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RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

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

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  1. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology RADIATION PROTECTION INDIAGNOSTIC ANDINTERVENTIONAL RADIOLOGY L 20: Optimization of Protection in Digital Radiology

  2. Topics • Introduction • Basic concepts • Relation between diagnostic information and patient dose • Quality Assurance 20: Digital Radiology

  3. 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

  4. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 20: Digital Radiology Topic 1: Introduction

  5. 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from conventional to digital radiology • Many conventional fluoroscopic and radiographic equipment have recently been replaced by digital techniques in industrialized countries • Digital radiology has become a challenge which may have advantages as well as disadvantages • Changing from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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 20: Digital Radiology Topic 2: Basic concepts

  13. Analogue versus digital Analogue: A given parameter can have continuous values Digital: A given parameter can only have discrete values 20: Digital Radiology

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

  22. Radiotherapy Department Outline of a basic PACS system 20: Digital Radiology

  23. 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

  24. 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

  25. 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

  26. The principle of PSP ADC PMT CB Trap Excitation Storage Emission 20: Digital Radiology

  27. Cassette and PSP PSP digitizer Workstation (Images courtesy of AFGA) 20: Digital Radiology

  28. Digital detector (Images courtesy of GE Medical Systems) 20: Digital Radiology

  29. 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

  30. 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

  31. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 20: Digital Radiology Topic 3: Relation between diagnostic information and patient dose

  32. 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

  33. 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

  34. 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

  35. 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

  36. An underexposed image is “too noisy” Exposure level 1,15 Exposure level 1,87 20: Digital Radiology

  37. 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

  38. 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

  39. 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

  40. 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

  41. 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

  42. 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

  43. 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

  44. 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

  45. 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

  46. 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

  47. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 20: Digital Radiology Topic 4: Quality Assurance

  48. 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

  49. 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

  50. 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

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