic range: 2bit depth = 216 = 65,536  Post-Processing: Image Display Adjustments By manipulating the window width and levels, the Radiographer can make all of the shades of gray available VISIBLE. Terminology Action Changes Window Width Contrast Inverse: (visibility of detail) ↑WW = ↓contrast Window Level Density Direct: (brightness) ↑ WL = ↑ density  Post-Processing: Magnification The larger matrix size monitors have better spatial resolution because they have smaller pixels. This allows magnification of a region of an image to “zoom in” to make the smallest anatomic objects visible.  Post-Processing: Image Flip Digital images may be reoriented by flipping them vertically or horizontally. Image Flipping is used to bring Additionally, newer technology allows for all-axis orientation correction.  Post-Processing: Image Inversion Pathology, tubes, and lines of often made more visible by applying image inversion, which results in a black appearance of bone and a white appearance of soft tissue.  Post-Processing: Image Subtraction Image subtraction, commonly used in Digital Subtraction Angiography (DSA), allows images taken in a series to “subtract” portions of the image to make other areas easier to see. Post-Processing: Pixel Shift Often used in conjunction with subtraction, pixel shift allows for correction of patient motion in a series of images. Post-Processing: Region of Interest Region of Interest allows for rescaling of the digital image and computing the mean pixel value for the area of interest.  Post-Processing: Additional Features  Edge Enhancement helps highlight fractures and small, high-contrast tissues.  Highlighting can be effective in identifying diffuse, non-focal disease.  Pan, scroll, and zoom allow for careful visualization of precise regions of an image.  See you next week!