RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

IAEAInternational Atomic Energy Agency

RADIATION PROTECTION INDIAGNOSTIC AND

INTERVENTIONAL RADIOLOGY

L 20: Optimization of Protection in Digital Radiology

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

IAEA 20: Digital Radiology 2

Topics

Introduction

Basic concepts

Relation between diagnostic information

and patient dose

Quality Assurance


Overview

• To become familiar with the digital imaging techniques in projection radiography and fluoroscopy, to understand the basis of the DICOM standard and the influence of the digital radiology on image quality and patient doses


Part 20: Digital Radiology

Topic 1: Introduction



Transition from conventional to digital radiology

Many conventional fluoroscopic and radiographic equipment have recently been replaced by digital techniques in industrialized countries

Digital radiology has become a challenge which may have advantages as well as disadvantages

Changing from conventional to digital radiology requires additional training


Transition from conventional to digital radiology

Digital images can be numerically processed This is not possible in conventional radiology!!.

Digital images can be easily transmitted through networks and archived

Attention should be paid to the potential increase of patient doses due to tendency of: producing more images than neededproducing higher image quality not

necessarily required for the clinical purpose


Radiation dose in digital radiology

Conventional films allow to detect mistakes if a wrong radiographic technique is used: images are too white or too black

Digital technology provides user always with a “good image” since its dynamic range compensates for wrong settings even if the dose is higher than necessary


What is “dynamic range”?

Wide dose range to the detector, allows a “reasonable” image quality to be obtained

Flat panel detectors (discussed later) have a dynamic range of 104 (from 1 to 10,000) while a screen-film system has approximately 101.5


Characteristic curve of CR system

HR-IIICEA Film-Fuji Mammofine

CR response

Air Kerma (mGy)

0.001 0.01 0.1 1

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Intrinsic digital techniques

• Digital radiography and digital fluoroscopy are new imaging techniques, which substitute film based image acquisition

• There are intrinsic digital modalities which do not have any equivalent in conventional radiology (CT, MRI, etc).


Digitizing conventional films

Conventional radiographic images can be converted into digital information by a “digitizer”, and therefore electronically stored

Such a conversion also allows some numerical post-processing

Such a technique cannot be considered as a “ digital radiology” technique.



Topic 2: Basic concepts



Analogue versus digital

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Digital: A given parameter can only have discrete values

Analogue: A given parameter can have continuous values

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What is digital radiology?

In conventional radiographic images, spatial position and blackening are analogue values

Digital radiology uses a matrix to represent an image

A matrix is a square or rectangular area divided into rows and columns. The smallest element of a matrix is called ”pixel”

Each pixel of the matrix is used to store the individual grey levels of an image, which are represented by positive integer numbers

The location of each pixel in a matrix is encoded by its row and column number (x,y)


Different number of pixels per image: original was 3732 x 3062 pixels x 256 grey levels (21.8 Mbytes). Here, resized at 1024 x 840 (1.6 MB).


Different number of pixels per image: original was 3732 x 3062 pixels x 256 grey levels (21.8 Mbytes). Here, resized at 128 x 105 (26.2 kB).


Different number of pixels per image: original was 3732 x 3062 pixels x 256 grey levels (21.8 Mbytes). Here, resized at 64 x 53 (6.6 kB)


The digital radiology department

In addition to the X-ray rooms and imaging systems, a digital radiology department has two other components:

A Radiology Information management System (RIS) that can be a subset of the hospital information system (HIS)

A Picture Archiving and Communication System (PACS).


DICOM

• DICOM (Digital Imaging and Communications in Medicine) is the industry standard for transferal of radiological images and other medical information between different systems

• All recently introduced medical products should therefore be in compliance with the DICOM standard

• However, due to the rapid development of new technologies and methods, the compatibility and connectivity of systems from different vendors is still a great challenge


DICOM format images:

Radiology images in DICOM format contain in addition to the image, a header, with an important set of additional data related with:

the X ray system used to obtain the image the identification of the patient the radiographic technique, dosimetric details,

etc.


