Digital Imaging exam #5

Fill factor Higher fill factor, ____________contrast resolution (increased SNR) à Less exposure needed, ____________ patient dose

*Assessment of quality of a flat-panel detector

Printer

-Fine white lines à debris on mirror in the laser printer -*PRINTER... (NOT reader) à These artifacts are on the hard copy images

Look-Up Tables

...customizes the grey scale and brightness according to the specific anatomy to be demonstrated

What does DQE stand for?

...how efficiently a system converts the x-ray input signal into a useful output imagslide 39e,

Signal to Noise Ratio (SNR) ...

...how much noise can be tolerated in the image (decided by the radiologist and technologist) noise down: patient exposure up

dose creep

...the use of automatic rescaling without regard to appropriate exposure amount Dose Creep occurs when technologists begin to use more exposure than is necessary to avoid quantum mottle in their images.

what critical role does technique place

...to ensure that adequate signal reaches the detector system such that computer algorithms can be successful in making corrections and refinements to the image.

performance measurements

1)Numerical measurement - Spatial Resolution (lp/mm) - DQE 2)Observation of low-contrast objects in a contrast detail phantom

Indirect:

1)X-ray photons strike a scintillator 2)Scintillator converts photons into light 3)Light is converted into an electrical charge by a photodetector 4)Electrons stored in the capacitor within the DEL until time for line sequencing and read-out

Beam Shaping Optics

1. "Shaped" by special optics that keep the beam size, shape and speed largely independent of beam position. 2. Beam Deflector à moves laser beam rapidly back and forth across the imaging plate to stimulate the phosphors. Mirrors are used to ensure consistent beam position. 3. Light Collection Optics à Direct released phosphor energy to an optical filter and then to photodetector.

______________________________ modifies the contrast of the incoming data set according the selected_____________________during gradiation processing. _____________________ can further manipulate image contrast.

1. Automatic rescaling 2. look-up table 3. Windowing

1.Describe the basic construction of a photostimulable (PSP) cassette.

1. durable, lightweight plastic material housing 2. Antistatic Material (felt) •Protects against static electricity build-up •Dust collection •Mechanical damage to plastic 3. ^backed by thin sheet of aluminum. Absorbs backscatter.

Del level actions

1. flat panel array 2. Capacitor: stores electrical charges until read out 3. Detector Surface: capture signal Thin FilmTransistor (TFT):1) isolates each DEL2) reacts like a switch à sends electrical charges to image processor if closed circle data transmitted

five common types of PSP artifacts and their causes.

1.Imaging Plate Artifacts 2.Image Processing Artifacts 3.Plate Reader Artifacts 4.Printer Artifacts Operator Artifacts (PICNIC Errors

Charge Coupled Device (CCD)

1.XR Photons interact with scintillation material 2.Signal transmitted by lenses or fiber optics to the CCD 3.During transmission, lenses reduce image size 4.Image is transferred to small capacitor where light is converted into an electrical charge 5.Charge released sequentially line by line and sent to ADC

Describe the imaging plate size selection process. *!! THE SIZE OF THE MATRIX DETERMINES THE SIZE OF THE PIXELS

14 x 17 cassette Bigger Pixels lesser Spacial Resolution 10 x 12 cassette Smaller Pixels Greater Spatial resolution

Amorphous Selenium (a-Se thickness and detail ____________ proportional

500 um thick=radiography 250 um thick= mammo

If the system needs to display 4 lp/mm, then it must sample the image receptor at _ pixels/mm.

