Image Production - Chapter 10

Digital Image Receptors

Systems that use storage phosphors to temporarily store energy representing the image signal are called

Computed Radiography (CR)

Systems that have detectors that directly or indirectly capture and read out an electronic image signal are called

Digital Radiography (DR)

How do DR systems work?

The image-forming radiation is captured and transferred to a computer from the detector array for viewing at the control panel

The physical component consisting of multiple detectors that efficiently absorb the transmitted radiation and accurately convert it to an electrical signal for display on a computer workstation is a

Detector Array

What are the 2 different types of Indirect Capture?

Charge-Coupled Device (CCD)

Thin Film Transistor (TFT)

Which type of indirect capture uses a light-sensitive semiconducting device that generates an electrical charge when stimulated by light and stores this charge in a capacitor?

Charge-Coupled Device -CCD

Which type of indirect capture uses electrical components layered onto a glass substrate that includes the readout, charge collector and light-sensitive elements?

Thin Film Transistor (TFT)

A material that absorbs x-ray energy and emits visible light in response

Scintillator

A process in which several CCD detectors abut (touch) to create one larger detector

Tiling

True or False: Tiling is a major advantage of CCDs

False - disadvantage because it results in seams with unequal response

True or False: Indirect Capture methods result in higher resolution

False - loss of resolution

True or False: Gadolinium Oxysulfide spreads significantly more light than Cesium Iodide

True

What is the photodiode layer made of on the thin film transistor array?

Amorphous Silicon

What is the photodiode layer (photoconductor) made of in the direct capture method?

Amorphous Selenium

What is the main difference between Indirect and Direct capture methods?

Direct capture does not use a scintillator; it directly converts x-rays into an electrical signal

A graphic representation of a data set is called; Represents the number of digital pixel values versus the relative prevalence of those values in the image

Histogram

Which axis of the histogram represents the amount of exposure

The X axis

Which axis of the histogram represents the incidence of pixels for each exposure level?

The Y axis

A process in which a computer analyzes the histogram using processing algorithms and compares it to a preestablished histogram specific to the anatomic part being imaged

Histogram Analysis

Established values within histogram models that determine what part of the data set should be incorporated into the displayed image

Values of Interest (VOI)

What is it called when the computer adjusts the image based on the comparison of the histograms and corrects for “over or under exposure”; a process of mapping the grayscale to the VOI to present a specific display of brightness

Rescaling

True or False: With these systems, only those detector pixels that received x-ray exposure contribute to the image

True

True or False: With digital receptors, the response to exposure is nonlinear and the range of exposures is limited

False - response to exposure is linear and range of exposures is wide (wide dynamic range)

The range of exposure intensities that an image receptor can respond to and use to acquire image data is called

Dynamic Range

The range of exposures that produce acceptable images and information within an acceptable radiation dose to the patient is called

Exposure Latitude

Primary Display workstations are

used for official interpretation of images; Radiologist work stations

List the guidelines for primary workstations

Max. Luminance levels of at least 171 cd/m2

Contrast response that meets the standards of the Association of Physicists in Medicine

A minimum of 8-bit luminance resolution

Minimal veiling glare

Minimizing reflections from and levels of ambient light sources

What do postprocessing functions allow for?

Manual manipulation of the displayed image

True or False: Digital radiography uses a 6-bit dynamic range?

False - 14

How many shades of gray can the human eye see?

30

In CR systems, Fuji, Philips, and Konica use ______ numbers, and the value is _____ related to the exposure to the plate

Sensitivity (S); inversely

Carestream (Kodak) uses ___ numbers, and the value is _____ related to the exposure to the plate and the changes are ______ expressions.

EI (exposure index); directly related; logarithmic expressions

Agfa uses _____ numbers; the value is _____ related and changes are ____ expressions

log mean (lgM) numbers; directly related; logarithmic expressions;

What do DR systems use as an indicator of exposure?

DAP - dose-are product

A measure of exposure in air, followed by computation to estimate absorbed dose to the patient

Dose-area product (DAP)

True or False: DAP reflects both the dose to the patient and the total volume of tissue being irradiated

True

What is DAP measured by?

a dose meter in the collimator

What does the DAP value depend on and how is it expressed?

