Module 4 - IP&E II

three-dimensional imaging, or 3-D imaging

CT way of manipulating image data sets - body parts can be rotated, tilted or angled in every direction to make a more specific diagnosis.

Initial Scintillators used with a Photomultiplier Tube - CT

Cadmium tungstate Calcium fluoride Bismuth germinate

Phosphor Types

Calcium Tungstate Non-calcium Tungstate Rare Earth

film emulsion characteristics are determined by

Concentration of silver halides Coating weight Types and the ratio of halides Sensitizers and dyes Mixing and ripening

Characteristics of Intensifying Screens

Conversion Efficiency Absorption Efficiency Detective QUantum Efficiency (DQE)

Flat-Panel Detector Blur is due to

DEL size - contributes to the image blur present in a flat panel detector receptor. The larger DELs in a flat panel detector cause more image blur.

The actual matrix of the flat-panel detector is composed of

DELs

Geometric Blur is impacted by

Focal Spot Size Source-to-image receptor distance (SID) Object-to-image receptor distance (OID)

Higher spatial frequencies represent

a larger number of viewable objects in an image. A higher DQE, which indicates that a receptor is more efficient in converting the input x-ray signal, means lower exposure is required.

film responds best to

a limited range of exposure values.

Single emulsion films typically are used in

high detail images, such as extremity work or mammography.

Our goal is to pick an image receptor with the

highest absorption efficiency, greatest conversion efficiency and greatest DQE to achieve the lowest patient dose and the highest quality image.

The lower sampling frequency equates to a

larger pixel size and lower spatial resolution.

an increased OID

leads to greater magnification.

a decrease in OID

leads to less object magnification.

An intensifying screen composed of smaller crystals produces an image with

less blur because less light diverges from the intensifying screen crystals.

Charge-coupled Device (CCD) Characteristics

made up of the following components: Scintillator Mirror (if so equipped) Focusing Lens Integrated Circuit Device

The digital image is a

matrix of numbers, known as pixels, that corresponds to the intensity of the x-ray beam that strikes a particular area.

A digital image is composed of a series of rows and columns known as a

matrix.

absorption efficiency

measure of how efficient the phosphor is absorbing the xray photons

DEL size does not determine

radiation exposure

as the pixel size used to produce an image decreases,

recorded detail increases

reflective layer

redirects light photons from intensifying screen crystals that will NOT strike the film emulsion back toward the film surface. Reduces pt dose.

screens with high relative speeds typically

reduce or limit the maximum details that a radiographic image can record

The size of the detector element, or DEL, determines the

spatial resolution of a flat panel detector element. This isn't a receptor characteristic that a technologist can change. you cant change DEL size with the flat panel detector because it's fixed.

The photostimulable phosphor

stores the energy deposited by the x-ray beam in the plate.

Brightness in a digital image is equivalent to

"density" in an analog image.

Higher Brightness in a digital system is the same as

"insufficient density" in an analog system

Digital imaging allows up to

100 percent overexposure, above the ideal, which provides no visual cue to a technologist that the patient was overexposed. In this respect, we may be using more radiation than necessary to create images with a digital receptor, or not enough. Therefore, we need to develop an ideal exposure level so that we can use exposure latitude to produce an ideal image.

Sampling frequencies for PSP based image receptors range from

5 pixels/mm up to 20 pixels/mm.

a mottled appearance that occurs if the exposure is less than

50 percent of the desired amount. So, in essence, if the desired exposure is 10 mAs and you use 5 mAs, the image will end up mottled or noisy. Therefore, noise is the visual cue for an underexposed image.

Patient Dose and Detective Quantum Efficiency

A higher DQE, which indicates that a receptor is more efficient in converting the input x-ray signal, means lower exposure is required.

Relative speed can be increased by increasing the factor(s)

Absorption efficiency Conversion efficiency Phosphor layer thickness Phosphor packing density Phosphor crystal size Adding a reflective layer

Digital Images are Evaluated Based On

Brightness Contrast Image Blur Exposure Indicator Image Noise

film resolution

an assessment of how accurately the film is able to record the details of the structure being imaged.