8

Which indirect image capture phosphor is unstructured (turbid)? A. Gadolinium Oxysulfide (Gd2O2S) B. Cesium Iodide (CsI)

A. Gadolinium Oxysulfide (Gd2O2S)

Which statement is true of Cesium Iodide detectors? A. Has a higher DQE B. Air pockets allow light to escape laterally - reducing efficiency C. Has lower spatial resolution D. Allowed for more rugged detector use in portable equipment

A. Has a higher DQE

What type of x-ray absorption material is used in direct capture systems? A. Photoconductor B. Scintillator C. Glass Substrate

A. Photoconductor

As the fill factor increases, DQE will... A. increase B. decrease C. be unaffected

A. Increase

T/F: The flat-panel array composed of DELs is the same regardless of direct or indirect image capture systems. A. True B. False

A. True

Which quality assessment measurement takes into account both the sensitive and nonsensitive portions of the DEL? A. Fill Factor B. DEL Pitch

A. fill factor

Explain the process of reading and erasing the imaging plate. Reader: Point Scan A. Single laser point radiates imaging plate B. several linear laser units and optical light collection lenses

A., Light collecting equipment photomultiplier tube

The more data in the bins, the _____ accurate representation in the image of the original object. A.more B.less C.same

A.more

What is the term used to describe the process of converting a light signal into an electrical signal? A. Quantization B. Digitization C. Electricalization D. Amplificazation

B. Digitization

Which DEL component is responsible for isolating each DEL until it is ready to send the electrical charges to the image processor? A. Detector Surface Area B. Thin Film Transistor (TFT) C. Capacitor D. Glass Substrate

B. Thin Film Transistor (TFT)

The Nyquist Theorem must be at least ___ the sampling frequency to prevent alaising. A.1x B.2x C.4x D.8x

B. 2x

What color of light is emitted when the trapped electrons relax into a lower energy level after being released from the active layer of the PSP plate? A.Red B.Blue C.Green D.Yellow

B.Blue

Which image capture system uses Amorphous Selenium (a-Se) as an x-ray absorption material? A. Direct B. Indirect

B.direct

active element of psp cassette

Barium Fluorohalide

qualities altered by final display in digital imaging

Brightness, contrast, noise, sharpness and magnification

What process discharges information from the DELs sequentially for read-out? A. Amplification B. ADC C. Line Scanning Sequence D. Scintillation

C. Line Scanning Sequence

Imaging Plate

CRACKS // ADHESIVE TAPE // STATIC // BACKSCATTER

Matching Up Exams Cassette-Based Cassette-less

Cassette-Based: •Barcode must be scanned and connected to patient position from the examination menu. •Barcode may be linked with patient information bar code on order Cassette-Less: •Match the exam to the patient through the worklist •No bar code on the plate

Digitization

Converting light signal to an electrical signal Conversion of the light emitted from the storage phosphor into an electrical signal representative of each pixel's average gray value.Sampled and digitized to specific location within the image matrix and displays specific brightness.

Which of the following is a major component of spatial resolution in CR imaging? A. Sampling Frequency B. Translation Speed C. Laser Beam Spot Size D. Phosphor Layer Thickness

D. Phosphor Layer Thickness

DEL pitch

DEL Pitch measures from the center of one DEL to the center of an adjacent DEL. Unlike Fill Factor, DEL pitch includes both sensitive and nonsensitive areas.

Why is it important to process cassettes shortly after exposure.

Deterioration begins almost immediately. Latent image loses 25% of its energy in 8 hours -

Define a thin-film transistor (TFT) flat-panel digital image detector. X-Ray Absorption Material+Flat-Panel Array

Direct -> Photoconductor -> amorphous selenium (a-Se) Indirect -> Scintillator -> amorphous silicon (a-Si)

Beam Shaping Optics, what does it do

Ensures same beam output no matter the location... Results in consistent spatial resolution and keeps the laser beam at a constant size, shape, speed, and intensity

Explain the process of reading and erasing the imaging plate. whole process

Exposure Photostimulation of Imaging Plate Enter Imaging Plate into Reader Laser excites electrons to release latent image Photodetector captures emitted light (ADC) Signal Digitized Plate Erased Preprocessing, Processing and Forwarding the Image

fast scan direction / slow scan direction

FAST SCAN DIRECTION movement of the laser across the imaging plate"scan" SLOW SCAN DIRECTION movement of the imaging plate through the reader "translation" or "sub-scan direction" slide 21, ch 34