Digital radiology process

Image acquisition Image processing Image display

Importance of viewing conditions

Image archiving (PACS) Image retrieving

Importance of time allocated to retrieve images


RadiotherapyDepartment

Outline of a basic PACS system


Image acquisition (I):

Phosphor photostimulable plates (PSP).

• So called CR (computed radiography)

• Conventional X-ray systems can be used

Direct digital registration of image at the detector (flat panel detectors).

• Direct conversion (selenium)

• Indirect conversion (scintillation)


Computed Radiography (CR)

• CR utilises the principle of photostimulable phosphor luminescence

• Image plate made of a suitable phosphor material are exposed to X-rays in the same way as a conventional screen-film combination

• However unlike a normal radiographic screen, which releases light spontaneously upon exposure to X-rays, the CR image plate retains most of the absorbed X-ray energy, in energy traps, forming a latent image


A scanning laser is then used to release the stored energy producing luminescence

The emitted light, which is linearly proportional to the locally incident X-ray intensity over at least four decades of exposure range, is detected by a photo multiplier/ADC configuration and converted to a digital image

The resultant images have a digital specification of 2,370 x 1,770 pixels (for mammograms) with 1,024 grey levels (10 bits) and a pixel size of 100 mm corresponding to a 24 x 18 cm field size

Computed Radiography (CR)


The principle of PSP

Excitation Storage Emission

CB

Trap

ADCPMT


(Images courtesy of AFGA)

PSP digitizer

Cassette and PSP

Workstation


(Images courtesy of GE Medical Systems)

Digital detector


Image acquisition (II)

Other alternatives are:

Selenium cylinder detector (introduced for chest radiography with a vertical mounted rotating cylinder coated with selenium)

Charge Coupled Devices (CCD) The image of a luminescent screen is

recorded with CCD cameras or devices and converted into digital images


Digital fluoroscopy

• Digital fluoroscopic systems are mainly based on the use of image intensifiers (I.I.)

• In conventional systems the output screen of the I.I. is projected by an optical lens onto a film. In digital systems the output screen is projected onto a video camera system or a CCD camera

• The output signals of the camera are converted into a digital image matrix (1024 x 1024 pixel in most systems).

• Typical digital functions are “last image hold”, “virtual collimation”, etc.

• Some new systems start to use flat panel detectors instead of image intensifier.



Topic 3: Relation between diagnostic information and patient dose



Image quality and dose

• Diagnostic information content in digital radiology is generally higher than in conventional radiology if equivalent dose parameters are used

• The wider dynamic range of the digital detectors and the capabilities of post processing allow to obtain more information from the radiographic images


Tendency to increase dose ?

In digital radiology, some parameters that usually characterize image quality (e.g. noise) correlate well with dose

For digital detectors, higher doses result in a better image quality (less “noisy” images)

Actually, when increasing dose, is the signal to noise ratio which is improved

Thus, a certain tendency to increase doses could happen specially in those examinations where automatic exposure control is not usually available (e.g. in bed patients).


Computed radiography versus film screen

• In computed radiography (CR) the “image density” is automatically adjusted by the image processing, no matter of the applied dose.

• This is one of the key advantages of the CR which helps to reduce significantly the retakes rate, but at the same time may hide occasional or systematic under or overexposures.

• Underexposures are easily corrected by radiographers (too noisy image).

• Overexposures cannot be detected unless patient dose measurements are performed


Underexposure results in a “too noisy” image Overexposure yields good images with

unnecessary high dose to the patient Over range of digitiser may result in uniformly

black area with potential loss of information

Exposure level 2,98 Exposure level 2,36


Exposure level 1,15 Exposure level 1,87

An underexposed image is “too noisy”


Exposure level

Some digital systems provide the user with a so called “exposure level” index which expresses the dose level received at the digital detector and orientates the operator about the goodness of the radiographic technique used

The relation between dose and exposure level is usually logarithmic: doubling the dose to the detector, will increase the “exposure level” to a factor of 0.3 = log(2).


