Chapter 8 - Digital Imaging

Question 1

photostimulable phosphor (PSP)

Answer 1

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.

Question 2

radiolucent

Answer 2

material that does not absorb primary x-rays

Question 3

5 Requirements to view digital images

Answer 3

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

Question 4

fog on the image

Answer 4

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.

Question 5

Flat-panel detectors

Answer 5

Flat-panel detectors (FPDs) consist of either a scintillation screen or a photoconductor, which converts the x-ray photons directly into electrical signals.

Question 6

Indirect conversion

Answer 6

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.

Question 7

scintillator

Answer 7

A scintillator is a device that glows when hit by the high-energy x-ray photons, and is often made of cesium iodide.

Question 8

photodiode

Answer 8

A photodiode made of amorphous silicon then converts the light into an electric signal

Question 9

detector elements [DELs]

Answer 9

Detectors with high fill factors present higher spatial and contrast resolution.

Question 10

analog-to-digital converter (ADC)

Answer 10

The ADC takes the stored charge and converts it into digital values. From there, it is sent to the computer for processing and viewing.

Question 11

charged coupled device (CCD)

Answer 11

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.

Question 12

complementary metal oxide semiconductor (CMOS)

Answer 12

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.

Question 13

Direct conversion

Answer 13

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

Question 14

Dead pixels

Answer 14

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.

Question 15

Spatial resolution

Answer 15

A basic definition of spatial resolution is the amount of detail or sharpness of an image as seen on the monitor.

Question 16

Contrast resolution

Answer 16

Contrast resolution is the ability to distinguish anatomical structures of similar subject contrast, such as liver–spleen and gray matter–white matter.

Question 17

dynamic range

Answer 17

Dynamic range is the response of the detector to different levels of radiation exposure.

Question 18

Signal-to-noise ratio (SNR)

Answer 18

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.

Question 19

quantum mottle

Answer 19

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.

Question 20

modulation transfer function (MTF)

Answer 20

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.

Question 21

histogram

Answer 21

The histogram is basically a graph of the minimum and maximum signals in the image.

Question 22

look-up table (LUT)

Answer 22

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.

Question 23

Window level

Answer 23

controls the density in the image. In digital imaging the word brightness is used in place of density.

Question 24

Window width

Answer 24

controls the contrast in the image.

Question 25

Electronic cropping

Answer 25

also known as masking or shuttering, is used to blacken out the white collimation borders. This eliminates the glare to the eyes.

Question 26

image stitching

Answer 26

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.

Question 27

Image annotation

Answer 27

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.

Question 28

Edge enhancement

Answer 28

Edge enhancement is a processing technique in which images can be made sharper and have greatly increased contrast; however, it does introduce some noise.

Question 29

Smoothing

Answer 29

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.

Question 30

rescaling

Answer 30

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.

Question 31

postprocessing

Answer 31

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.

Question 32

DICOM

Answer 32

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.

Question 33

DICOM gray-scale function

Answer 33

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.

Question 34

Informatics

Answer 34

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.

Question 35

Artifact: Moiré pattern

Answer 35

This artifact occurs when the grid lines are not aligned with the laser scanning frequency of the CR reader (Fig. 8.17).

Question 36

Artifact: Light spots

Answer 36

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.

Question 37

Artifact: White line

Answer 37

White line artifacts appear along the length of travel on the image due to dust on the light guide.

Question 38

Artifact: Histogram analysis error

Answer 38

may be caused by any of the following: improper collimation, improper technique, beam alignment error, scatter, and extreme subject density differences.

Question 39

Artifact: Phantom or ghost images

Answer 39

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).

Question 40

Artifact: Scratches or tears

Answer 40

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).

Question 41

Artifact: Extraneous line patterns

Answer 41

are linear lines caused by noise in the image reader electronics. They can run lengthwise or crosswise (Fig. 8.20).

Question 42

Artifact: Fogging from background or scatter radiation

Answer 42

is caused by the CR plates being much more sensitive than the former film.

Question 43

picture archival and communication system (PACS)

Answer 43

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.

Question 44

Patient ID

Answer 44

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.

Question 45

The most important monitor tests include:

Answer 45

*Viewing surface and airflow
*Image quality and appearance using a test pattern
*Geometric distortion
*Luminance, reflection, noise, and glare
*Resolution