Increase DQE =

Decreased Patient Dose

Direct exposure

Designed for direct exposure to x-ray photons or electrons Higher silver content

Spectral Matching - film emulsion

Designed for exposure to light from a source Blue sensitive Green sensitive UV sensitive Infrared sensitive

How a CT works

During a CT scan exposure, the scintillator-based detector converts x-ray photons into light that exits the patient. The photomultiplier or photodiode then converts the light into an electrical signal. Then the output signal from the photomultiplier tube or the photodiode is processed and the digital signal transferred to a computer to create the image. This action occurs thousands of times as the digital image forms in CT and the computer processes images, so detectors must quickly convert light photons to an electrical signal and accept the next signal.

Dye reduces the

Dye that is mixed in the phosphor materials is another aspect of intensifying screens. Divergence of the light photons emitted by intensifying screen crystals prior to reaching the film.

CT Image Reconstruction

Each x-ray acquired during a CT examination is measured as it transmits through the patient along a line from the x-ray tube to the detector array. A reference detector measures the unattenuated intensity of the x-ray beam during the scan. Each measurement is given a unique exposure quantity based on its location in the CT scanner. With this information, numerous reconstruction algorithms can be used to reconstruct the image data into a viewable image. Filtered backprojection reconstruction is the most widely used algorithm in clinical CT scanners.

The output phosphor produces light from x-ray photons. True or false

FALSE

The photocathode produces electrons from x-ray photons. True or False

FALSE

Why change to digital?

Improved structure visualization due to increased contrast resolution Increased image throughput - ability to share images between depts easily and quickly Post acquisition image processing - Post-acquisition image processing allows us to manipulate an image and view things differently. After processing an analog image, it's difficult to alter its appearance. For example, in orthopedic work technologists produce two images using film/screen to visualize both soft tissue and bones. Digital imaging requires only one image to distinguish between bone and tissue. Ability to electronically transmit images Large dynamic range - the digital image receptor responds to a wide range of exposure values to create diagnostic images. Whereas an analog imaging system requires a very precise technique. A digital system is more forgiving in terms of the technique required to produce an image of diagnostic quality.

Increase Absorption Efficiency =

Increased DQE

Increase Conversion Efficiency =

Increased DQE

CCD Advantages over Film/Screen

Increased Dynamic Range - wide range of exposure settings Increased Sensitivity - respond to lower light levels from a scintillator than other types of receptors, which makes them suitable for low dose imaging. Fast Response Times - produce an image quickly, and instantly refresh for the next image. This makes them excellent image receptors for fluoroscopic applications.

Larger Receptor Size with fixed pixel size =

Larger Matrix

Smaller Pixel Size in a fixed receptor size =

Larger Matrix

Spatial Resolution measurements

Line pair/mm or Cycles/mm

Digital Image Spatial Resolution

Measured by the size of the pixel used to create an image.

Characteristics of Photostimulable Phosphors

Phosphor Types Conversion Efficiency Absorption Efficiency System Speed/Speed Class Detective Quantum Efficiency

Relative speed decreases when

Phosphor dye is added

Digital Image Receptors

Photostimulable Phosphor Flat Panel Charge-coupled Device (CCD)

Photostimulable phosphor

Photostimulable phosphor stores the energy from its interaction with an x-ray beam. The PSP plate is found in two types of systems. In a cassette-based system, the cassette holds the PSP plate. A non-cassette based, or cassette-less, system places the PSP plate inside a large piece of equipment, such as a movable Bucky. This image shows a chest unit, or a table type device, with the PSP receptor inside an enclosed area. A technologist never touches or handles a plate in a cassette-less device. Whereas, a radiographer can open the cassette and look at a PSP plate in a cassette-based system.

Image Blur types

Receptor Blur Geometric Blur Motion Blur

In digital fluoro

Remnant radiation that has passed through the patient first strikes the input phosphor of an image intensifier. The input phosphor gives off light photons that strike the photocathode, causing electrons to be released.

Flat-Panel Receptor Characteristics

Scintillator Based (Indirect Detection) Flat-Panel Receptor Non-scintillator Based (Direct Detection) Flat-Panel Receptor

Flat-Panel Detectors types

Scintillator-based: uses amorphous silicon and a thin film transistor (TFT) array - converts the x-ray beam to light and then that light is converted into electrons to create an image. Non-scintillator based: uses amorphous selenium and a TFT array - converts the x-ray beam into electrons to create an image.

