Image Acquisition Exam 2 (Workbook)

Workbook

CR (Computed Radiography) is defined by the image receptor having no __ connection to processor - The cassette must be physically _ to and inserted in the reader/processor

Direct, carried

DR (Direct-capture Digital Radiography) doesn’t require the IR to be carried to a processor separate from the exposure unit - Rather, the IR is directly __, either electrically or electromagnetically (by radio waves) to the processor

Connected

For DR, the radiographic table or chest board houses a permanent array of miniature radiation detections. The fixed nature of this receptor system limits flexibility in __

Positioning

What are the 2 main advantages of DR

Its high compatibility with PACS systems and increased departmental efficiency for work flow

The typical size of detector elements for a DR system is about __ microns or _

100 microns or 1/10th a mm (1/10th the size of a pinhead)

DR systems are based upon an __ of microscopic pixel elements each containing its own transistor

Active-matrix array

The term pixel refers to elements of a visual image or picture, and is not appropriate for __ elements that are components of the DR image receptor

Detector

A voxel is a volume element within the

Patient

A __ is a detector element in the IR

Del

A pixel is a __ element in the final image

Picture

To form a digital radiograph, information from different voxels within the patient is collected by the dels of the imaging machine and computer-processed to become the __ of the final image

Pixels

Dels apply to both forms of __ imaging (direct and indirect capture)

DR

Pixels apply to the light image emitted by the __ plate in a CR reader prior to computer processing

Phosphor

What are the 3 components of a DR del

Semiconductor detection surface

Microscopic capacitor

TFT (thin film transistor for electrical gate switching)

An active matrix __ is a panel made up of hundreds of detector elements, each del containing its own thin film transistor (TFT)

Array

Some have come to refer to dels as TFTs, and active-matrix arrays as __ arrays. This is inaccurate nomenclature

TFT

The TFT is but one __ of each del - a “gate” through which electrical charge held by the del is discahrged out into the system upon readout of image data

Componet

The __ detection surface of a del can be primarily sensitive to either xrays (if made of selenium), or light (if made of silicon)

Semiconductor

The efficiency factor for any type of detector element is measured by physicists as its __

Detective quantum efficiency (DQE)

The percentage of a dels square area devoted to the semiconductor detection layer is called the dels __ factor

Fill

A higher fill factor provides (better or worse) contrast resolution and spatial resolution

Better

Because the capacitor and TFT cannot be shrunk, smaller dels have a lower fill factor, which then necessitates an increase in

Radiographic technique

The heart of the del is the microscopic electronic __. The amount of charge stored on it will eventually be represented as the pixel value for each pixel

Capacitor

In each del, a thin film transistor which acts as a switching __ to release the built up electrical charge when the del is read out

Gate

In a direct-conversion DR system, reach xray or light ray penetrating into a dels layer of amorphous selenium will ionize one of its molecules, creating an __ pair

Electron-hole

While the top of electrode in the del attracts freed electrons to drift upward, a negative charge placed on the del electrode below causes the positively charged __ to drift downward

Holes

Positive __ from the lower electrode is accumulated in the capacitor

Charge

In the active matrix array, gate lines are controlled by the __ driver, which controls the order in which the dels are read out

Address

When a bias voltage is applied, the TFT “gates” open up sequentially and cause the stored-up charge from each del in succession to flow out down a __ line to an amplifier, which then sends it to the computer

Data

Indirect conversion DR systems, developed before direct conversion systems, use amorphous __ rather than amorphous selenium, because the dels must absorb light rather than xray photons

Silicon

For indirect conversion DR systems, the active-matrix array is overlaid with a __ screen made of cesium iodide or gadolinium oxysulfide

Phosphor

The indirect conversion DR systems phosphor screen __ or fluoresces when exposed to x-rays, emitting light that will strike the TFT detectors below

Scintillates

Although vertical crystals in the phosphor form light __ that confine with dispersion of the light somewhat, the resolution achieved is still not as good as with the direct conversion system

