For nearly 100 years now, the photographic film has been used to record images For over 60 years, intensifying screens have been used with x-ray films to obtain high quality images with lower radiation doses Very recently it has become possible to record x-ray images without the use of conventional film-screen systems (CR and DR systems) But even now, radiography using film-screen technology accounts for about 65 % of all diagnostic examinations
Define the key terms used in digital imaging List the equipment needed to perform digital imaging Explain the computed radiography (CR) digital system Explain the digital radiography (DR) system Explain PACS and recent advances in digital radiography
Any Imaging Technique has following steps –1. Image Acquisition2. Image Processing3. Image Display4. Image Storage
Radiography Analog Digital(Conventional)Scanner Computed Direct Digital (X-ray Radiography Radiographydigitizer) (CR) (DR or DDR)
A conventional system uses an intensifying screen to create a latent image on x-ray film. The film is then processed, creating a manifest image that can be interpreted by a physician. It is later stored in the file room (physical storage for archival)
Method is film-based. Method uses intensifying screens. Film is placed between two screens. Screens emit light when x-rays strike them. Film is processed chemically. Processed film is viewed on view-box (lightbox).
10% of films are not available when we want them! 15% of films are “hard” to locate or find! 25% of films are “misplaced” or not retrievable (misfiled). 10% of films are lost (referrals, residents, etc.) Recent study – physicians spend two weeks/year (100 hours/year) trying to locate or find the films they need. Cost in physician time is estimated from $60 to $80 per study.
Radiography Analog DigitalScanner Computed Direct Digital (X-ray Radiography Radiographydigitizer) (CR) (DR or DDR)
Definition –Digital Imaging is any modality / method of imagingthat creates an image that can be viewed or stored on acomputer.
Concept began in the 1950s. Early PACS systems were developed by the military to send images between Veterans Administration hospitals in the 1980s. Early process involved scanning radiographs into the computer and sending them from computer to computer. Images were then stored in PACS. Computed and digital radiography followed.
Only recently, it has become technically possible and economically feasible to challenge film technique for projection radiography Made possible by certain pre-requisite technological advances such as - high luminance and high resolution display monitors high performance computers / workstations
Radiography Analog DigitalScanner Computed Direct Digital (X-ray Radiography Radiographydigitizer) (CR) (DR or DDR)
A scanner is used to convertexisting analog images into adigital formatNot cost-efficientHence, seldom used (oldavailable films need to beconverted into digital format)
Definition - Computed Radiography (CR) is a process of capturing radiographic data from a Conventional X-ray machine and processing the data digitally to produce crisp and high quality radiographic images 1. Image Acquisition 2. Image Processing 3. Image Display 4. Image Storage
Light weigth aluminium Plastic Steel frame The front panel made up of low attenuation carbon fiber
Approximately 1 mm thick Protective layer Phosphor layer Anti-halo and reflecting layer Base Backing layer
Fluorinated Polymer Material – Protects the Phosphor layer Protective layer Phosphor layer Anti-halo and reflecting layer Base Backing layer
Europium-activated Barium-fluorohalide (BaFX: Eu+2) Protective layer Phosphor layer Anti-halo and reflecting layer Base Backing layer The phosphor crystals are usually cast into resin material to give them the form of plates
To reduce scatter Protective layer Phosphor layer Anti-halo and reflecting layer Base Backing layer
PET – Polyethylene Teraphtalate Protective layer Phosphor layer Anti-halo and reflecting layer Base Backing layer
Protects the base from damage Protective layer Phosphor layer Anti-halo and reflecting layer Base Backing layer