Emulsions are characterized by

Speed Contrast - average gradient Spectral sensitivity - refers to the color of light that film emulsion responds to. Crystal grain size

Phosphor dye prevents some divergent light photons from interacting with the film, so

That reduction in the amount of divergent light that strikes the film emulsion produces an image with less blur and creates more sharpness.

Analog Receptor Spatial Resolution

The ability of a system to record adjacent small objects as separate and distinct.

Spatial Resolution

The ability of a system to record adjacent small objects as separate and distinct.

TFT Assembly

The individual DEL of a TFT assembly collects electrons that represent the exposure level, or x-ray intensity, in its area of a flat-panel receptor.

Fluoroscopic flat panel

The internal operation of this receptor is identical to the one used in flat-panel radiography. In other words, that scintillator or non-scintillator based technology is used to capture and record patient diagnostic information in real-time.

Integrated Circuit Device (CCD Chip)

The light from scintillator material strikes the silicon in the CCD silicon chip. The electrons then are collected by the CCD chip elements we refer to as pixels. The number of pixels for a given chip varies by manufacturer but may contain millions. Following exposure, the electrons that the CCD captures are analyzed and assigned digital values based on how many electrons each pixel captures. These values then transfer to a computer and display on a monitor.

solid-state detectors

The newer CT scintillation detectors

Thin film transistor

The portion of the flat-panel detector receptor that is responsible for collecting individual electrons

receptor size

The receptor size is made to fill the field of view of the display monitor.

Scintillator Based Detector

The scintillator-based detector converts x-ray photons into light after they exit the patient. These detectors are assembled in a straight line array like the one shown on this page. The detectors used in CT scanning are much larger than DELs found in flat-panel detectors. Detectors used in CT scanners measure greater than 1 mm-by-1 mm, whereas a DEL measures about a tenth of that size. The x-ray beam is tightly collimated to the detector array.

fan beam - CT

The shape of this beam is due to the effects of the collimator located in front of the x-ray tube. - provides a very thin 'slice' of x-rays that form a wedge shape

System speed, or speed class

The term applied to a digital imaging system based on the amount of exposure that photostimulable phosphor type receptors receive. The terminology for this value varies by the manufacturer.

a large acceptance angle

This indicates a CT scintillator's ability to accept an x-ray not directed perpendicular to the individual detector. This capacity increases the detector's ability to absorb radiation and makes for a more responsive detector.

Charge-coupled Device

Uses a scintillator and a CCD to create the image. The CCD image receptor uses a scintillator, so it needs a material that gives off light when the x-ray beam strikes it. But it also requires using a CCD to interact with the light to create an image. Considered indirect.

photostimulable phosphor image receptor captures image data in the following manner

X-ray photon energy strikes the phosphor crystals, causing electrons within the crystals to move from their normal orbital location to a higher energy level. This action forms a latent image in the receptor. Some, but not all, of the electrons raised to a higher energy level spontaneously return to their resting energy state by emitting energy as light. A laser light source scans the receptor plate to extract image data. The laser light exposure then causes electrons at high energy states (those that form the latent image) to release their energy as light and return to a lower energy resting state. The light released from the photostimulable phosphor following laser scanning is collected and used to record the patient image. The exposed photostimulable phosphor plate goes through a final erasure step following exposure and scanning. This action causes any residual electrons at a higher energy level to return to their normal resting state following a scan.

Detective quantum efficiency (DQE)

a measure of a receptor's ability to create an output signal that accurately represents the input signal (x-ray beam)

PSP absorption efficiency

a measure of how effective the phosphor is at absorbing the x-ray photons

Conversion efficiency

a measure of how efficient the phosphor is at converting xray photons into light

Film speed

a measure of the amount of density produced for a given amount of exposure

film contrast

a measure of the film's ability to display the differences between adjacent densities within the image

CCDs need

a scintillator to produce light photons before an image is captured - May be composed of Cesium Iodide (CsI) or Gadolinium Oxysulfide (Gad2O2S)

A digital image is composed of

a series of rows and columns known as a matrix.

a smaller focal spot results in

a smaller penumbra. Therefore, less blur is created around the object and its edges are much easier to recognize.

