Ch.4 Image Acquisition And Evaluation

Front 1

The amount of blackness on a given area of a radio- graphic image (brightness)

Back 1

Density

Front 2

controls the current through the x-ray tube and, consequently, the number of x-rays produced at the anode; also controlled by window level

Back 2

Density is directly controlled by mAs—mAs

Front 3

Controls the energy of x-rays produced; also has a direct impact on density, although not proportional; governed by the 15% rule

Back 3

kVp

Front 4

Affects density through the inverse square law

Back 4

SID

Front 5

Absorb scatter radiation; this has an impact on the density of the imag

Back 5

Grids

Front 6

Limits the size of the x-ray beam; affects density by reducing the amount of scatter produce

Back 6

Beam restriction

Front 7

Has some effect on density when a large field is being exposed

Back 7

Anode heel effect

Front 8

Few gray tones, mainly black and white image; may also be referred to as short-scale contrast; produced at lower kV

Back 8

High contrast

Front 9

Many gray tones on image; may also be referred to as long-scale contrast; produced at higher kV

Back 9

Low contrast

Front 10

what is controlled by kVp—kVp controls differential absorption of the x-ray beam by the body because of its control of x-ray beam energy

Back 10

Contrast

Front 11

Ultimately controlled by window width and bit dep

Back 11

contrast

Front 12

Controls image blur; a small focal spot should be used whenever possible to keep x-rays coming off the anode from the smallest possible are

Back 12

Focal-spot size

Front 13

what Controls magnification and image blur; the longer the SID, the better the recorded deta

Back 13

SID

Front 14

any misrepresentation of an anatomic structure on an image receptor that alters its size and/or shape

Back 14

Distortion

Front 15

— magnification; caused by excessive object-to- image distance (OID) or insufficient source-to-image distance (SID)

Back 15

Size distortion

Front 16

Lead strips separated by aluminum interspacer

Back 16

Construction:

Front 17

The height of the lead strips divided by the distance between the lead strips: grid ratio 1⁄4=H

Back 17

Grid ratio

Front 18

Number of lead strips per inch (or centimet

Back 18

Grid frequency

Front 19

Lead strips are parallel to one anoth

Back 19

Linear grids

Front 20

Lead strips are angled to coincide with divergence of the x-ray bea

Back 20

Focused grids

Front 21

Decreased density along the periphery of the film caused by absorption of image-forming ray

Back 21

Grid cutoff

Front 22

Distance at which focused grid may be used (also called focal distance or focal range

Back 22

Grid radius

Front 23

Measure of grid’s ability to en- hance contras

Back 23

Contrast improvement factor

Front 24

Expressed as the ratio of primary radiation trans- mitted through the grid to secondary radiation transmitted through the gri

Back 24

Grid selectivity

Front 25

Amount of mAs exposure increase necessary to compensate for the absorption of image-forming rays and scatter in the cleanup process (also called Bucky factor)

Back 25

GCF

Front 26

Do not move during the exposur

Back 26

Stationary grids

Front 27

Reciprocate (move back and forth) during expo- sure; eliminate the visibility of grid li

Back 27

Moving grids

Front 28

Result is normal density in the middle of the radiograph with decreased density on the side

Back 28

Upside-down grid

Front 29

Result is image-forming rays absorbed all across the radiographic field, with cutoff (decreased density) visible over the entire radiograp

Back 29

Off-level grid

Front 30

Cutoff visible, more to one side of the radiograp

Back 30

Lateral decentering

Front 31

Normal density in the middle of the ra- diograph with cutoff visible on the sides

Back 31

Grid-focus decentering

Front 32

Radiation protection through decreased repeat exposures; more consistent image quality

Back 32

Primary purpose

Front 33

1) fixed kVp–variable mAs, (2) vari- able kVp–fixed mAs, (3) variable kVp–variable mAs

Back 33

Types of technique charts

Front 34

produced when the analog signal is sent through an analog- to-digital converter (ADC) to convert information into numerical data

Back 34

DR

Front 35

Uses an IP made of a PSP; absorbs energy exiting the patient; IP is scanned with a laser; energy released is converted from an- alog to digital for postprocessing

Back 35

CR

Front 36

Convert exit radiation directly into electrical charges, speeding image constructio