Image Lag

Faint image from previous exposure is visible.Results from previous image's signal not getting completely erased. Trapped charges create a type of 'double exposure' artifact

AD

Imaging plate scanned with red light in a raster pattern. Laser energy needs to be high enough to energize trapped electrons à escape the active layer à emit visible blue light as they relax into lower energy levels. Lines of light intensity from imaging plate are detected by a photodetector that amplifies light and sends to an ADC

describe the process that makes indirect image capture systems different from direct image capture system

Indirect: photodetector, lights from scintillator and generates an electrical charge direct: photodiode, JOB: absorb the electrons and generate electrical charges

Nyquist Theorem More Data in our bins (more sampling frequencies)à ___________________________________of signal coming off the plate!

MORE ACCURATE REPRESENTATION

what affects spacial resolution

Phosphor Layer up, resolution up (?) Pixel Size up, resolution decreases

What is the term used to describe when light is given off when the x-ray strikes the PSP plate?

Photostimulation

Operator (PICNIC)

Problem In Chair...Not in Computer.

erasure steps

Removes Images 1. Returns electrons to lower energy state (removes image from plate) 2. Automatic process after plate is read 3. Plate flooded with light to remove any electrons still trapped from Plate 1. Erase Cycle on Reader (Special Process 2. Should be performed once a week to remove buildup of background signal and scatter Should be performed once a week to remove buildup of background signal and scatter

erasure

Returns electrons to lower energy state (removes image from plate)

dell pitch equation

SR= 1/2P

del pitch formula

SR=1/2p Spatial Resolution = 1/ 2 times the del pitch

Explain the process of photostimulation in the imaging plate.

Same processes as conventional film/screen up until image is recorded. Patient is positioned the same •Body part aligned with IR •Patient exposed using proper kVp, mAs and SID

Explain the process of reading and erasing the imaging plate. Reader: Line Scan

Simultaneous stimulation of imaging plate line by line with a scanning module that contains several linear laser units and optical light collection lenses. CCD linear array photodetector

Automatic Rescaling

Software that allows images to have uniform density and contrast, regardless of the amount of exposure.

Photostimulation

Some light is given off when XR strikes PSP -Fluorohalides luminsesce from XR interaction à -Not all of the energy is released with the light -'Trapped' energy forms latent image

Certain amount of exposure remains trapped so that the imaging plate can never be completely erased. T OR F

TRUE

Define a thin-film transistor (TFT) flat-panel digital image detector.

The absorption material is the part that decides if the IR is direct or indirect technology

Describe the imaging plate size selection process. CR Plate Selection Size

The entire cassette is read by the CR reader Smallest imaging plate possible = highest sampling rate

Digital imaging systems have greater dynamic range. T/F

True

Describe a gadoliniumoxysulphide detector.

Turbid Unstructured Air pockets allow light to escape laterally before it reaches the TFT surface, reducing efficiency and lowering spacial resolution

Image latitude

amount of error that can be made in exposure factor choice and still result in the capture of a quality image

Which alternative system has the higher fill factor? A. CCD B. CMOS

ccd

1st layer of indirect system

cesium iodine scintillator

Which alternative system is more susceptible to image noise? A. CCD B. CMOS

cmos

Which image capture system has its own individual amplifiers per DEL? A. CCD B. CMOS

cmos

If DQE increases, patient dose... A. increases B. decreases C. is unaffected

decreases

Gain Calibration

flat fielding, corrects flaws in the detector

higher fill factor = _________qde

higher

Explain the process of photostimulation in the imaging plate. where does photostimulation take place in?

imaging plate

In Signal to Noise Ratio, as the noise decreases, patient exposure... A. increases B. decreases C. is unaffected

increases

Direct, what happens @ the del level

look at slide 19

Does kVp still control image contrast?

