image production - Image Acquisition & Evaluation (Pt. 2)

What is Automatic Exposure Control (AEC)?

terminates exposure when sufficient radiation to provide the required optical density has reached the image receptor

What are the components of the AEC system?

• Radiation Detector (Chamber/cell)

• Comparator

What is the purpose of the AEC comparator?

Receives detector signal and terminates exposure once a certain amount of radiation is reached

What is the backup timer?

Terminates exposure within 6 seconds or 600 mAs, whichever comes first

Use of AEC requires proper ______

• Cell Selection
• Patient Positioning
• CR Centering
• Collimation

When would AEC not be used?

• Orthopedic Devices
• Narrow Anatomy
• Peripheral Anatomy
• Table-Top

What is the Density/SNR Selection?

Controls IR exposure by controlling how much radiation is detected before exposure is terminated

What is Spatial Resolution?

The ability to distinguish two separate objects close together

Spatial Resolution is also known as ________

High Contrast Resolution or Recorded Detail

What are the categories that affect spatial resolution?

• Procedural Factors
• Image Receptor Factors
• Monitor Display

What is Umbra?

the distinctly sharp area of shadow or the region of complete shadow

What is the penumbra?

geometric unsharpness around the periphery of the image

What is Penumbra affected by?

• Large OID
• Short SID
• Large Part Size
• Shape
• Motion
• Angulation
• large Focal Spot Size

What is the Matrix Size?

Square arrangement of numbers in columns and rows, and in digital imaging, the numbers correspond to discrete pixel values

the larger the matrix, the ______ the pixels or DELs the ________ the spatial resolution

Smaller; Greater

What is a Detector Element (DEL)?

Makes up portion of DR IR, collects image data from beam to send to computer for processing

Bigger the size of the DEL, _____ the Fill Factor

Lower

What is DEL Pitch?

Distance between center of a DEL to center of adjacent DEL

Smallerhe DEL Pitch, ______ the Spatial Resolution

Greater

What is the fill factor?

The percentage of each DEL that is detects radiation informationF

How is spatial resolution measured?

line pairs per millimeter (lp/mm)

The higher the lp/mm, the ______ spatial resolution is

Greater

What is Sampling Frequency?

The number of pixels sampled per millimeter as the laser scans each line of the imaging plate

What is the Nyquist frequency?

The rate at which spatial resolution is limited to

How is the Nyquist frequency determined?

1/2 the sampling frequency;
2 pixels/mm = 1 lp/mm

What is the monitor's resolution?

Spatial resolution determined by pixel matrix

What is the modulation transfer function (MTF)?

The ability of a detector to transfer the modulation of the input signal at a given spatial frequency to its output); measures visibility of detail

High MTF at High Spatial frequencies means ______ Spatial Resolution

Greater

Smaller Objects have ______ Spatial Frequency and ______ Contrast

High; Low

What is Contrast?

the difference in brightness between the light and dark areas of an image

What is contrast resolution?

The ability to distinguish anatomic structures of similar subject contrast

low contrast image

displays many intermediate grays or color shades with no true white or black areas

High Contrast Image

bright whites, deep blacks, narrow range of shades between the whites and blacks

What is gray scale?

The number of brightness levels or shades of gray visible on an image

Short Gray scale = ______ Contrast

High

Long Gray scale = ______ Contrast

Low

What are the types of Contrast?

• Radiographic/image/displayed
• Subject Contrast
• IR, Inherent, Detector Contrast

What is image contrast?

Contrast of the final radiograph as a result

What are the main factors of image contrast

• kVp (Film)
• Computer Processing

What is subject contrast?

The degree of differential absorption resulting from the differing absorption characteristics of the tissues in the body

What are the factors of subject contrast?

• Part (Size, Shape, Density, Tissue)
• Pathology
• Radiation Quality
• Contrast Media
• Scatter
• Noise

What is Detective Quantum Efficiency (DQE)?

how efficiently a system converts the x-ray input signal into a useful image signal

What is the relationship between DQE and Exposure?

High DQE means a more efficient detector that requires less exposure

Why don't all detectors have high DQE?

Higher DQE = ____ Noise and _____ Spatial Resolution

Less; Greater

Higher DQE = _____ patient dose

Lower

DQE is greater when ____

• Lower kVp
• Higher IR Exposure

What is image receptor contrast?

the range of densities that the image receptor is capable of recording

What is the dynamic range?

Contrast, as displayed on a monitor; a range of X-ray signals that is recorded

Low Dynamic Range = _____

Low Exposure, Quantum Mottle

high Dynamic Range = _____

High Exposure, Saturated

What is exposure latitude?

Range of exposures which produce quality images at appropriate patient dose

What is a Saturated image?

More radiation that can be processed, black image

What is Bit Depth?

total number of possible brightness levels that can be assigned to any given pixel in digital image

How is bit depth calculated?

2^n

What is an analog signal?

Data transmission that is continuously varying in proportion to the intensity of a given signal

What is a disadvantages of an analog signal?

Signal Distortion due to amplification, attenuation, or electronic noise

What is a Digital Signal

data transmission in which the signal varies in discrete steps and not in a continuous manner

What is the advantage of a digital signal?

Ability to manipulate the signal without loss of data

What is Noise?

Non-informational data in digital imaging that decreases contrast

What is Electronic Noise?

Noise from Internal Electronic signal fluctuations due to external sources

What is Structured Noise?

Tiling; outline of internal DR microchip will appear

What is anatomical Noise?

Normal patient anatomy that is present but of not of interest

What is quantum Noise?

uneven distribution of photons due to lack of uniform radiation exposure

What is signal-to-noise ratio (SNR)?

A method of describing the strength of the radiation exposure compared with the amount of noise apparent in a digital image

A high SNR = ______ Spatial Frequency and ______ Contrast

High; High

What is contrast to noise ratio (CNR)?

Measures how distinguishable structures are relative to noise

Low CNR means that ______

Tissues blend together

Which imaging modality displays the greatest contrast resolution?

MRI

What value can contrast resolution be measured in?

• Signal-to-Noise Ratio (SNR)
• Contrast-to-Noise Ratio (CNR)