10/25 - Digital Detectors, Image Quality & Processing

Direct digital radiography (DR)

Converts x-ray photons directly into electrical charge without first producing light.

Direct DR detector material

Amorphous selenium (a-Se).

Direct DR sequence

X-rays → a-Se → electrical charge → TFT readout → ADC → digital image data.

Indirect digital radiography (DR)

Converts x-ray photons into light and then converts the light into electrical charge.

Indirect DR detector materials

A scintillator converts x-rays to light, and amorphous silicon (a-Si) photodiodes convert the light to electrical charge.

Scintillator

Converts x-ray photons into visible light.

Photodiode

Converts light produced by the scintillator into electrical charge.

Thin-film transistor (TFT)

Functions as an electronic switch that releases stored charge from each detector element for readout.

Analog-to-digital converter (ADC)

Converts the analog electrical signal into digital numerical data.

Indirect DR sequence

X-rays → scintillator → a-Si photodiode → electrical charge → TFT readout → ADC.

Computed radiography (CR)

Uses a photostimulable phosphor (PSP) imaging plate to store the latent image.

Photostimulable phosphor (PSP) plate

Stores energy from the x-ray exposure until stimulated by a laser in the CR reader.

CR reader laser

Stimulates the PSP plate, causing stored energy to be released as light.

Photomultiplier tube (PMT)

Detects and amplifies light released from the PSP plate and converts it into an electrical signal.

CR readout sequence

PSP plate → laser stimulation → light emission → PMT → electrical signal → ADC → digital image.

Pixel

The smallest picture element in a digital image.

Matrix

An arrangement of pixels in rows and columns that forms the digital image.

Field of view (FOV)

The physical area of anatomy represented in the digital image.

Pixel size = FOV ÷ matrix size.

Pixel size formula

Pixel size decreases and spatial resolution improves.

What happens when matrix size increases while FOV remains constant?

Spatial resolution improves.

What happens when pixel size decreases?

Spatial resolution

The ability of an imaging system to display small, closely spaced structures as separate objects.

Spatial frequency

Describes the size and spacing of image detail and is commonly expressed in line pairs per millimeter (lp/mm).

High spatial frequency

Represents small, closely spaced structures or fine detail.

Low spatial frequency

Represents larger structures or coarse detail.

The largest lp/mm value.

Which lp/mm value represents the highest spatial frequency?

Modulation transfer function (MTF)

Measures how accurately an imaging system transfers object detail and contrast into the recorded image.

High MTF

Indicates better preservation of image detail and better spatial resolution.

MTF of 1.0

Represents perfect transfer of object contrast and detail.

MTF generally decreases.

What happens to MTF as spatial frequency increases?

Detective quantum efficiency (DQE)

Measures how efficiently an image receptor converts incoming x-ray information into a useful image signal.

High DQE

Indicates greater detector efficiency and the ability to maintain image quality with less receptor exposure.

DQE generally decreases.

What happens to DQE as spatial frequency increases?

Signal-to-noise ratio (SNR)

Compares useful image signal with unwanted image noise.

High SNR

Indicates strong useful signal compared with noise and produces better image quality.

Quantum noise (quantum mottle)

Grainy image appearance caused by an insufficient number of x-ray photons reaching the detector.

Increase receptor exposure appropriately, most directly by increasing mAs.

How can quantum noise be reduced?

SNR increases because more photons reach the detector and quantum noise decreases.

What happens to SNR when mAs increases?

Scatter radiation

X-ray photons that change direction after interacting with matter and may reach the image receptor as unwanted exposure.

Scatter versus quantum noise

Scatter is unwanted redirected radiation; quantum noise results from too few photons reaching the detector.

Dynamic range

The range of exposure values that an image receptor is capable of detecting and recording.

Exposure latitude

The range of exposure techniques that can still produce an acceptable diagnostic image.

Dynamic range versus exposure latitude

Dynamic range describes what the detector can record; exposure latitude describes how much exposure can vary and still produce a usable image.

Exposure indicator (EI)

A numerical value that reflects the amount of radiation reaching the image receptor.

Deviation index (DI)

Indicates how far the actual detector exposure differs from the target exposure.

DI of 0

Actual detector exposure equals the target exposure.

Positive DI

Detector exposure is greater than the target exposure.

Negative DI

Detector exposure is less than the target exposure.

Histogram

A graph showing the distribution of pixel or exposure values within a digital image.

Histogram X-axis

Represents pixel values or signal intensity values from low to high.

Histogram Y-axis

Represents the frequency, or number of pixels, at each value.

Histogram analysis

The computer evaluates the distribution of pixel values to identify the useful exposure data for image processing.

Automatic rescaling

Adjusts acquired pixel values so the displayed image has appropriate brightness despite variations in receptor exposure.

Look-up table (LUT)

Assigns processed pixel values to displayed gray shades and determines the appearance of image contrast.

Histogram versus rescaling versus LUT

Histogram analyzes the pixel values; rescaling adjusts the values; the LUT determines how those values are displayed as gray shades.

Digital image brightness

Primarily controlled by image processing and display settings rather than directly by mAs.

mAs in digital imaging

Primarily affects photon quantity, detector exposure, SNR, quantum noise, and patient dose.

mAs.

A digital image appears grainy because too few photons reached the receptor. Which technical factor should be increased?

DQE.

Which detector characteristic describes how efficiently x-ray photons are converted into useful image information?

Spatial resolution.

Which image-quality characteristic describes the ability to display small structures separately?

MTF.

Which image-quality characteristic describes how well object detail is transferred to the image?

Dynamic range.

Which term describes the range of exposure values a detector can record?

Exposure latitude.

Which term describes the range of techniques that still produces an acceptable image?

Y-axis.

A histogram question asks how many pixels contain a specific signal intensity. Which axis represents this?

X-axis.

A histogram question asks for the range of signal intensity values. Which axis represents this?

LUT.

Which processing function maps pixel values to displayed gray shades?

Automatic rescaling.

Which processing function helps maintain relatively consistent displayed brightness despite exposure differences?

Detector underexposure relative to the target.

A negative deviation index indicates what?

Detector overexposure relative to the target.

A positive deviation index indicates what?