Channels

Amorphous or non-crystalline forms of silicon and selenium are used for the TFT detectors because they can be coated onto the active-matrix array in finely controlled __

Thickness

In comparing the two approaches, we find that the direct conversion system produces higher __ in the image but the indirect system results in less _

Spatial resolution, patient dose

The CR cassette is designed in most respects to be used just as screen __ were used for film-based radiography and has a somewhat similar construction

Cassettes

The active phosphor later of CR imaging plate is supported by a firm base, usually made of __

Aluminum

An anti-halo layer added to the phosphor prevents the particular color of light used by the laser beam scanning the plate from __ through to the reflective layer while allowing light emitted by the phosphor itself to pass through

Penetrating

Both the front and back panels of the CR cassette are lined with what material that minimizes the build-up of static electricity and cushions the plate from minor jolts

Felt

For most CR systems, the phosphor plate housed in the cassette has only one single emulsion surface and must be placed facing __ in the cassetee

Forward

CR phosphors use one of a small number of barium-fluorohalide compounds which possess a unique property called __ phosphorescence

Stimulated

A latent image is stored by the CR phosphor plate in the form of electrons that are trapped in __ centers of barium flurohalide crystals doped with europium

F

Electrons trapped in F centers are freed from their atoms during an exposure when xrays __ the atoms

Ionize

The photostimulable plate actually glows __ times: When do those happen?

2 times

1st during xray exposure as some electrons immediately fall back into an atomic shell

2nd when re-stimulated by the laser beam in the CR reader (processor)

The immediate emission of light under stimulation is called

Fluorescence

The delayed emission of light sometime after initial stimulation is called

Phosphorescence

Only a very small percentage of the electrons becomes trapped in F centers, and the glow of the phosphor under laser stimulation is very __ and must be amplified by the electronics in the CR reader

Dim

An electron trap or F center is actually an additional __ band added between the valence band and the conduction band of atoms in certain molecules

Energy

Technically, while the conduction band is considered to be “outside” the atom, this new band is a metastable energy state that resides __ the atom. This is the F center

Within

The CR reader (processor) uses suction cups to remove the exposed phosphor plate from its cassette, then moves it by a series of __ through the different sections in the processor

Rollers

Unlike the elements of a DR detector plate, the laser beam of the CR reader is __ in shape, so the laser spot must overlap pixels that are being recorded as the plate is read

Circular

As the phosphor plate moves through the CR reader, the crosswise direction in which the laser beam scans across the plate is called

Fast scan direction

The direction that the phosphor plate itself is moving through the CR reader is called the

Subscan or slow-scan direction

Light emitted by the phosphor plate is changed into an electronic signal by a __ tube

Photomultiplier

The PM tube is a __ plate attached to an electronic amplifier

Photocathode

The photocathode is a layer of material which releases electrons when light strikes it, through the __ effect

Photoelectric

The amplifier section of a PM tube consists of a series of __ plates, which can be switched back and forth between positive and negative charge to continuously accelerate and multiply the electron stream

Dynode

The PM tube avoids picking up reflected light from the laser beam in 2 ways: First, the tube is sensitive to a different __ of light (blue-violet) and second, the PM tube is positioned at a different angle than any reflection of the laser beam

Color

The wavelengths of the red laser beam and the emitted blue phosphor light are far enough apart that there is effectively an optical __ between them, preventing the PM tube from detecting reflected laser light as noise

Barrier

After scanning, the phosphor plate moves into the __ section of the processor, where bright white light is used to completely remove any residual image

Eraser

Each exposure made on a CR phosphor plate must be identified with the appropriate __ information before the image is read in the processor

Patient

CR plates are approximately __ times more sensitive than the older film/screen cassettes to scatter and background radiation accumulated during storage

10

Radiographers should be careful to erase any cassette prior to use if there is any chance it has been in storage for __ days or more