The Imaging Plate looks like the intensifying screens found in Conventional film-screen cassettes They are made of photostimulable phosphors Instead of emitting light immediately when exposed to X-rays, the photostimulable phosphor has the special property of storing the X-ray energy in a latent form and releasing the same when stimulated by a laser energy in the CR Reader / Digitizer – photo Phosphorescence (c/w - fluorescence)
When phosphors are stimulated with X-ray photon energy, electron hole pairs are produced In effect, Europium is excited to a higher energy level (excited state) leaving behind a hole / vacancy
Energy absorbed by the imaging plate must be transformed into electrical charges, which are then recorded and digitized. Read out Process
The readout process should start immediately after exposure because the amount of stored energy decreases over time (latent image completely disappears by 24 hrs – spontaneous phosphorescence) Stimulation with a scanning laser beam releases electrons Fallingback electrons emit luminescent light (phosphorescence)
633 nm 390 nm Typical wavelength of the stimulating laser is 633 nm (usually helium-neon laser) Typical wavelength of the emitted light is 390 nm(BLUE)
The emitted light intensity is proportional to the original incident X-ray intensity The emitted light is captured with an optical array and a photomultiplier tube, the signals amplified and digitized (Analog to Digital converters - ADC) The residual image is erased from the plate by an intense light source, which returns all electrons to their original state. This makes the plate ready to be reused for new exposures
How many times can we use a StoragePhosphor Plate?• The life of a phosphor plate depends on how carefully it is handled. Physical damage to the plate will limit its useful life• If properly cared for, a plate will produce thousands of images• Imaging Plates are known to last more than 50000 Exposure Cycles !!!!!
Cassette with Imaging Plate MA TRIXLR 3300Rx Exposure Printing Network Processing server Identification Digitizer
Radiography Analog DigitalScanner Computed Direct Digital (X-ray Radiography Radiographydigitizer) (CR) (DR or DDR)
INTRODUCTION In CR, image acquisition is a two-stage process wherein image capture and image read out are done separately. Direct digital radiography (DR) systems, on the other hand, use detectors that have a combined image capture and image read out capability. Cassette-less system
There are different types of DR Systems available depending on the type of detectors used in them (a) Flat panel detector (FPD) based systems (b) Charge coupled device (CCD) array based system
Requires a photoconductor Converts x-ray photons into electrical X-ray charges by setting electrons free. Directly Typical photoconductor materials include amorphous selenium, lead iodide, lead oxide, thallium Electrical bromide, and gadolinium compounds. signals The most commonly used element is selenium.
Selenium-based direct conversion DR systems are equipped with either a selenium drum or a flat-panel detector (FPD).
A rotating selenium-dotted drum, which has a positive electrical surface charge, is exposed to x-rays. During exposure, a charge pattern proportional to that of the incident x-rays is generated on the drum surface and is recorded during rotation by an analog to- digital converter.
Amorphous selenium–based direct conversion DR systems.1. A rotating selenium-dotted drum with a positive electrical surface charge isexposed to x-rays. 2. Alteration of the charge pattern of the drum surface is proportional to theincident x-rays.3. The charge pattern is then converted into a digital image by an analog-to-digital (A/D) converter.
Selenium Drum….. Several clinical studies have confirmed that selenium drum detectors provide good image quality that is superior to that provided by screen-film or CR systems. (ADVANTAGE) However, because of their mechanical design, selenium drum detectors are dedicated thorax stand systems with no mobility at all. (DISADVANTAGE)
A newer generation of direct conversion DR systems make use of selenium-based flat-panel detectors. These detectors make use of a layer of selenium with a corresponding underlying array of thin-film transistors (TFTs). The principle of converting x-rays into electrical charges is similar to that with the selenium drum, except that the charge pattern is recorded by the TFT array, which accumulates and stores the energy of the electrons.