Back 36

DR direct detectors

Front 37

Convert exit radiation first to visible light and then to electrical charge

Back 37

DR indirect detectors

Front 38

Digital image: Composed of rows and columns called a _____

Back 38

matrix

Front 39

Picture element; smaller part of a digital image; is a shade of gray representing a vox

Back 39

Pixel

Front 40

Volume element; an area of tissue in the patien

Back 40

Voxel

Front 41

Shows the radiographer the distribu- tion of pixel values (indicating low, proper, or high expos

Back 41

Histogram (graphic display)

Front 42

Used by the com- puter to reconstruct the image specific to the examination that was performe

Back 42

Processing algorithm (mathematical formula)

Front 43

Subtraction, edge enhancement, contrast enhancement, black/white reversa

Back 43

Postprocessing

Front 44

Level of detail or sharpness on the CR imag

Back 44

Spatial frequency resolution

Front 45

Histogram of brightness level values from image acquisition that can be used to correct or enhance luminance v

Back 45

LUT

Front 46

Source of noise in the image; caused by too few x-ray photons hitting the image recepto

Back 46

Quantum mottle (“grainy” or “noisy” image)

Front 47

May be caused by dust on the IP; improper use of grids may cause a Moire ́ pattern; scatter radiation is easily imaged in digital imagin

Back 47

Digital image artifacts

Front 48

Standard protocol used for blending PACS and various imaging modaliti

Back 48

DICOM

Front 49

layers have the same structure as single crystals over short distances but are less ordered over larger distances...

provide uniform x-ray detection over the large areas needed by flat- panel x-ray detectors...

Direct-conversion detectors use this...

The a-Se can be deposited onto amorphous-silicon TFT arrays...

Back 49

Amorphous selenium (a-Se)

Front 50

make flat-panel detectors possible...

Early semiconductor technology required single-crystal silicon, which limited the size of electronic devices to the largest single crystal that could be grown....

has enabled fabrication of flat-panel thin-film transistor (TFT) arrays large enough to be used as the basis for all flat-panel x-ray detectors....

Back 50

Amorphous silicon (a-Si)

Front 51

Software function maps the gray scale to the values of interest (VOI) in the histogram.

This feature provides image brightness that is at a prescribed level over a large range of exposure.

With some digital systems the image brightness will be consistent for a 50 to 100 change in exposure.

Back 51

Automatic rescaling (Auto Ranging, rescaling, scaling, normalization)

Front 52

The available gray scale for image acquisition and display.

is equal to 2n, where n is the number of bits.

cannot be changed after equipment is purchased and is a vendor-specific system characteristic

(i.e., 8 bits 1⁄4 256 shades of gray, 10 bits 1⁄4 1024 shades of gray, 12 bits 1⁄4 4096 shades of gray).

Back 52

Bit depth

Front 53

A photographic detector. None are in use except for intraoral dental imaging.

Back 53

Complementary metal-oxide semiconductor (CMOS)

Front 54

The smallest exposure change (signal difference) that can be captured by a detector.

limited by the dynamic range and the quantiziation (number of bits per pixel) of the detector.

Increased ____ is considered one of the major advantages of digital receptors

counteract the lower spatial resolution of many digital systems.

Back 54

Contrast resolution

Front 55

An indicator of the potential “speed class” or dose level required to acquire an optimal image.

obtained by comparing the image noise of a detector with that expected for an “ideal” detector having the same signal-response characteristics.

The only source of noise in an ideal detector results from the incident x-ray quantum statistics.

Back 55

Detective quantum efficiency (DQE)

Front 56

describe the useful image acquisition area of an imaging device

Cassette-less digital systems have a fixed OV, which makes some projections difficult, while cassette-based CR systems have flexible FOVs like screen/film.

Back 56

Detector size or field of view (FOV)

Front 57

the smallest resolvable area in a TFT- or CCD-based digital imaging device

Back 57

Detector element (DEL)

Front 58

1. Made of polyester base and emulsion composed of silver halide crystals suspended in gelatin 2.Speedandcontrastoffilmarebasedonthesizeofthesilverhalide crystals and thickness of the emulsion

3. Film should be stored at temperatures no greater than 68 F to 70 F with humidity 40% to 60%

4. Stored film should be protected from radiation, fumes, outdating, and light and placed on end

5. Care must be taken when handling film to prevent pressure marks and static

Back 58

Film

Front 59

1. Made of polyester base with an active layer containing phosphors that produce visible light when struck by x-rays

2. Normally mounted inside the cassette in pairs for use with double- emulsion film

3. Contact between the screens and the film must be perfect; poor contact results in localized loss of recorded detail and localized blurring on the image

