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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY L 20: Optimization of Protection in Digital Radiology Topics Introduction Basic concepts

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radiation protection in diagnostic and interventional radiology

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

RADIATION PROTECTION INDIAGNOSTIC ANDINTERVENTIONAL RADIOLOGY

L 20: Optimization of Protection in Digital Radiology

topics
Topics
  • Introduction
  • Basic concepts
  • Relation between diagnostic information and patient dose
  • Quality Assurance

20: Digital Radiology

overview
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

20: Digital Radiology

transition from conventional to digital radiology
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  • Many conventional fluoroscopic and radiographic equipment have recently been replaced by digital techniques in industrialized countries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

20: Digital Radiology

transition from conventional to digital radiology6
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 needed
    • producing higher image quality not necessarily required for the clinical purpose

20: Digital Radiology

radiation dose in digital radiology
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

20: Digital Radiology

what is dynamic range
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

20: Digital Radiology

characteristic curve of cr system
Characteristic curve of CR system

3.5

3

2.5

2

1.5

1

0.5

0

HR-III

CEA Film-Fuji Mammofine

CR response

Density

0.001 0.01 0.1 1

Air Kerma (mGy)

20: Digital Radiology

intrinsic digital techniques
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).

20: Digital Radiology

digitizing conventional films
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.

20: Digital Radiology

analogue versus digital
Analogue versus digital

Analogue: A given parameter can have continuous values

Digital: A given parameter can only have discrete values

20: Digital Radiology

what is digital radiology
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)

20: Digital Radiology

slide15

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).

20: Digital Radiology

slide16

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).

20: Digital Radiology

slide17
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)

20: Digital Radiology

the digital radiology department
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 CommunicationSystem (PACS).

20: Digital Radiology

dicom
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

20: Digital Radiology

dicom format images
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.

20: Digital Radiology

digital radiology process
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

20: Digital Radiology

slide22

Radiotherapy

Department

Outline of a basic PACS system

20: Digital Radiology

image acquisition i
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)

20: Digital Radiology

computed radiography cr
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

20: Digital Radiology

computed radiography cr25
Computed Radiography (CR)
  • 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

20: Digital Radiology

the principle of psp
The principle of PSP

ADC

PMT

CB

Trap

Excitation

Storage

Emission

20: Digital Radiology

slide27

Cassette and PSP

PSP digitizer

Workstation

(Images courtesy of AFGA)

20: Digital Radiology

slide28

Digital detector

(Images courtesy of GE Medical Systems)

20: Digital Radiology

image acquisition ii
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

20: Digital Radiology

digital fluoroscopy
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.

20: Digital Radiology

part 20 digital radiology31

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

Part 20: Digital Radiology

Topic 3: Relation between diagnostic information and patient dose

image quality and 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

20: Digital Radiology

tendency to increase dose
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).

20: Digital Radiology

computed radiography versus film screen
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

20: Digital Radiology

slide35

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

20: Digital Radiology

an underexposed image is too noisy
An underexposed image is “too noisy”

Exposure level 1,15

Exposure level 1,87

20: Digital Radiology

exposure level
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).

20: Digital Radiology

risk to increase doses
Risk to increase doses:
  • The wide dynamic range of digital detectors allows to obtain good image qualitywhile 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.

20: Digital Radiology

digital fluoroscopy39
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

20: Digital Radiology

difficulty to audit the number of images per procedure
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.

20: Digital Radiology

actions that can influence image quality and patient doses in digital radiology 1
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).

20: Digital Radiology

actions that can influence image quality and patient doses in digital radiology 2
Actions that can influence image quality and patient doses in digital radiology (2)
  • 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)

20: Digital Radiology

actions that can influence image quality and patient doses in digital radiology 3
Actions that can influence image quality and patient doses in digital radiology (3)
  • 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.

20: Digital Radiology

influence of the different image compression levels
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)

20: Digital Radiology

digital radiography initial pitfalls 1
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).

20: Digital Radiology

digital radiography initial pitfalls 2
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

20: Digital Radiology

important aspects to be considered for the qa programs in digital radiology 1
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

20: Digital Radiology

important aspects to be considered for the qa programs in digital radiology 2
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

20: Digital Radiology

displaying of dose related parameters 1
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).

20: Digital Radiology

slide51

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

20: Digital Radiology

displaying of dose related parameters 2
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.

20: Digital Radiology

reference levels
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

20: Digital Radiology

initial basic quality control
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

20: Digital Radiology

optimisation technique
Optimisation technique

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

20: Digital Radiology

optimization technique for abdomen ap
Optimization technique for Abdomen AP

Simulation with TOR(CDR) + ANSI phantom

81 kVp, 100 cm (focus-film distance)

1.6 mGy

20: Digital Radiology

optimisation technique for chest pa
Optimisation technique for Chest PA

Simulation with TOR(CDR) + ANSI phantom

125 kVp, 180 cm (focus-film distance)

* Grid focalised at 130 cm

0.25 mGy

20: Digital Radiology

image quality comparison
Image quality comparison

20: Digital Radiology

routine qc programme
Routine QC programme
  • 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

20: Digital Radiology

qc equipment
QC equipment
  • Available
    • TOR(CDR) image quality test
    • Photometer
    • Densitometer
    • Dosimeters
  • Needed
    • CR image quality test object
    • SMPTE image test
    • Pencil type photometer

20: Digital Radiology

workload with cr
Workload with CR
  • High
    • Image quality with test object
    • CRT evaluation (monitors)
  • Low
    • Rejection rate analysis
    • Image devices: film-screen, dark rooms,...

20: Digital Radiology

summary
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

20: Digital Radiology

where to get more information 1
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.

20: Digital Radiology

where to get more information 2
Where to Get More Information (2)
  • 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.

20: Digital Radiology

where to get more information 265
Where to Get More Information (2)
  • 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).

20: Digital Radiology