A selenium-based flat-panel detector system1. Incident x-ray energy is directly converted into electrical chargeswithin the fixed photoconductor layer2. read out by a linked TFT array beneath the detective layer
Greater clinical usefulness, since the detectors canbe mounted on thorax stands and Bucky tables(ADVANTAGE)Another promising clinical application ofselenium-based flat-panel detectors is in the fieldof Mammography (ADVANTAGE)Studies indicate that the image quality providedby selenium-based flat-panel detectors isequivalent to that provided by other flat-paneldetectors and selenium drum detectors(ADVANTAGE)
Charged Coupled Device (CCD) is used A CCD is a light-sensitive sensor for recording images that consists of an integrated circuit containing an array of linked or coupled capacitors. X-ray energy is converted into light by a scintillator such as Thallium doped cesium iodide. The amount of light emitted is then recorded by the CCD, and the light is converted into electrical charges. CCD Lens Slot scan coupled
An array consisting of several CCD chips forms a detector area similar to that of a flat-panel detector. Optical lenses are needed to reduce the area of the projected light to fit the CCD array, which subsequently converts the light energy into electrical charges
One drawback of the lens system is a decrease in the number of photons reaching the CCD, resulting in a lower signal-to-noise ratio (SNR) and relatively low quantum efficiency.
• Slot-scan CCD systems make use of a special x-ray tube ( tungsten anode) with a collimated fan-shaped beam, which is linked to a simultaneously moving CCD detector array having a matching detector width.• The combination of a small collimated beam and a concordant detector reduces the impact of scattered radiation in the image, since much of this radiation will escape without detection.• Relatively low quantum efficiency of slot-scan CCD systems can be offset by the resulting lower image noise.
The exposure time to the patient is about 20 msec, and the readout process takes about 1.3 seconds. Because of the need for ﬁxed installation, slot scan CCD systems are dedicated to chest radiography, mammography, or dental radiography.
Advantages –• Relatively cheaper• Individual defective components can be replaces rather than changing the entire detector• Upgradeable to future innovations Limitations – • Bulky design • Relatively small CCD arrays (2-5 cm) than the typical projected X-ray areas – hence require demagnification and optical coupling • Optical system – more signal noise • Thermal noise in CCD can degrade mage quality • Repeated exposure to X-rays may damage optical system
Small overall design, which allows integration into existing Bucky tables or thorax stands Image generation with flat-panel detectors is almost a real-time process, with a time lapse between exposure and image display of less than 10 seconds. Consequently, these systems are highly productive, and more patients can be examined in the same amount of time than with other radiographic devices.
Indirect conversion DR systems are “sandwich” constructions consisting of a scintillator layer, an amorphous silicon photodiode circuitry layer, and a TFT array. When x-ray photons reach the scintillator, visible light proportional to the incident energy is emitted and then recorded by an array of photodiodes and converted to electrical charges. These charges are then read out by a TFT array similar to that of direct conversion DR systems.
CsI or Gd2O2SIndirect Conversion with a Flat-Panel Detector / TFT
The scintillators (in FPD) usually consist of CsI or Gd2O2S CsI based FPD - are highly vulnerable to mechanical load because of their fine structure, these systems cannot be used outside of fixed installations and therefore lack mobility. The advantage of CsI-based scintillators is that the crystals can be shaped into 5–10 micrometer wide needles, which can be arranged perpendicular to the surface of the detector. This structured array of scintillator needles reduces the diffusion of light within the scintillator layer As a result, thicker scintillator layers can be used, thereby increasing the strength of the emitted light and leading to better optical properties and higher quantum efﬁciency Gd2O2S based FPD - resistant to mechanical stress as are storage phosphors and hence are portable.
Key features of Direct Digital Conversion X-rays > Electrical signals No intermediate light production Detector material – Amorphous Selenium Maintains high resolution of images as photoconductor thickness is increased Moderate DQE (effficiency) for conventional radiography but high DQE for mammography KV range Very sensitive to ambient temperature variations
X-rays > Light > Electrical signals Used Phosphors – Thallium doped Cesium Iodide (CsI) or Gadolinium Oxy-Sulphide (GdO2S) More light scatter, so less spatial resolution Generates poorer resolution images as phosphor thickness is increased High DQE (Efficiency) for Conventional range KV range Less sensitive to ambient temperature changes
ViewingFor an image on a screento have the qualityapproaching that of a filmimage, a special monitormust be used with aresolution of 1024 x 1024pixels
After exposure and readout, the raw imaging data must be processed for display on the computer. Greatly influences the way the image appears to the radiologist . AIM - to improve image quality by reducing noise, removing technical artifacts, and optimizing contrast for viewing. Spatial resolution (the capacity to define the extent or shape of features within an image sharply and clearly) cannot be influenced by the processing software because it is dependent on the technical variables of the detector (eg, pixel size).