Multi-planar Reconstruction

ability to manipulate image data to display images of the same subject in different ways without performing additional imaging. For example, due to the way data is acquired during the exam, images may be reconstructed in sagittal, coronal, axial or oblique planes.

improved structural visualization

ability to more easily view a structure inside the body using a digital imaging system

Gas filled CT detectors have a smaller

acceptance angle than scintillator based CT detectors

equalization

allows us to see structures above and below the diaphragm. Doing so on this image provides the means to assess a tube pathway through the chest and placement in the abdomen. When we work with an analog image receptor, we can't look at heart, mediastinum and lung fields to see below the abdomen in one image to verify tube placements. Hence, the benefit of digital imaging is the ability to see more aspects of an image with one exposure instead of producing multiple exposures.

Dynamic range refers to

an image receptor's ability to respond to an exposure; (contrast in digital imaging)

The lower the exposure level needed to achieve a specific optical density reflects

an increase in the receptor speed.

When comparing two receptors, the receptor with the highest DQE can produce

an optimal image with less radiation exposure.

motion blur

any motion of a part, tube or image receptor blurs an image, making it unacceptable.

Each CT gas filled detector is quite small,

approximately 1.5 mm wide. The individual detector coincides with the pixel size on the digital image. Smaller gas-filled detectors have smaller pixels.

Thicker phosphor produces an image with greater

blur because light photons can diverge, or spread out, over a greater surface area before reaching the film emulsion. This results in the blurring of object edges on film.

In digital, we lose the visual cues of

brightness and contrast that we're accustomed to in working with an analog image receptor. The end result is a tremendous potential to overexpose a digital receptor by believing that an image is acceptable when that's not the case.

How is a film turned into a digital image

by using a film digitizer, which scans an analog radiograph and assigns individual values for the various shades of gray in the image. At that point, the image has been converted to a digital image with individual values for each area instead of shades of gray.

Modern CT scanners use

calcium tungstate, gadolinium and yttrium as scintillators and a photodiode converts the light photons to an electrical signal. This increases CT detector array efficiency and improves image quality. The newer scintillation detectors are called solid-state detectors.

Gas-filled computed tomography detectors

capture the x-ray photons ionized by the gas inside a chamber as the photons exit a patient. The ionized gas produces an electron signal that produces the digital signal and creates the image. The gas used in these detector arrays is either xenon or krypton. The acceptance angle is smaller than the acceptance angle found in scintillator-based CT detectors, meaning that gas-filled detectors are less sensitive to radiation.

thin film transistors

complex circuit device that collects electrons emitted from either amorphous selenium or amorphous silicon. Within that assembly are areas referred to as DELs, or detector elements. DELs collect the electrons that represent individual components of a digital image.

PSP - a type of digital image receptor

contains photostimulable phosphors

Why is film/screen called analog

continuous series of gray that the signal records on the receptor

Intensifying Screen

converts the remnant x-ray beam exiting the patient into light, which interacts with the film to create the latent image. This film is the recording device that captures the latent image using the light given off by the intensifying screen.

scintillator

converts x-ray photons into light, which are emitted from the scintillator and interact with a photoconductive material typically made of amorphous silicon to convert the light photons into electrons. The electrons created in the amorphous silicon then migrate to thin film transistors and produce an electric signal. The illustration on this page shows the three-step process associated with the image capture that occurs in a scintillator flat-panel receptor. conversion efficiency, absorption efficiency and detective quantum efficiency. are important characteristics of any receptor we use to create an image. they affect the performance of all scintillator-based flat-panel detectors.

Increasing SID

decreases the blur in an image because the x-ray beam photons diverge less.

A decrease in conversion efficiency, absorption efficiency and detective quantum efficiency

decreases the exposure required to create an image. And in doing so, they decrease patient exposure and comply with ALARA.

Increasing conversion efficiency, absorption efficiency and detective quantum efficiency

decreases the exposure required to create an image. And in doing so, they decrease patient exposure and comply with ALARA.

characteristics of the analog image a radiographer evaluates are

density, contrast, recorded detail and distortion

cassette-based

describes the process of creating a digital image using a cassette. The cassette-based digital image receptor uses storage phosphor technology to create the image.

cassette-less

describes the process of creating an image when a technologist does not touch a cassette.

