Chapter 8 - Digital Imaging Flashcards
photostimulable phosphor (PSP)
The plate contains a photostimulable phosphor (PSP) that stores the latent image of the body part until it is processed. The phosphor absorbs the energy of the x-rays and, with it, the image of the body part. Most IP phosphors are made of barium fluorohalide with europium.
radiolucent
material that does not absorb primary x-rays
5 Requirements to view digital images
To process and view the image, five additional equipment components are required: a CR reader unit, a limited operator computer workstation, a computer system with monitors for the radiologist to view the images, a printer if images need to be printed on film, and a computer server to store the images
fog on the image
The IP is also sensitive to background radiation and should be erased if it is not used in 48 hours; otherwise there will be fog on the image.
Flat-panel detectors
Flat-panel detectors (FPDs) consist of either a scintillation screen or a photoconductor, which converts the x-ray photons directly into electrical signals.
Indirect conversion
Indirect conversion DR is a two-step process in which the x-ray energy is first converted into light and then converted into an electric signal. This method uses a scintillator to convert the x-ray energy into light.
scintillator
A scintillator is a device that glows when hit by the high-energy x-ray photons, and is often made of cesium iodide.
photodiode
A photodiode made of amorphous silicon then converts the light into an electric signal
detector elements [DELs]
Detectors with high fill factors present higher spatial and contrast resolution.
analog-to-digital converter (ADC)
The ADC takes the stored charge and converts it into digital values. From there, it is sent to the computer for processing and viewing.
charged coupled device (CCD)
The charged coupled device (CCD) is a type of indirect conversion detector. In this system, there is no photodiode and instead the CCD converts the light from the scintillator to the electric signal. The CCD device uses optics and light and therefore is often referred to as a CCD camera.
complementary metal oxide semiconductor (CMOS)
Another type of indirect conversion detector. CMOS detectors convert light into electrons very similar to CCD technology. With CMOS, the electrons are stored in capacitors. The semiconductor is a solid chemical element or a compound that allows excellent control of electrical current. The most common CMOS semiconductor is silicon. An ADC is used with this detector to convert the image into digital format.
Direct conversion
Direct conversion is a one-step process. Detectors convert the x-ray energy directly to an electric signal through an amorphous selenium detector without the light conversion
Dead pixels
This may cause a loss of patient information. The manufacture of the pixels is so complex that it is inevitable that the detector array will suffer some damage. Dust, scratches, and interactions between materials can occur, resulting in some defective pixels. These pixels may be malfunctioning or dead, that is, not functioning at all. As the detector ages, the number of dead pixels increases but may not be detected if they are located on the edges of the panel.
Spatial resolution
A basic definition of spatial resolution is the amount of detail or sharpness of an image as seen on the monitor.
Contrast resolution
Contrast resolution is the ability to distinguish anatomical structures of similar subject contrast, such as liver–spleen and gray matter–white matter.
dynamic range
Dynamic range is the response of the detector to different levels of radiation exposure.
Signal-to-noise ratio (SNR)
Signal-to-noise ratio (SNR) describes the ability of the digital system to convert the x-ray input electric signal into a useful radiographic image. Signal refers to the useful information in the image. Noise refers to the amount of information that is not useful.
quantum mottle
Quantum mottle occurs when there are not enough photons in the detectors to provide a high-quality image, which is usually the result of mA or kVp set to low.
modulation transfer function (MTF)
MTF is used to measure the capacity or accuracy of the digital detector to pass its spatial resolution characteristics to the final image. An MTF of 0 represents no signal and therefore no image. An MTF of 1.0 would represent a body part image perfectly.
histogram
The histogram is basically a graph of the minimum and maximum signals in the image.
look-up table (LUT)
The LUT is a file of stored images for each projection. These LUT files are referenced during processing. The LUT is used as a base image reference when adjustments are made on an image.
Window level
controls the density in the image. In digital imaging the word brightness is used in place of density.