Contrast enhancement - makes anatomic structures more visibleand distinguishableContrast reduction - results in smoothing of the structures
Provides sharperdelineation of thefine structures ofbones.
1. Initially acquired raw data without any processing2. Contrast enhancement makes anatomic structures morevisible and distinguishable3. Contrast reduction results in smoothing of the structures4. Edge enhancement provides sharper delineation of the finestructures of bones
Positioning Markers Add predetermined text or free text Zoom and roam image Invert image Show/hide histogram (exposure details) Advanced measurement options (Orthopedic Application) Stitching for full leg/full spine
Caution - If one feature is being improved, others may be suppressed, so that unintended and unwanted masking of diagnostically relevant features may occur. Consequently, image processing must be optimized carefully for each digital radiography system. Image processing software is usually bundled with the detector and cannot be replaced by other software (in DDR).
Pixel Size, Matrix, and Detector Size Spatial Resolution Modulation Transfer Function Dynamic Range Radiation Exposure
Pixel Size, Matrix, and Detector Size: Digital images consist of picture elements, or pixels. The two-dimensional collection of pixels in the image is called the matrix, which is usually expressed as length (in pixels) by width (in pixels) Maximum achievable spatial resolution is defined by pixel size and spacing. The smaller the pixel size (or the larger the matrix), the higher the maximum achievable spatial resolution. The overall detector size determines if the detector is suitable for all clinical applications. Larger detector areas are needed for chest imaging than for imaging of the extremities
Spatial Resolution: Definition - Spatial resolution refers to the minimum resolvable separation between high-contrast objects. In digital detectors, spatial resolution is defined and limited by the minimum pixel size. Increasing the radiation applied to the detector will not improve the maximum spatial resolution. On the other hand, scatter of x-ray quanta and light photons within the detector influences spatial resolution. Therefore, the intrinsic spatial resolution for selenium- based direct conversion detectors is higher than that for indirect conversion detectors.
The older generations of storage phosphors did not have diagnostically sufficient intrinsic spatial resolution For digital mammography, the demanded diagnostic spatial resolution is substantially higher indicating the need for specially designed dedicated detectors with smaller pixel sizes and higher resolutions
Modulation Transfer Function: Definition - capacity of the detector to transfer the modulation of the input signal at a given spatial frequency to its output . At radiography, objects having different sizes and opacity are displayed with different gray-scale values in an image. MTF has to do with the display of contrast and object size. More specifically, MTF is responsible for converting contrast values of different-sized objects (object contrast) into contrast intensity levels in the image (image contrast). MTF is a useful measure of true or effective resolution, since it accounts for the amount of blur and contrast over a range of spatial frequencies.
Dynamic Range Dynamic range is a measure of the signal response of a detector that is exposed to x-rays . In conventional screen-film combinations, the dynamic range gradation curve is S shaped within a narrow exposure range for optimal film blackening . Thus, the film has a low tolerance for an exposure that is higher or lower than required, resulting in failed exposures or insufficient image quality. For digital detectors, dynamic range is the range of x-ray exposure over which a meaningful image can be obtained. Digital detectors have a wider and linear dynamic range, which, in clinical practice, virtually eliminates the risk of a failed exposure.
Detective Quantum Efficiency (DQE): Deficiency - Efficiency of a detector in converting incident x- ray energy into an image signal. Calculated by comparing the signal-to-noise ratio (SNR) at the detector output with that at the detector input as a function of spatial frequency . Dependent on radiation exposure, spatial frequency, MTF, and detector material. High DQE values indicate that less radiation is needed to achieve identical image quality; increasing the DQE and leaving radiation exposure constant will improve image quality.