DELs

detector elements. DELs collect the electrons that represent individual components of a digital image.

Nyquist Frequency

determines the level of spatial resolution for an image receptor - theorem that states that when you determine the spatial frequency, spatial resolution, you must sample the signal at twice the desired sampling frequency. For example, if your goal is to visualize 2.5 line pairs per millimeter, then you must sample the image receptor at 5 pixels per millimeter. If you need to see 5 line pairs per millimeter, then sample the image receptor at 10 pixels per millimeter. When it samples at 20 pixels per millimeter, it resolves at 10 line pairs per millimeter. An important aspect here is that it takes longer to see an image display when sampling frequency increases because the image receptor is gathering more information.

charge-coupled device

differs from the photostimulable phosphor and the two flat panel devices that we've discussed in that it requires a scintillator or other material to produce light. A mirror directs that light onto a focusing lens and then to the CCD, or integrated circuit device.

Automatic rescaling is the reason

digital imaging systems produce images with such wide exposure variations.

Why is digital called digital

discrete values

Non-scintillator Based Flat-Panel Receptors

doesn't use a material that converts an x-ray beam into light. Instead, non-scintillator image receptors use amorphous selenium to convert the x-ray beam to electrons that thin film transistors then collect. Both scintillator and non-scintillator flat-panel receptors use thin film transistors. Note that the non-scintillator-based flat-panel receptor relies on a two-step image capture process rather than the three-step process associated with scintillator-based flat panel receptors. Once again, conversion efficiency, absorption efficiency and detective quantum efficiency are key characteristics of this image receptor type.

Dual emulsion - dual screen systems

don't record the high level of image detail of single emulsion - single screen systems. But those systems require much less exposure than a single emulsion - single screen system to produce a diagnostic image.

The range of exposure values that a receptor responds within is known as

dynamic range.

Receptor Blur cause is in the

equipment.

Lower Brightness in a digital system is the same as

excessive density in an analog system

the digital system and its response to overexposure requires

exposure greater than 100 percent to 200 percent of the desired value. Image contrast is lost when this occurs and largely depends on the examination performed.

screens with a thicker phosphor layer are classified as

faster screens -

Non-scintillator based

flat panel type: uses amorphous selenium and a TFT array. The non-scintillator based detector converts the x-ray beam into electrons to create an image.

Scintillator-based

flat panel type: uses amorphous silicon and a thin film transistor (TFT) array. A scintillator-based flat-panel detector converts the x-ray beam to light and then that light is converted into electrons to create an image.

A lens

focuses the light produced by the scintillator onto a CCD.

Pixel size is measured

from side to side within an individual pixel.

Pixel pitch measures

from the center of one pixel to the center of an adjacent pixel and includes any gaps between them.

CT scanner components

gantry

The input phosphor in digital fluoro

gives off light photons that strike the photocathode, causing electrons to be released. These electrons strike the output phosphor, causing light photons to be emitted.

CT scanners can produce images with

greater contrast than many image receptors used in diagnostic imaging.

A large focal spot produces

greater image blur than a small focal spot.a large area of penumbra created around an object, which is an area of unsharpness or 'blur.'

greater pixel density has a

greater resolution

gantry

holds an x-ray tube and a detector array that acts as the digital image receptor. The gantry can be tilted to match the patient angle or the anatomy of interest. The gantry also contains laser mounted positioning lights to properly adjust the patient on the table prior to beginning an exam.

F-trap

holds electrons elevated to a higher than resting energy level.

The size of phosphor crystals determines

how much light the crystals give off. Larger crystals produce more light than smaller crystals.

A reflective layer increases

image blur when used with an intensifying screen due to additional divergent light photons striking the film.

The sampling frequency of a PSP plate controls

image blur with the photostimulable phosphor digital receptor. If the sampling is low frequency, you'll see more blur than when sampling at a higher frequency. A low sampling frequency suffices for imaging large parts, such as a pelvis. A higher sampling frequency, on the other hand, provides greater spatial resolution for examining extremities. Otherwise, blur would obscure fine structures.

The likelihood that blur will appear in an image

increases along with the thickness of a phosphor layer.

a decrease in SID

increases both beam divergence and magnification.

