Digital imaging ch. 22/23/24

FLAT-PANEL DETECTOR EVOLUTION

Referred to as Digital Radiography (DR)

Introduced in 1995

Manufacturers striving to improve quality and functionality

Flat panel detector is commonly used term

• “panel”

Direct detector configuration

Use a photoconductor (convert x-ray photons into an electronic signal)

indirect detector configuration

Uses scintillator (converts incoming x-ray photons to light)

direct vs indirect conversion diagram

INDIRECT DETECTORS (TWO STEP PROCESS)

1. Use a scintillator to convert x-ray photons to light first

2. Light energy converted to electronic signal

Electronic signal collected by two possible methods

• Thin Film Transistor (TFT)

• Charge-Coupled device (CCD)

INDIRECT DETECTORS

Amorphous silicon requires a scintillator

Scintillation material of two possible types

• Cesium iodide (CsI)

• Gadolinium oxysulfide (Gd2O2S)

Isotropic light emission

layers of digital detector

ELECTRONIC SIGNAL CAPTURE

Employs any of three (3) technologies

• Thin film transistor (TFT)

• Charge-coupled device (CCD)

• Complimentary metal oxide semiconductor (CMOS) (not used)

Designed to convert light energy into electrical signal

• Transmission along detector circuit pathways

• Analog-to-digital convertor (ADC)

what does TFT stand for

THIN FILM TRANSISTORS

THIN FILM TRANSISTORS (TFT)

• Used in both indirect and direct detectors for Electronic readout of signal

• Collected as an array or matrix of pixel-size detector elements (DEL’s)

• Pixel size directly related to DEL size

• Readout is pixel-by-pixel and column by column basis

TFT FILL FACTOR

• Fill factor is determined by percentage of DEL consisting of sensing material

•  80 % fill factor then 20% covered by electronics.

• Fill factor affects image quality

• Higher fill factor = higher (better) spatial and contrast resolution (absorbing more photons)

higher fill factor=more pixels waiting for signal

what does CCD stand for

CHARGE-COUPLED DEVICE

CHARGE-COUPLED DEVICE (CCD)

• CCD is a photodetector (gathers signal)

• Released line by line to the ADC (gets send to here after 1st step)

• Electronic signal then sent to computer for processing

what does CMOS stand for

COMPLEMENTARY METAL OXIDE SEMICONDUCTOR

COMPLEMENTARY METAL OXIDE SEMICONDUCTOR (CMOS)

• Similar to a CCD, attached to scintillator material

• CMOS sensors are more susceptible to noise when compared to CCD

• Light sensitivity tends to be lower.

not used anymore

DIRECT DETECTORS

• Use amorphous selenium (a-Se) as photoconductor

• No scintillating layer

• Direct action of x-ray photons converted to electronic signal in a-Se layer

• Charges collected by TFT for readout

best DQE (sensitivity to radiation)

IMAGE ACQUISITION, PROCESSING, AND DISPLAY

• Regardless of conversion type, result is a latent image that needs to be read out.

• Readout completed in a logical sequence

• One difference only exposed elements are used for image (only area that is gathering photons, that we collimate to, gets processed and shows up on the image)

• Image processing similar to CR processing

• DR panels capable of pre-processing and post-processing

• Pre-processing permits DEL calibrations

- Image Display is the final step just like CR

Portable detectors

• Wireless

• Or Hard-wired with tether cable

Integral detectors

• Typically, hard-wired to radiographic equipment

ex. wall Bucky

AUTO-DETECTION TECHNOLOGY

DR panels typically require a communication with x-ray generator (need to select correct detector, need to activate it)

Interface is vendor-specific

• Can be expensive

• Limits versatility of panel usage in various rooms

Newer DR panels eliminate the traditional generator interface

• Employ new “auto-detection” technology

• Commonly referred to as “trigger panels”

Panels are wireless

No true generator interface (not plugged in)

TFT is charged by on-board capacitor prior to exposure (on imaging plate)

Presence of x-radiation hitting panel “triggers” TFT to collect signal

Exposure terminated by manual timing or automatic exposure control (AEC) (machine stops once it gets proper EI number)

Data read-out completed; panel recharges for next exposure

DR PANEL PRECAUTIONS

Respect the level of sophistication a DR panel represents

• Far more than a cell phone

Damaged panels sent to manufacturer for analysis

• DR panel “autopsy”

Fluid Invasion

• Encase in protective bags

Weight load restrictions and bending

• Standing feet, portable exams on obese patients, etc

Avoid panel dropping

• Drop protection insurance

Clean panels according to manufacturer recommendations

Newer panels have less weight and improved durability

It is your responsibility to know and understand DR panel restrictions from the manufacturer

Digital Exposure Considerations

•Traditional parameters of radiographic image formation still apply

•SID, OID, SOD, Inverse Square Law

•Focal spot size

•kVp

•mAs

•Higher mA, shorter exposure times

•Tighter Collimation (better image)