Depends on the exposure factors and field size and is expressed in centigray-meter squared

Represents the exposure at the detector relevant to the region being imaged and is defined by the signal-to-noise ratio

EI

The target reference exposure obtained from a properly exposed image receptor

Target Exposure Index

A measure of the deviation of the EI from projection-specific target EI values

Deviation Index

Which value indicates to the radiographer that proper exposure is taking place for the body part being examined by indicating above or below the ideal value determined for that part

Deviation Index

An expression of the radiation exposure level that is required to produce an optimal image; a measurement of the efficiency of an IR in converting the x-ray exposure it receives to a quality radiographic image

Detective Quantum Efficiency DQE

True or False: The lower the DQE, the lower the radiation exposure required to produce a quality image, thereby decreasing patient dose

False - the Higher the DQE

What happens to the image if the DQE is too high?

It results in a noisy (grainy) image because not enough photons are hitting the IR

An expression of how clearly a very faint object appears in the image

SNR

True or False: DQE is a measure of how well the SNR is preserved in an image

True

Any undesired fluctuation in the brightness of an image is

Image Noise (quantum noise plus the electronic components of digital imaging systems)

____ ___ is defined as equal to ½ the Nyquist Frequency

Spatial Resolution

The highest spatial frequency (number of line pairs per mm) that a digital detector can record and is determined by the sampling frequency of CR systems and the DEL spacing of DR systems

Nyquist Frequency

What are the 2 measures of Spatial Resolution?

Limiting Spatial Resolution (LSR)

Modulation Transfer Function (MTF)

The ability of a detector to resolve small structures and is measured using a bar pattern

Limiting Spatial Resolution

LSR is limited by

the number of pixels and pixel size

True or False: MTF is not as accurate as LSR

False - LSR is not as accurate as MTF because it depends on the contrast of the target and the exposure and display conditions

A measure of the ability of the system to preserve signal contrast (display the contrast of anatomic objects varying in size) as a function of spatial resolution and describes the fraction of each component that preserves the captured image

Modulation transfer function (MTF)

Which method is considered to be the ideal expression of digital detector image resolution?

MTF

What is the primary factor influencing radiographic contrast with digital systems?

The Lookup Table (LUT)

Histograms of luminance values used as a reference to evaluate the input intensities and assign predetermined grayscale values - VOI

Lookup Tables (LUTs)

At what stage does rescaling occur?

at the Lookup Table

The adjusting of the image by the computer program to present an image of predetermined brightness is called

Rescaling

True or False: Digital receptors are less sensitive to low energy radiation

False - more sensitive

What 2 ways can you effectively control scatter radiation’s effect on the image?

Collimation

Grid Use

How many test patterns can be used to assess display device performance?

26

This test looks at general image quality and for the presence of artifacts

Overall visual assessment TG18-QC

This test checks for a variation in the shape of the displayed image from the original image

Geometric Distortion

This test evaluates the ambient light contribution to the light reflected by the display monitor

Reflection TG18-AD

This test assesses the displayed luminance values versus the input values from the display system

Luminance Response

This test evaluates the image for nonuniformity and effects of viewing different angles

Luminance Dependencies

This test assesses the systems ability to display images of different parts of an image with high fidelity

Resolution

What is the average size of a digital radiography study?

38 MB

An electronic network for communication between the image acquisition modalities, display stations and storage

Picture Archiving and Communication System (PACS)

A common language used by different systems for communication

Digital Imaging and Communication in Medicine (DICOM)

A PACS system consists of

Acquisition (imaging modalities)

Display (viewing and workstations)

Storage (archive server)

A 1-megapixel monitor is sufficient for ….

A 2-megapixel monitor is sufficient for…

A 5-megapixel monitor is sufficient for …

1 - general viewing by non-interpreting physicians

2 - general interpretation by a radiologist

5 - interpretation of digital mammograms

What are the challenges of PACS

Storage of data over decades

Expansion and maintenance

changes in vendors

True or False: Vendor neutral archives (VNAs) allow for images and data from different systems and in different formats to be stored using a singular system on common infrastructure

True

The component of PACS that handles the workflow of the system moving images back and forth between viewing stations

Image Manager

Storage is classified as

Online

Nearline

Offline

Data are stored on magnetic hard drives with access times in milliseconds and transfer times in range of tens and hundreds of megabytes per second

Online

A tape or jukebox uses robotic arms to retrieve the tapes automatically and insert them into a drive to read or write data; can access data within 60 seconds and is able to transfer data at a few megabytes per second

Nearline

A removable tape or optical media will be stored on a shelf in a catalog and retrieved manually

Offline