no, kVp controls subject contrast. Procedure algorithms/LUT now control image contrast.

can automatic exposure fix too little exposure

no, quantum mottle occurs

Dead Pixels

non functioning pixels, caused by Caused by: •Dust •Scratches •Static Discharge •Chemical Corrosion •Age

Describe the construction of direct and indirect TFT flat-panel detector systems.

photoconductors = direct scintillators= indirect

Purpose of the laser

release stored electrons

the only quality that typically has not been altered by digital processing

shape distortion

Describe a thallium doped cesium iodide detector.

structured. Tubules act as a guide for light from the scintillation layer, almost no light spread, increasing efficiency, increasing spacial resolution

Image Latitude: Dynamic range

the ability of an imaging system to respond to varying levels of exposure (grays available to respond to any given exposure)

State the Nyquist Theorem.

the sampling frequency must be greater than 2x the frequency of the input signal

Direct

x ray photons -> electrical signal 1. X-ray photons absorbed by photoconductor 2. Photoconductor converts the photons into an electrical signal 3. Electrical signal is stored in the DELs 4. Information is sequentially discharged and read-out with dedicated electronics

Software programs identify and isolate ______ pixels

àINTERPOLATION METHOD "fills in" the dead pixels with information using the surrounding pixels as a guide à GAIN CALIBRATION

Describe the purpose of each layer of the imaging plate

•1 •PROTECTIVE LAYER •Thin, tough clear plastic à protects phosphor layer •2 •PHOSPHOR LAYER (Active Layer , PHOTOSTIMULABLE PHOSPHOR) •Barium Fluorohalides work here à "traps" electrons during exposure •3 •LIGHT REFLECTIVE LAYER (Some detail is lost in this step) •Sends light forward once released from reader // Black, reduce light spread •4 •CONDUCTIVE LAYER •Absorbs and reduces static electricity •5 •SUPPORT LAYER •Semi-rigid material for plate strength •6 •LIGHT SHIELDING LAYER •Prevents light from erasing data and from leaking through backing (decreasing SR) •7 •BACKING LAYER •Soft polymer à Protects back of cassette

What can't automatic rescaling correct

•Cannot change the pixel count in each grey's bin (essentially change the shape of the histogram) •Can only alter the grey value of x number of pixels in that bin •Can only align grey scale or contrast partially by aligning the high low and average values •Is able to perfectly align image brightness

Explain the application of CCD in Radiography

•Digital Fluoroscopy •Stereotactic Breast Biopsy (mammo) •Digital Mammography •Some General Radiography •Whole Body Scanner (trauma and forensic applications)

Image Processing(Algorithm Errors)

•Incorrect LUT selected •Inappropriate technique •kVp •mAs •Collimation •Grid selection Inappropriate positioning (operator error leading to an algorithm error

Describe the basic construction and function of a CMOS.

•Light photons stored in individual capacitors within each pixel. •Each pixel has own amplifier that converts light photons into electrical signal.(instead of having the amplifiers as part of the line sequencing read-out process in TFT capture) •Created voltage from amplifiers is converted by the ADC.

Low-noise, high-sensitivity amplifiers perform:

•Read-out •Amplification •Analog-Digital Conversion

Aliasing

•Sampling occurs less than twice/cycle •Results in lost information and a fluctuating signal •When the signal is reproduced, frequencies above the Nyquist frequency cause aliasing •MOIRE EFFECT (same effect as grid errors)

Explain the process of reading and erasing the imaging plate.

•Using Laser to Read the Plate •Digitizing the Signal •Erasing the Image •Preprocessing, Processing and Forwarding

Plate Reader

•White lines parallel to direction of plate travel •Double imaging plates loaded into single cassette •Insufficient erasure after overexposure •Incorrect alignment of grid lines to laser scan direction (Moiré)