Optical Density (OD)

measures a film's degree of opacity. measures the overall blackening within an image and the amount of light transmitted through the film to the viewer's eye. The greater the density, the less light is transmitted through the film to the viewer's eye.

Pixel density

measures the number of pixels contained within a unit area. Increased pixel density means that there are more pixels in a given measured area

The CT table

moves patients through the opening during an exam. The table is made of materials that support the patient while attenuating the least amount of the beam. Because the beam must penetrate the table, along with anything else on the table during the exam, it's important to clear the table of potential artifacts.

Packing density refers to the

number of crystals packed into a given area of an intensifying screen.

CT console

outside the room but within view of the patient and equipment. Most CT operations are controlled from the operator console outside the room. These operations include angling the gantry, moving the table into and out of the gantry, setting technical factors and initiating a scan. Once a patient is properly positioned on the table, there should be little or no need to re-enter the examination room.

Exposure indicators help to identify

over-exposure and under-exposure.

100% of the available diagnostic information is found in the

patient

exposure doesn't change

pixel size.

The micron (µm) measures

pixel size. 1 millimeter = 1 thousand micrometers.

The dynamic range of a receptor is

refers to an image receptor's ability to respond to different exposure levels. the number of signal values that the receptor is capable of capturing. A digital receptor responds to a tremendously large exposure range, while the analog receptor's range is more limited.

flat panel detectors are discrete elements that create image information without losing

resolution

radiography systems that image large body parts typically employ

screens with high relative speeds

exposure indicators

set of numbers that confirm that a correct exposure is used in producing a digital image. In some ways this replaces the visual cues of density and contrast that are used in analog imaging systems, and provides additional guidance to evaluate digital images.

CT Image Receptor - Advantages

significantly reduced scatter strikes the detector assembly due to the tightly collimated beam. This means that CT demonstrates differences among low contrast structures that can't be seen on digital radiographs. A cross section of an abdomen, for example, clearly demonstrates individual organs in the abdominal cavity that couldn't easily be seen on a diagnostic radiograph. The scatter produced inside a patient during a regular radiographic examination creates too much radiation for the image receptor to be able to resolve the contrast. Therefore, the viewer's eye can't differentiate among the organs of the abdomen to this extent.

extremity cassettes typically have screens with

smaller phosphor crystals and a slower speed in order to maximize image detail.

An inverse, or indirect relationship, exists between

source-to-image distance, or SID, and image blur.

Increased absorption efficiency increases

the DQE of a receptor.

PSP Conversion efficiency

the ability of storage phosphor to convert the signal exiting the pt into trapped electrons

rare earth phosphors provide

the ability to use lower kVp values and enhance the recorded image contrast.

an intensifying screen's relative speed

the amount of exposure necessary to produce an optical density of 1.0 above what is called "base plus fog.

Relative speed measures

the amount of exposure needed to achieve a specific optical density.

The number of electrons deposited in the DEL corresponds to

the amount of radiation that strikes each area.

analog image receptor has three components

the cassette, an intensifying screen and film

Characteristics of Computed Tomography

the first modality to use digital imaging when it was instituted in radiology departments during the mid 1970s. A CT scanner uses an x-ray tube, ionizing radiation and digital image receptors to digitally create a radiographic image. Those images can be electronically manipulated, stored and sent throughout the medical center just like all digital images

Image Noise

the grainy appearance of an image - too few photons interacted with the receptor to produce the image

the more pixels we sample on a plate

the greater recorded detail we can achieve with a digital image receptor

the greater the number of signal values that a receptor is capable of capturing,

the greater the receptor's dynamic range.

The lower the amount of radiation required

the higher the stated relative speed

it takes longer to see an image display when sampling frequency increases because

the image receptor is gathering more information.

The receptor with the highest DQE requires

the least exposure but may result in a greater amount of image noise.

the higher the packing density

the more light a screen produces and the faster an intensifying screen is labeled

Filtered backprojection reconstruction

the most widely used algorithm in clinical CT scanners.

using a higher sampling frequency

the pixel size is smaller and we increase the spatial resolution

Relative speed is determined by

the radiation needed to produce a predetermined film density, typically a density of 1.0 + Base + Fog, for a given screen compared to a standard or 'par speed' screen/film combination. The lower the amount of radiation required - the higher the stated relative speed. One way to change the speed of a screen is to adjust the phosphor layer thickness. the thicker plate produces more light than the thinner plate. Because the thicker plate produces more light, less radiation is required to produce a given density on a film. Therefore, screens with a thicker phosphor layer are classified as faster screens.

film latitude

the range of exposures over which a film is able to produce an image with the desired diagnostic density

exposure latitude refers to

the range of exposures that will create an optimal image.