The ideal detector - DQE of 1, meaning that all the radiation energy is absorbed and converted into image information. In practice, the DQE of digital detectors is limited to about 0.45 at 0.5 cycles/mm.
Radiation Exposure: The higher DQE values of most digital detectors compared with screen-film combinations suggest that, besides providing better image quality, digital detectors have the potential for substantially lowering patient exposure without a loss of image quality. The most obvious way to minimize patient exposure is to greatly reduce the number of failed exposures and requisite additional images. This reduction is made possible by the wider dynamic range of digital detectors compared with conventional screen-film combinations.
Unlike storage-phosphor systems, in which the possibility of exposure reduction is limited, DR systems offer a significantly higher potential for general exposure reduction because of their far superior quantum efficiency. Several studies have shown that a considerably lower exposure is required for equivalent depiction of anatomic details with flat-panel detectors than with storage phosphor systems and screen-film combinations. In summary, reduction of exposure in flat-panel detector digital radiography is possible, to some extent regardless of the clinical situation.
A Picture Archiving and Communication System is an inter and intra-institutional computation system that manages - data acquisition , transfer, storage, distribution display and interpretation of medical images, all integrated with various digital networks .
Radiologists need to view, archive, and retrieve patients images. they also need to retrieve and recall complex and rare pathologic, anatomic, and radiologic knowledge and compile, retrieve, and consult medical records and reports. Finally, they have to be able to communicate results to referring physicians and colleagues.
Components of PACS • Scheduling, Digital acquisition devices • Demographics, Network Short term-rapid access • Patient information, and Long term-slowcomponents • Billing database access Database server Duplicate-off site for disaster • Reports recovery Archival system Radiology information system / Hospital Information System (RIS / HIS) Soft copy display Early remote access (for the referring clinician)
Once correctly installed, no information is lost and available when needed Comparison with previous examinations available at all times Simultaneous viewing in different places Retrieval easy Post processing manipulations Film budget reduced Tele-radiology
Technically complex , Dedicated maintenance program required Training required No fall back after installation Failure of individual workstations or acquisition components will affect functions and data flow in the local PACS branch, failure of the PACS controller or main PACS archive server can cripple the entire PACS operation.
The rapid acquisition of images can result in latent signal from one exposure lingering into the readout of subsequent exposures, producing what appears to be an incomplete erasure of the previous image, known as Image lag / Ghosting. Mainly a DR artifact because of rapid image acquisition ability
Flawed Gain Calibration Detector Lag Backscatter (in portable DR due to thin backside shielding – to make it lighter)
(a) Tomosynthesis: Multiple low dose exposures are given from various angles while the X-ray tube moves in an arc and the detector remains stationary. Multiple images with different focal zones are possible to be created by addition of these low dose images after pixel shift. It emphasizes contrast in a particular layer of a region of body. Generated images can be viewed singly or as a cine loop. It is considered useful in Chest, IVU studies and mammography
(b) Dual-energy imaging: By using a high and low kilo-voltage technique, two datasets are created. Soft tissues and bones can be separately depicted by this method. Dual-energy techniques are most effective when both images are acquired simultaneously. Similar results are obtained with two exposures within a very short period of time. This is useful in chest radiography, particularly for the evaluation of partially calcified nodules and pleural plaques.
(C) Computer aided diagnosis (CAD) software programs: These are important in early detection of cancer of the lung and breast. The suspicious areas are marked by the software for review by the radiologist. The efficiency of CAD software program is related to its sensitivity and specificity profile. The main advantage of CAD is that it permits a radiologist to avoid overlooking diagnostically significant findings.
(d) Automatic image stitching: Useful in determining precise measurements in lengthy anatomical regions like the spine or lower limbs. The largest flat-panel DR plates available today are 43 × 43 cm. Using these detectors, only a limited portion of the body part can be imaged at one given time, thus making these detectors inadequate for studying the whole spine or the entire lower limb. To overcome this problem, multiple sequential exposures at different patient positions are acquired in a still patient. Automatic stitching is then performed to reconstruct a larger composite image. This special software enables pixel shift and overlap.