Window width
controls the contrast in the image.
Electronic cropping
also known as masking or shuttering, is used to blacken out the white collimation borders. This eliminates the glare to the eyes.
image stitching
When anatomy or the area of interest is too large to fit on one IR, multiple images can be “stitched” together using a special computer program, a technique called image stitching.
Image annotation
Image annotation allows the limited operator to add text that is useful to have on the image. This can include information such as time, exposure technique, or patient position.
Edge enhancement
Edge enhancement is a processing technique in which images can be made sharper and have greatly increased contrast; however, it does introduce some noise.
Smoothing
Smoothing is another type of processing technique. With this technique each pixel’s frequency is averaged with the surrounding tissue’s pixel values. This is done to remove noise, which can be bothersome to the radiologist. Less contrast is also seen in the image. This technique is useful in viewing very small structures and the fine details of bone.
rescaling
When the x-ray exposure is greater or less than what is required to produce an image, automatic rescaling occurs. Automatic rescaling means that images are produced with uniform density and contrast, regardless of the amount of exposure.
postprocessing
Two common postprocessing techniques are subtraction and contrast enhancement. With the subtraction technique (Fig. 8.14), the computer can remove anatomy such as the bones or organs. With the contrast enhancement technique (Fig. 8.15), contrast can be changed from very high to very low; however, the more an image is postprocessed, the less information is transferred to the physician.
DICOM
Digital imaging and communications in medicine (DICOM) is a universally accepted standard for exchanging medical radiographic images within the institution and in the many areas where the images are viewed.
DICOM gray-scale function
The DICOM gray-scale function provides methods for calibrating a particular viewing monitor display system for the purpose of presenting images consistently on different display monitors and printers.
Informatics
Informatics in medical imaging is closely tied together. Each patient has an electronic medical record (EMR) or sometimes it is referred to as the electronic health record (EHR). In radiology, a specific radiology information system (RIS) is tied to these records for reports of the patients x-ray examinations.
Artifact: Moiré pattern
This artifact occurs when the grid lines are not aligned with the laser scanning frequency of the CR reader (Fig. 8.17).
Artifact: Light spots
usually caused by dust or other foreign material on the IP. CR phosphor plates can be cleaned, but this must be done carefully according to the manufacturer’s recommendations to avoid permanent damage.
Artifact: White line
White line artifacts appear along the length of travel on the image due to dust on the light guide.
Artifact: Histogram analysis error
may be caused by any of the following: improper collimation, improper technique, beam alignment error, scatter, and extreme subject density differences.
Artifact: Phantom or ghost images
may appear as a result of incomplete IP erasure. This artifact requires troubleshooting of both the CR plate preparation system and the display systems. Extreme overexposure may require two erasure cycles to completely remove the image (Fig. 8.18).
Artifact: Scratches or tears
are permanent artifacts caused by damage to CR plates. Replacement of CR plates is expensive, but it is the only solution because these artifacts cannot be repaired (Fig. 8.19).
Artifact: Extraneous line patterns
are linear lines caused by noise in the image reader electronics. They can run lengthwise or crosswise (Fig. 8.20).
Artifact: Fogging from background or scatter radiation
is caused by the CR plates being much more sensitive than the former film.
picture archival and communication system (PACS)
The system consists of an extensive networked group of computers, servers, and archives (Fig. 8.21). A PACS will contain all the digital images that are produced in the department, including CT, MRI, ultrasound, and nuclear medicine images. This extensive computer system allows multiple users inside and outside the department to view images.
Patient ID
Every image, including digital images, must include four items of information: the patient’s name or institution ID for a given patient, a birth date or institution ID specific for a given patient, the date of the examination, and the name and location of the x-ray facility.
The most important monitor tests include:
*Viewing surface and airflow
*Image quality and appearance using a test pattern
*Geometric distortion
*Luminance, reflection, noise, and glare
*Resolution