4. Poor film-screen contact may be verified by radiographing a wire-mesh screen

Back 59

Intensifying Screens

Front 60

1. Developer solution in developer tank converts exposed silver bromide crystals (latent image) to black metallic silver (visible image)

2. Fixer solution in fixer tank clears and removes unexposed silver bromide crystals, stops development, and hardens the emulsion 3. Water in the wash tank removes chemicals remaining on film; film is dried in dryer system

Back 60

Automatic Processing and Quality Assurance

Front 61

Moves film through the processor and agi- tates the chemistr

Back 61

Transport system

Front 62

Adds fresh developer and fixer solution for each film as it is fed into the processo

Back 62

Replenishment system

Front 63

Agitates and filters developer solution; stabilizes solution temperature; prevents stratification of chemi- cals in the tan

Back 63

Recirculation system

Front 64

Dries film at approximately 120 F after it leaves wash ta

Back 64

Dryer system

Front 65

The range of exposures over which a detector can acquire image data.

The increased allows more anatomical structures to be captured during an exposure.

Typical will respond to exposures as low as 100 mR and as high as 100mR.

Back 65

Dynamic range

Front 66

The range of receptor exposures that provides a quality, low-noise image at an appropriate patient exposure consistent with ALARA.

Back 66

Exposure latitude

Front 67

A measure of the ability of the imaging system to preserve signal contrast as a function of the spatial resolution. ]

Every image can be described in terms of the amount of energy for each of its spatial frequency components

describes the fraction of each component that will be preserved in the captured image

the ideal expression of the image quality provided by a detector.

Back 67

Modulation transfer function (MTF)

Front 68

The highest spatial frequency that can be recorded by a digital detector.

determined by the pixel pitch.

half the number of pixels/mm.

Back 68

Nyquist frequency

Front 69

An electronic element that converts light into charge. With indirect TFT detectors this is ac- complished by a light-sensitive amorphous silicon photodiode on top of the TFT array

Back 69

Photodiode

Front 70

Amorphous selenium TFT detectors

converts x-ray energy directly to charge. This charge can be directly “read out” by the TFT array.

not necessary with a-Se detector

Back 70

Photoconductor

Front 71

A material that absorbs x-ray energy and re- emits part of that energy as visible light.

indirect TFT flat-panel detectors use

Two modern are cesium iodide and gadolinium oxysulfide.

Back 71

Scintillator

Front 72

Noise, especially quantum noise, ultimately limits our ability to see an object’s edge (signal difference);

describe the edge conspicuity of a particular object under well-defined exposure conditions.

measure of the efficiency in which the incident exposure is preserved in an image

Back 72

Signal-to-noise ratio (SNR)

Front 73

A process whereby several flat-panel detectors are joined to obtain one larger detector.

results in segments that have unequal response requiring flat- field correction for flat-panel detectors.

Back 73

Tiling

Front 74

controls the number of electrons passing from cathode to anode in the xray tube

Back 74

mAs

Front 75

controls the quantity of xrays produced at the anode

Back 75

mAs

Front 76

controls the amount of radiation exiting the xray tube

Back 76

mAs

Front 77

any combination of mA and time that produce the same mAs value result in the same density on the radiograph

Back 77

reciprocity law

Front 78

directly affects density, although not in a directly prop. relationship

Back 78

kVp

Front 79

if SID is doubled intensity _____ four times

Back 79

decreases

Front 80

with SID, if distance is halved intensity ____ four times

Back 80

increases

Front 81

what is the density maintenance formula

Back 81

mAs 1 / mAs 2 = SID 1 ^2 / SID 2^2

Front 82

this controls differential absorption of the xray beam by the body because of its control of xray beam energy

Back 82

kVp

Front 83

as kVp increases, contrast __________.

as kVp decreases, contrast __________.

Back 83

decreases, increases

Front 84

less scatter fog from grids result in ____ gray tones. which increase contrast

Back 84

fewer

Front 85

contrast ______ as filtration increases.

Back 85

decreases

Front 86

used to indicate the increase in mAs needed when converting from nongrid to grid status

Back 86

grid conversion factor