(e) Mobile DR: This is in general a 17 × 14-inches flat panel detector (FPD) connected by a cable to a mobile x-ray system having a monitor. The use of mobile DR systems is hampered by the fragility of the FPDs and the high costs. A mobile DR system, when compared with an FSR system, avoids problems related to the availability, storage, transportation and disposal of films and chemicals.
(f) Wireless FPDs: Wirelessly transfers image data to the DR system (Pixium 3543, Thales) It has no cables and does not interfere with surrounding machines. Typically a 17 x 14-inch image is made available within 3 sec. This allows radiography of difficult regions of the body like the axilla or the TM joint and enables radiography in unusual positions as in a flexed knee, or in a limb with limited mobility due to contractures.
Some of the drawbacks of CR systems, namely cassette handling, long read out time of PSP plates, low DQE and poor resolution Have been addressed by newer innovations and technological advances.
(A) Automated CR systems with fast readout: Automated CR systems reduce the readout time less than 10sec In these systems there is no cassette handling, leading to totally automatic image data acquisition
(B) Newer phosphors for PSP plates: Commercially available PSP plates have unstructured phosphor like rubidium chloride or barium fluorohalides doped with Europium. A needle-shaped phosphor cesium bromide, has been newly introduced, for example, in Konica Minoltas Regius 370 Upright DR, and is considered more efficient due its structured configuration of crystals. It reduces light diffusion because of the needle shaped configuration that acts as light guide. In addition the newer phosphors are more efficient with an increased DQE.
(C) Mobile CR systems: Mobile X-ray unit with an integrated CR reader. They are easy to use and offer quick image availability in less than 25 sec.
Relatively cheaper (c/w DR) Can be incorporated into existing radiography system Cassettes and imaging plate relatively more sturdy (c/w DR) Relatively more time for image processing (c/w DR) Less DQE (c/w DR)
Increased workflow Integrated high power X-ray system of 30-100 KW – very short exposure times – eliminating motion blur Reduction in radiation dose possible as per ALARA principle Presets available for various anatomical studies Automatic Exposure Control (AEC) facility Automatic filter, focal spot size and tracking for easy positioning
High initial cost Some radiographic views are difficult to obtain as detectors are generally not free to be placed in any position Fragile detectors – careful handling
ENTRANCE SURFACE DOSE STANDARD RADIOGRAPHIC EXAM USING SFR,CR,DDR G compagnona, Balani et al: BJR NOV 2006SFR – Screen Film RadiographyCR – Computed RadiographyDDR – Direct Digital Radiography
NIMS FIGURES AT A GLANCE SEPT 2009-AUG 201O - CONVENTIONAL SEPT 2010-AUG 2011 - DIGITAL
Analog CR DR (conventional)Installation Cost + ++ ++++Radiation reduction not possible possible Definite reduction possibleFragility - + +++Spatial resolution + + ++ (high quality monitors)DQE + +++ ++++Dynamic range + (‘S’ shaped +++ (Linear & +++ (Linear & curve) wide) wide)Post processing Not possible possible possiblePACS and teleradiology Not possible possible possibleImage display time +++++ ++ (upto 25 sec) + (< 10 sec)
Conventional Radiography is evidently the last of the radiology modalities to embrace and incorporate digital technology. The future of radiography will be digital. CR is a simple and cost effective technology that permits use of existing radiographic equipment. It has been suggested that for moderate workload (upto 70-90 films per day), a CR system is adequate. High cost of a DR system is justified only when the workload is much beyond this level. Lastly, a change over to digital technology is essential to create a fully digital filmless radiology department and fully reap the benefits of implementing RIS and PACS programs.
Advances in Digital Radiography: Physical Principles and System Overview - May 2007 RadioGraphics, 27, 675-686. Artifacts in Digital Radiography - AJR January 2012 vol. 198 no. 1 156-161. Christensen’s : physics of diagnostic radiology. Internet.