•Grids

•Filtration

•Patient size and condition

•Old rules of kVp and contrast do not apply, in the traditional sense

•mAs and image density no longer applicable

•Total amount of exposure to the detector is converted to electronic “SIGNAL”

-machine knows what to use, quits when done

Higher kVp’s now compared to analog imaging

• Due to higher k- edge of detector 15% Rule of kVp/mAs 

Increase of kVp values reduce entrance skin exposure (ESE) to patient, and lower mAs values

mAs values that are too low introduce quantum mottle (noise)

what does SID stand for

source to image distance

what does OID stand for

object to image distance

what does SOD stand for

source to object distance

15% rule of kVp/mAs

if you want your image to change, you need to increase by 15% kVp or 50% mAs

Exposure Technique Systems

Exposure technique systems as important as ever

Image quality is referenced to an Exposure Indicator #

EI# can vary between manufacturers/rooms

Departments must equip staff with both exposure technique charts and target EI#s

Target EI#s established as a standard of care

Two distinct technique systems

• Fixed-kVp, variable mAs

• Variable kVp, fixed mAs

Fixed kVp system best suited for digital receptors

Fixed kVp System

• Requires an optimum kVp for various body parts

• Ensures desired image contrast

• mAs adjustments based upon the optimum kVp

• mAs changes increments of 30%

• Every 5 cm thickness change requires a 30% change in mAs (increase/decrease)

Variable kVp System

• Requires a threshold kVp

• Typically 30 kVp

• kVp change for each 2-cm change in part thickness

• (2 kVp × part cm) + 30 kVp = new kVp

• 15% Rule applies with this system

Establishing Technique System

• Done as a Five (5) Step Process

• Test exposures using a phantom Consult QC tech and /or Rad. for acceptability

• Create trial exposure chart using range of large and small sizes

• Place technique chart into clinical trial

• Clinical fine tuning of trial chart exposures, Referencing

clinical images and part measurements

• Re-examine chart for accuracy and continue making adjustments over extended period

• Image acceptability primarily based upon image noise

• EI #s used as a reference point

Exposure Techniques

• Image noise directly related to exposure

• Digital post-processing software more tolerant of over exposure

• Underexposure has less exposure latitude

• Dose Creep! (overexposing still looks ok)

• Technologists should employ “the highest kVp within the optimal range for the position and part, coupled with the lowest amount of mAs as needed to provide an adequate exposure to the image receptor (ASRT 2012 Best Practices In Digital Radiography)

Techniques Systems and ALARA

• ALARA compliance based upon Target Exposure Index (EIT) values

• Target values need to be established by department

• Deviation Index (DI) Values based upon EIT

• EI# and DI# can be displayed as part of DICOM header on images

Deviation Index (DI)

• Created by AAPM

• Establishes a range of EI# variances based upon EIT

• Scale in increments

• +25 on positive side of zero (overexposure) 25% increase is +1

• −25 on negative side of zero (underexposure) 25% underexposed is -1

• DI values +3 or greater considered ALARA violation and cause for concern (75% overexposed)

Exposure Factors and DR Image Quality

Image quality a function of several factors

• Noise, artifacts, resolution, and DQE

Scatter/secondary production primarily a function of mAs, rather than kVp

Underexposure can produce photon starvation

Extreme overexposure can yield data drop due to DEL (detector element) saturation

Assessing DR Image Quality

Look at image in “totality”

• Positioning and relevant anatomy on image

• Quantum Noise or mottle

• Collimation

• Motion distortion

• Resolution

• Brightness/contrast

• Artifacts

Use EI# as frame of reference for exposure to detector

If you are satisfied with the image appearance and the EI# supports the look of the image, feel comfortable in sending image for interpretation

Digital Processing Considerations

• From Analog to Digital re: image density

• Grossly overexposed images can still look good!

• Quantum mottle

• Minimum Exposure Index

Benefits to Digital Processing:

• increased latitude (can see a lot more shades of gray)

• increased margin for error

Electronic Masking

• Post-processing function

     • Manual or automatic

     • Removes nonclinical data from image file

• Also known as “cropping”

• Intended to eliminate extraneous brightness from image edges

• Reduces veil glare (white around the edges)

• No impact upon image resolution

• NOT a SUBSTITUTE for collimation

• Can impact accuracy of EI# in older CR/DR systems

• Incidental findings on images, important to radiologist

• ASRT Position Statement on Electronic Masking

• “that a digital image should not be cropped or masked such that it eliminates areas of exposure from the image that are presented for interpretation” (ASRT 2015).

Electronic Annotation

• Used to indicate image acquisition conditions

• upright, decubitus, inspiration, expiration, semi-erect, etc.

• ABSOLUTELY not a substitute for lead markers to indicate laterality (Lt versus Rt)

• Convenient “cut and paste” feature

• Valuable for radiologist

• ASRT Position Statement

• As a best practice, the ASRT White Paper recommends “consistently using lead anatomic side markers captured on the original image during the x-ray exposure” (2012).

Tenets of Radiologic Images

Medical radiographs are considered a legal document just as all contents of a patient’s medical record.

A medical image of a patient is a pictorial record of the patient’s anatomy and

medical condition.

A medical image is an image of the patient at a single moment in the patient’s medical timeline.

The accuracy of medical image interpretation is a function of the quality of the image created and includes technological and human components being optimized.

Radiologists expect that department routines and procedures are followed when creating medical radiographs, and any variance explained completely.

Radiologists assume that images are produced in an ALARA-compliant manner.

Diagnostic Yield

important information on the req. Varies between imaging modalities

diagnostic efficacy

how well our images represent the patient

image fidelity

LCR

Low-contrast resolution

LCR and Artifacts

• Due to spectral response of digital detectors

• Wider dynamic range of photon energies detected by receptor

• Yields additional image gray shades that can be very subtle

• Produces artifacts on DR images

• Clothing, hair braids, sheets, transfer devices, positioning aids, skin keloids, etc.

• Unexpected radiographic findings need an explanation to radiologist

Data Drop Artifacts

• Due to DEL saturation and excessive exposure

• Produces misrepresentation of patient information on image

• Several correction methods

• Post-processing and pixel recovery

• Tissue bolusing

• Collimation (not masking)

• Compensating filtration

Digital Artifacts

All image artifacts need to have an explanation and mechanism for removal or elimination

Image “ghosting or phantom image (can still see previous image under new image)

Electronic artifacts

Scratches or tears

Digital

Artifacts

Opaque spots on image (dust)

White lines

Light spots on images

Drop out (pixels drop out)

Backscatter fogging (quantum mottle)

Reticulation

CR transport artifacts

Informatics

• Devices and processes

Platforms

• Particular device to pursue informatics

BMI

Biomedical informatics

• Platforms used for medical purpose applications

PACS

picture archiving and communications system

HIS

Hospital information system

RIS

Radiology information system

EMR

Electronic medical record

EHR

Electronic Health Record

PACS Network

• Separate PACS network allows for large data files to be moved quickly

o LAN uses Ethernet

• Bandwidth of 10 megabits per second (Mbps)

o WAN slower

PACS Difficulties

LARGE FILES

LIMITED BANDWIDTH

SPECIAL LANGUAGE

PROBLEMS INTERFACING WITH HIS

HIS Hospital information systems

Database containing all patient medical record information except for radiology

RIS Radiology information systems

Radiology-specific database

Electronic Health Records (EHR)

• Electronic version of an individual patient’s collection of medical documents

• Patient portal

• Seamless stream of patient information that is shared in a secure fashion by healthcare entities

• Integrated with PACS, HIS and RIS in a holistic approach to patient care

• Vendor neutral archive (VNA)

VNA

vendor neutral archive

DICOM

digital imaging and communications in medicine

DICOM Standards

• Originated in 1980sby ACR and National Electrical Manufacturers Association (NEMA)

• Standard format for communicating imaging files around the world

NEMA

National Electrical Manufacturers Association

HIPAA

Health Insurance Portability and Accountability Act

• Medical information must be encrypted

confidentiality

HL-7

• Provides standards of interoperability between stakeholders

IHE

Integrating Healthcare Enterprise

Hardware

Mainframe, central processing unit (CPU), memory, input devices, output devices, BUS

Software

Computer programs, applications, operating systems (OS)

LAN

Local Area Network

WAN

Wide Area Network

Hard-copy image display

Images on film

Laser printers and dry processors

Soft-copy display

Flat screen monitors

Monitor quality greatly affects interpretation

Must be assessed regularly using approved QC methods

•resolution, luminance, contrast, bit depth, uniformity, and glare

CRT

cathode ray tube monitor

LCD

liquid crystal display monitor

LCD Flat Panel Monitors

• Light source shines on individual pixels

• Liquid crystal and hydrogenated amorphous silicon TFT s

•  Located between glass plates

Data Storage

Image storage requirements are huge

Images typically archived 5–7 years (Pediatric and litigation images indefinitely)

Terabyte storage a typical requirement (1 Tb = 10,000 Gb, 10,000,000 Mb)

Storing and sharing medical images done via cloud computing

Data Compression effectively manages data volumes in radiology (Lossless versus lossy compression)

Short-term storage

• Local hard drive

• PACS server and redundant array of independent discs (RAID)

Long-term storage

• Jukebox of disks or tapes (not used anymore, use cloud)

Workflow and Security

Understanding clinical informatics workflow

Computerized provider order entry (CPOE)

Clinical decision support system (CDSS)

Medical record & image privacy is critical

EHR security must adhere to National guidelines

CPOE

computerized provider order entry

CDSS

clinical decision support system