Digital Imaging Receptors - Q1

analog

film-screen imaging is called ________ imaging because the physical image varied continuously and was proportional to the intensity of the x-ray beam striking it

analog signal

this is the continuous wave of electrical current (raw data) used to send data thru a circuit board or wire, it cannot be seen with your eyes; it is the temporary electrical signal from each detector showing the exposure values that will tell the pixels how brightly to glow in the image on the monitor

digital

both computed radiography (CR) & direct radiography (DR) are _______. DR image receptors use electronic detectors to convert x-ray energy into electrical (analog) signal. This raw data is then converted to binary language (a matrix of bits) for processing. The image on the monitor is made of a continuous physical pattern of light representing the data & is thus considered an analog

computed radiography

cassette based systems that used storage scintillation phosphors to temporarily store energy made by the exposure (latent image); data was extracted in a CR reader to get the manifest image to appear on a monitor

direct radiography (DR)

no extra processing step to see the image; replaced cassettes w/ imaging detector arrays that capture, read, & display on a monitor

indirect digital capture radiography

converts x-rays to visible light using a scintillator, then to electrical signals, which may reduce spatial resolution

direct digital radiography

converts x-rays directly into electrical charges, preserving higher spatial resolution by avoiding scintillation

scintillation

materials like cesium iodide or gadolinium oxysulfide convert x-ray energy into light first, then electrical signal

1. FPD-TFT

2. CCD

3. CMOS

3 types of indirect capture collection IRs

charge-coupled device

CCD

charge-coupled device (CCD)

a light-sensitive integrated circuit that collects light, converts it to electrical charge related to the exposure and stores it in a capacitor; also used in digital cameras

complementary metal-oxide semiconductor system

CMOS

complementary metal-oxide semiconductor system (CMOS)

converts light into electrical signal using photosensitive pixels, then to digital with on-chip circuitry (each has its own amplifier, photodiode, storage capacitor, & transistors)

flat plate detector w/ thin film transistor

FPD-TFT

flat plate detector w/ thin film transistor (FPD-TFT)

a TFT array consists of semiconductors on a glass substrate on which a thin layer pf photodetector (a-silicon) is painted to collect light (capture element) and convert it to electrical charge in the capacitor. The TFT transmits the analog signal to the ADC. This is what modern diagnostic systems use

fiber optics or lenses

light is transmitted via ________ or _______ to the CCD sensor underneath

CCD sensor converts the light to electrical charge & transfers the charge across the sensor in a sequential manner to the analog-to-digital convertor (ADC) to be digitized

CCD charge transfer & digitization

analog-to-digital convert

ADC

• require a separate ADC

• limited in size

• slower to process

• bulkier

• potential geometric distortion & resolution loss

CCD system limitations

hygroscopic

cesium iodide (CsI) is _________: it absorbs water so it must be sealed well

crystaline silion (c-Si)

in CMOS, each sensor has its own photodetector, ____________ to convert the light to electrical signal, stores it in capacitors & digitize it (no separate ADC is needed)

• efficiency & speed benefits: rapid image requisition & works well with AEC due to pixel-level processing

• limitations in Size & Sensitivity: smaller size & lower light sensitivity limit CMOS detectors’ spatial resolution in large-field imaging compared to CCD

• Clinical Usability & Applications: CMOS detectors are suited for smaller receptors & specialized imaging (dentistry & mammo)

about CMOS detectors

1. DELs/pixels collect light from the CsI & convert it to electrical signal

2. sent thru channels to be converted to digital signal, which requires a separate ADC

How CCDs work

1. each DEL/pixel amplifies the light from the crystalline Si

2. converts it to electrical charge & then to digital signal- no separate ADC (analog to digital converter) is required

how CMOS work

• all of the pixels can be devoted to light capture

• the output’s uniformity (a key factor in image quality) is high

• very light sensitive, better fill factor, responds to low light intensitites

• wide dynamic range

CCD advantages

• unequal response due to tiling

• CsI is hygroscopic (absorbs moisture)

• expensive to manufacture

• consumes a lot of power

CCD disadvantages

• each pixel has a digital conversion to output digital bits (no ADC necessary)

• less power consumption

• less expensive to manufacture

• work well with AEC

• fast acquisition times

CMOS advantages

• reduced area available for light capture

• uniformity is lower

• less light sensitivity & resolution

• more noisy

CMOS disadvantages

flat panel detectors (FPD)

are IRs made up of a detector matrix coated with photoconductor (direct) or scintillator (indirect) on a glass substrate

detector elements (DELs)

the detector matrix contains a photodiode that absorbs electrons & stores electrical charges. _______ are different than pixels but function similarly as data captured is specific to position & exposure value in the IR or monitor

thin film transistors (TFTs)

are electronic switches located on each DEL that capture & store the charge, & then send charges to the computer in an orderly fashion

1. capture element

2. storage capacitor

3. TFT switch

each DEL has 3 components:

capture element

either a photoconductor/photodiode like a-Se (direct) or photodetector like a-Si (indirect)

storage capacitor

to store the electrical charges

TFT switch

release the electrical charges to the ADC

fill factor

the percent (%) of a DELs area that can detect x-ray exposure, so larger fill factors are preferred. The detection area of a DEL is made smaller by the capacitor & TFT

reduces

bc the capacitor & switch are fixed in size regardless of the size of the DEL, reducing the DEL size _______ the total fill factor %

better

when reducing DEL size, smaller DELs have less exposure detection areas but have _______ spatial resolution

CsI or GdOS; CsI

scintillators in indirect FPD-TFT systems are made up of __________; convert x-ray photons to light, _______ has less spread & is preferred

amorphous silicon (a-Si)

beneath the scintillator, in indirect FPD-TFT systems, __________ photodetectors are painted in a thin layer directly on the DELs to convert light into electrical charges stored in capacitors for image readout

thin film transistors (TFTs)

____________ are layered onto a glass substrate & act as switches to read stored charges row by row, to send to the ADC (indirect FPD-TFT system)

• image quality

• cost

• size

• ** most common clinical radiography detectors

indirect FPD-TFT systems clinical & educational relevance

• NO scintillator: no light spread so preserves the fine image details for highest spatial resolution

• Direct conversion technology: Direct FPD-TFT systems convert x-rays directly to electrical charges using an amorphous selenium (a-Se) as a photoconductor for uniform detection of exposure over a large surface area

• Top Bias Electrode Layer: forces the electrons created in the a-Se down to the TFTs where the charge is stored in capacitors

• Charge Collection & Readout: an electric field guides electrical charges to capacitors, which are read out by TFT arrays for digitization and processing the same Indirect FPD-TFT receptors

• Clinical Importance: Direct DR systems offer superior image quality critical for applications like mammo with minimal resolution loss

Direct FPD-TFT systems

spatial resolution

the sharpness & accuracy of the anatomic structural lines; is characteristic of a digital system & equal to ½ the Nyquist frequency

Nyquist frequency

the highest spatial resolution (# of lp/mm) that a digital detector can capture w/ out distortion; determined by DEL spacing of DR systems

modulation transfer function

MTF

modulation transfer function (MTF)

a system’s ability to display contrast of objects of varying size as a function of spatial resolution; this is most commonly used to express the ideas expression of digital detector image resolution

limiting spatial resolution

LSR

limiting spatial resolution (LSR)

detector’s ability to resolve small structures, depends on the contrast of the target, exposure & display conditions so not as accurate as MTF

pixel

picture element of the system (CR) & display monitors

pixel pitch

distance btw 1 pixel or DEL & the center pf adjacent pixel (measured in microns)

DEL

detector elements in IR; one TFT = 1 pixel

matrix

rows & columns (grid) of pixels in a digital image area; this is fixed in the devices we are using (IR, monitor, etc.)

voxel

CT volume element, determined by the size of pixel & thickness of slice

FOV (field of view)

in CR the entire IP was read but in DR, only the exposed DELs are part of the image produced; collimated image

• the smaller the focal spot → the sharper the image

• minimize OID & maximize SID

• changes in distance require adjustment in mAs

• accurate positioning of the part & proper alignment of the part with AEC (if used) also remains critical

• pay attention to the EI values as an indicator of proper exposure

• review factors that may result in a histogram analysis error/collimation/positioning

• evaluate proper positioning and tube-part-receptor alignment

Spatial Resolution: Geometric Factors