2

Although digital processing has a remarkable ability to clean up fog densities caused during exposure, it is not able to correct fog densities caused __ to exposure during storage

Prior

“Pre-fogging” a cassette prior to use adds to the overall total amount of __ the computer must deal with, which can cause rescaling or graduation errors if it becomes overwhelming

Noise

Fog densities between fields from placing multiple exposures on one plate can lead to processing errors due to __ failure

Segmentation

In the bin of a mobile xray machine, care must be taken to regularly __ the cassettes

Rotate

In a CR reader, the only way to increase the sampling frequency (the number of samples taken per line) is to reduce the __ between them

Distance

For DR systems, this image sampling frequency depends only upon the del __, defined as the distance from the center of one del to the center of an adjacent del

Pitch

Del pitch is approximately equal to __ size

Del

Pixel pitch is approximately equal to __ size

Pixel

The __ the del or pixel size, the higher the spatial resolution

Smaller

Technically, for DR detectors, the del pitch includes any __ between dels

Spaces

The simplest definition for an image detail is the __ between two pixels with different values (ex. white and black)

Edge

It takes a maximum if __ pixels (with different values) to make a “detail”

2

The number of resolvable details is __ the number of available pixels

1/2

To measure sharpness, we use the unit of spatial frequency: Line pairs per millimeter, abbreviated __

LP/mm

With a pixel width of 0.1 mm, the spatial frequency is __ LP/mm - This is 5 details per millimeter for each row of pixels

5

The maximum spatial frequency is expressed by this formula

SR = 1/ 2 P

Since del pitch is essentially the same as the width of each del, the maximum spatial frequency is also inversely proportional to a doubling of the __ of the dels in millimeters

Width

A del pitch of 0.05 mm would yield __ LP/mm maximum spatial frequency

10

What is the spatial frequency for an imaging system with a del pitch of 0.03 mm

17

There are several different types of matrices:

The __ matrix of dels in a DR detector

The _ image matrix created by a CR reader that is sampling a PSP plate

The _ matrix array of hardware pixels (dots) in a display monitor

The _matrix of the displayed image itself

Hardware, light, hardware, light

The size of the matrix and the field of view can affect sharpness only is they alter the size of the actual __ or _

Pixels, dels

For any hardware matrix, the size of the dels or hardware pixels is fixed and therefore not subject to __. Therefore, the spatial resolution for these devices is also _ and does not change

Change, fixed

DR detector elements do not change in size regardless of detector plate __ or _ of the field

Size, collimation

DR dels range in size from __ microns

100-200

A 100 micron del produces a SF of about __ LP/mm

5

For a DR detector, the inherent spatial resolution is __

Consistent

In an LCD display monitor, each hardware pixel is composed of the intersection of 2 flat, transparent __ that conduct electricity

Wires

For a particular manufacturer and model, the __ of these pixels is fixed

Size

The hardware pixel size sets the __ resolution with which any image can be displayed on that brand of monitor

Maximum

For a particular brand, a larger monitor will have more pixels, but the size of the pixels is the __

Same

Sharpness is therefore __ with different size monitors

Consistent

For a DR detector, collimation of the xray beam results in a __ anatomical area being recorded on the detector plate. This results in a restricted (although magnified) _ displayed on the monitor

Smaller, field of view

Nonetheless, the inherent sharpness of DR is not changed by collimation, because __ size is unchanged

Del

For a display monitor, magnification of the image results in a __ anatomical area being displayed. Again, this is a restricted FOV displayed on the monitor

Smaller

Nonetheless, the inherent sharpness of the display monitor itself is not changed, because __ pixel size is unchanged

Hardware

To summarize, the hardware elements in DR detectors and display monitors have a fixed size, so these devices have consistent inherent spatial resolution regardless of changes to __ size or _

Matrix, FOV

A __ matrix is a matrix that can be changed within the physical area of the display

Soft