Risk to increase doses:

The wide dynamic range of digital detectors allows to obtain good image quality while using high dose technique at the entrance of the detector and at the entrance of the patient

With conventional screen film systems such a choice is not possible since high dose technique always results in a “too black” image.


Digital fluoroscopy:

In digital fluoroscopy there is a direct link between diagnostic information (number of images and quality of the images) and patient dose

Digital fluoroscopy allows producing very easily a great number of images (since there is no need to introduce cassettes or film changers as in the analogical systems).

As a consequence of that: dose to the patient is likely to increase without any benefit


Difficulty to audit the number of images per procedure

• Deleting useless images before sending them to the PACS is also very easy in digital fluoroscopy

• This makes difficult any auditing of the dose imparted to the patient

• The same applies to projection radiography to audit the retakes.


Actions that can influence image quality and patient doses in digital radiology (1)

• Ask for a significant reduction of noise (detector saturation in some areas, e.g. lung in chest images)

• Avoid bad viewing conditions (e.g. lack of monitor brightness or contrast, poor spatial resolution, etc)

• Improve insufficient skill to use the workstation capabilities to visualize images (window level, inversion, magnification, etc).



• Eliminate post-processing problems, digitizer problems, local hard disk, fault in electrical power supply, network problems during image archiving etc.

• Avoid loss of images in the network or in the PACS due to bad identification or others

• Reduce artifacts due to incorrect digital post-processing (creation of false lesions or pathologies)



• Promote easy access to the PACS to look previous images to avoid repetitions.

• Use easy access to teleradiology network to look previous images.

• Display dose indication at the console of the X ray system.

• Availability of a workstation for post-processing (also for radiographers) additional to hard copy to avoid some retakes.


Influence of the different image compression levels

Image compression can:• influence the image quality of stored images in the

PACS • modify the time necessary to have the images

available (transmission speed in the intranet)

A too high level of image compression may result in a loss of image quality and, consequently, in a possible repetition of the examination (extra radiation dose to the patients)


Digital radiography: initial pitfalls (1)

• Lack of training (and people reluctant to computers)

• Mismatching of image density on the monitor and dose level (and as a consequence, to increase doses).

• Lack of knowledge of the viewing possibilities on the monitors (and post-processing capabilities).

• Drastic changes in radiographic techniques or geometric parameters without paying attention to patient doses (image quality are usually good enough with the post-processing).


Digital radiography: initial pitfalls (2)

• The radiologist advice on the image quality should be taken into consideration before printing the images

• Lack of a preliminary image visualization on the monitors (made by the radiologist) may result in a loss of diagnostic information (wrong contrast and window levels selection made by the radiographer)

• The quality of the image to be sent (Tele-radiology) has to be adequately determined , in particular when re-processing is not available



Topic 4: Quality Assurance



Important aspects to be considered for the QA programs in digital radiology(1)

• Availability of requirements for different digital systems (CR, digital fluoroscopy, etc).

• Availability of procedures avoiding loss of images due to network problems or electric power supply

• Information confidentiality

• Compromise between image quality and compression level in the images

• Recommended minimum time to archive the images


Important aspects to be considered for the QA programs in digital radiology(2)

• Measurement of dosimetric parameters and records keeping

• Specific reference levels

• How to avoid that radiographers delete images (or full series in fluoroscopy systems)

• How to audit patient doses


Displaying of dose related parameters (1)

• Medical specialists should take care of the dose delivered to the patients referring to the physical parameters displayed (when available) at the control panel level (or inside the X-ray room, for interventional procedures)

• Some digital systems offer a color code or a bar in the previsualization monitor. This code or bar indicates the operator whether the dose received by the detector is in the normal range (green or blue color) or whether it is too high (red color).