Exposure latitude

the range of under- or overexposure that can occur in producing an acceptable image. The ultimate goal is to create an ideal image at the lowest possible level of patient exposure.

How frequently the signal extracted from the PSP is sampled determines

the recorded detail

Image blur

the result of motion during the examination.

How dyes work

the screen phosphor layer absorbing some light prior to the photons reaching the film surface. The downside is that phosphor dyes lower the relative speed of a given film/screen system, but they also provide greater sharpness, or resolution, in images. Phosphor dyes also are used to tailor the spectrum of light the screen delivers to a film's surface. You may be familiar with film/screen systems that rely on green light sensitive or blue light sensitive film. Intensifying screen phosphor layer dyes help match the light output of an intensifying screen to the spectral sensitivity of a radiographic film.

The brightness of a digital image is the equivalent of

the term density used to describe the appearance of analog images.

The major difference between a CT scanner and other types of digital image receptors

the way an image is acquired and analyzed after it's acquired. The x-ray beam used to create the CT image is a tightly collimated fan beam that rotates 360 degrees around the patient. A detector assembly collects the x-ray photons that exit the patient during this beam rotation. an x-ray tube rotates around the patient positioned in the middle of the rotational area. The detectors are located on the exact opposite side of the patient from the x-ray tube. This is a normal configuration for any general radiographic procedure. The x-rays pass through the patient and are collected by an image receptor. In a live demonstration of a CT scanner's abilities, you would see two other significant differences from general diagnostic imaging besides the fact that the tube moves. First, there is a thin beam of x-rays flowing from the x-ray tube and through the patient to strike the detectors. The shape of this beam is due to the effects of the collimator located in front of the x-ray tube. This collimator limits the size of the x-ray beam to a very thin 'slice' of x-rays that form a wedge shape. The shape of the beam in this type of CT scanner often is referred to as a 'fan beam.' The second quite noticeable difference on this image compared to a general radiograph is the resulting image. In a CT scan, the x-ray's path coupled with the detector array results in a transverse image of the patient's anatomy. As we will see later, a computer can reconstruct digital image data to provide virtually any type of desired projection. The detector assembly can be one of two types ─ either a scintillator-based or gas-filled detector.

Both scintillator and non-scintillator flat-panel receptors use

thin film transistors.

Indirect Flat-Panel Receptor (Scintillator Based)

three-step process associated with scintillator-based flat panel receptors.

two phosphor types available for photostimulable phosphors

turbid - phosphor crystals distributed throughout a phosphor layer. needle, or columnar - phosphor crystals arranged as needles or columns. - The chemical makeup necessary to growing needle, columnar, phosphor differs from turbid phosphor. Needle crystals result in less light divergence when stimulated compared to the turbid phosphor material. You may hear the term "light pipes" associated with needle phosphors.

scintillator-based flat-panel image receptor

use cesium iodide, which comes in a columnar needle formation, or gadolinium oxysulfide, as scintillators. The gadolinium oxysulfide comes in a turbid formation.

The exposure indicator is what provides

useful feedback to the technologist regarding exposures delivered to the image receptor Over- or underexposure = incorrect exposure indicator Correct exposure = correct exposure indicator

The way a CCD device forms an image

very different from how either a PSP or flat-panel image receptor creates images. The CCD system requires a scintillator, which may be either cesium iodide or gadolinium oxysulfide. Scintillators convert x-ray beams into light that strikes a mirror, which directs it to a focusing lens then onto the charge-coupled device. The charge-coupled device converts that pattern of light to an electric signal that's sent to an analog-to-digital converter. This creates a binary code of digital values that becomes a data set to form a digital image.

In systems that require maximum detail

we typically use intensifying screens with a low relative speed

quantum noise is the only visual cue that indicates

when the digital receptor is underexposed.

if you increase the DEL size,

you decrease spatial resolution.