• Example of bar in the image showing the level of dose received by the digital detector


Displaying of dose related parameters (2)

• The use of the radiographic and dosimetric data contained in DICOM header can also be used to auditing patient doses

• If radiographic (kV, mA, time, distances, filters, field size, etc) and dosimetric data (entrance dose, dose area product, etc) are transferred to the image DICOM header, some automatic on-line or retrospective analysis of patient doses can be performed and assessed against the image quality.


Reference levels

• In digital radiology, the evaluation of patient doses should be performed more frequently than in conventional radiology:• Easy improvement of image quality

• Unknown use of high dose technique

• Re-assessment of local reference levels when new digital techniques are introduced is recommended to demonstrate the optimization of the systems and to establish a baseline value useful for future patient dose assessment


Initial basic quality control

• A first tentative approach could be:• to obtain images of a test object under different

radiographic conditions (measuring the corresponding doses)

• to decide the best compromise considering both image quality and patient dose aspects


TOR(CDR) plus ANSI phantom to simulate chest and abdomen examinations and to

evaluate image quality

Optimisation technique


Optimization technique for Abdomen AP

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High cont. (n) Low cont. (n) Resol. (lp/mm)

Simulation with TOR(CDR) + ANSI phantom

81 kVp, 100 cm (focus-film distance)

1.6 mGy


Optimisation technique for Chest PA

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High cont. (no.) Low cont. (no.) Resol. (lp/mm)

Simulation with TOR(CDR) + ANSI phantom

125 kVp, 180 cm (focus-film distance)

* Grid focalised at 130 cm

0.25 mGy


Exam. Type Resolution

(lp/mm)

Low contrast sensitivity threshold

High contrast sensitivity threshold

Conv 2.50 7 9 Abdomen

CR 3.15 9 9

Conv 3.55 8 6 Chest

CR 2.24 7 6

Conv 7.10 11 14 TOR(CDR)+1.5 mm Cu

CR 2.80 16 16

Image quality comparison


• Not affected by change to CR• Patient dose evaluation (when optimised)• Tube-generator controls (except. AEC)

• Affected by change to CR• Image quality evaluation with test object• Image quality evaluation with clinical

criteria• Image receptors (film-screen, viewing...)• Automatic processors• Image processing

Routine QC programme


• Available• TOR(CDR) image quality

test • Photometer• Densitometer• Dosimeters

• Needed• CR image quality test

object• SMPTE image test• Pencil type photometer

QC equipment


High

• Image quality with test object

• CRT evaluation (monitors)

Low

• Rejection rate analysis

• Image devices: film-screen, dark rooms,...

Workload with CR


Summary

• Digital radiology requires some specific training to benefit of the advantages of this new technique.

• Image quality and diagnostic information are closely related with patient dose.

• The transmission, archiving an retrieving of images can also influence the workflow and patient doses

• Quality assurance programs are specially important in digital radiology due to risk of increasing patient doses


Where to Get More Information (1)

• Balter S. Interventional fluoroscopy. Physics, technology and safety. Wiley-Liss, New York, 2001.

• Radiation Protection Dosimetry. Vol 94 No 1-2 (2001). Dose and image quality in digital imaging and interventional radiology (DIMOND) Workshop held in Dublin, Ireland. June 24-26 1999.

• ICRP draft on Dose Management in Digital Radiology. Expected for 2003.



• Practical Digital Imaging and PACS. Seibert JA, Filipow LJ, Andriole KP, Editors. Medical Physics Monograph No. 25. AAPM 1999 Summer School Proceedings.

• PACS. Basic Principles and Applications. Huang HK. Wiley – Liss, New York, 1999.

• Vañó E, Fernandez JM, Gracia A, Guibelalde E, Gonzalez L. Routine Quality Control in Digital versus Analog Radiology. Physica Medica 1999; XV(4): 319-321.



• http://www.gemedicalsystems.com/rad/xr/education/dig_xray_intro.html (last access 22 August 2002).

• http://www.agfa.com/healthcare/ (last access 22 August 2002).

Documents

RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY