Digital Radiography (DR) Lecture Notes PDF

DIGITAL RADIOGRAPHY (DR)  Learning outcomes in this lecture we will describe the different types of detectors used in DR discuss the benefits of digital radiography. Differentiate between direct and indirect image capture. briefly look at some image processing techniques used in DR....

DIGITAL RADIOGRAPHY (DR)  Learning outcomes in this lecture we will describe the different types of detectors used in DR discuss the benefits of digital radiography. Differentiate between direct and indirect image capture. briefly look at some image processing techniques used in DR. describe the application of DR in improving the diagnostic capability of medical imaging  What is a digital image? Analogue images Digital images Images are composed Images are made of of continuous pixels of discrete variations in optical number values, stored density and are viewed and viewed on a film or TV monitor. electronically as grey levels. You can’t “see” a digital image. You see a display made from a digital image. What is a digital radiograph? There are three practical digital radiography cassette systems: – Computed Radiography involves the digitisation of analogue images captured with an imaging plate. – Digital Radiography uses flat panel detectors to directly produce a digital image. (Direct and Indirect) – All systems are widely used Ideal digital radiography system Physical design Compatible size with film cassettes Immediate readout Cost effective Image Quality Spatial and contrast resolution as good as film Wide dynamic range DICOM compatible Digital acquisition system Imaging system  Image Acquisition Three main ways: –Film digitisation –Computed Radiography (CR) –Direct Radiography (DR) Workload steps in screening-Film Environment and CR DR Work Flow Example - PA & Lat. Chest Conventional Exam With Film Patient PA Chest First Film Last Film in Room Exposure Lat Exposure Out of Out of QC Release Patient 6:05 min Processor Processor Exposure and Positioning Repeats outpatients Exam Using Computed Radiography Patient PA Chest Cassette Read & in Room Exposure Lat Exposure to Process QC Release Patient 7:02 min Reader Cassette Minimal Repeats outpatients Exam Using Digital Detector Release Confirmed 62% PA Chest Lat Patient in Room Exposure Exposure Patient / Send Study 2:18 min* Reduction In & QC & QC To WS Exam Time Minimal Repeats * 1:26 min, outpatients only Source: “Methodist: Digital & Analog Chest Examination 1 Digital Rad = 2.6 Film-Screen Rooms Productivity Study Project”, August 2000. 2 Types of Digital Detectors.Direct Digital Detectors Indirect Digital Detectors Direct Radiography (DR) Fixed detector panel Less portable than CR More expensive BUT: – Faster results (On screen within seconds, less admin) – Lower dose due to advances in detector panel Typically fixed within the table or vertical bucky Amorphous silicon/selenium arrays 15 Direct v indirect detectors Acquisition time may have a bearing of tube loading  Indirect involves conversion of x-rays to light (CsI) and then light to electrical charge. This charge accumulates in a storage capacitor and is read out sequentially using an FET switching element (TFT array)  Direct converts x-rays to charge and is coated to a TFT array. What is Direct 17 Direct X-ray + electrodes Photoconductor - Flat Panel Detectors : Direct Layer of photoconductive material (amorphos silicon or amorphos selenium) Conductive electrode High voltage Photoconductor added electrode Capacitive storage element added to store charge Interacting x-rays produce charge which is attracted to the electrode and the capacitive storage element. Glass substrate Capacitive Active matrix array is Switching storage read out. element element Flat Panel Detectors: Photoconductors Detectors based on insulators or semiconductors are solid state analogs of gas ionisation chambers The electrons generated via ionisation are collected via the application of an electric field These are used to indicate the position of the incident radiation. Energy absorbed promotes electrons from the valence band to the conduction back where they can be collected Band gap is typically 1 or 2 eV Electrons must have sufficient lifetime to be collected before falling back into the valence band Such insulators are called photoconductors a-Se (a-Si) Flat Panel Detectors: Photoconductors Collected to Conduction Band anode before returning Forbidden Gap Eg Valence Band Indirect An indirect detector system is basically a scintillator based x-ray detector. X-rays interact with a phosphor causing it to emit light. Light collected by a photodetector (film emulsion or silicon photodiode. Indirect X-ray Light Scintillator Photodetector Flat Panel Detectors : Indirect Phosphor layer eg Gd2O2S :Tb or scintillator eg CsI: Tl Placed in intimate contact with an active Phosphor or matrix array scintillator X-rays converted to light photons and the charge stored in each pixel element. The magnitude of the charge in each pixel is the latent image. Glass substrate Photosensitive Active matrix array is Switching storage read out. element element Flat Panel Detectors: Phosphors Detectors employ a phosphor initially to absorb x-rays and produce light Again electrons are excited from the valence band to the conduction band. Many electrons return to the valence band through a local energy state in the phosphors forbidden gap Create by using small amounts of impurities called activators This causes light to be emitted Light photon is about 2-3 eV For a 60 keV photon can produce about 3600 light photons (nb. not full conversion due to other energy loss processes) Gd2O2S:Tb (Gadox) and CsI:Tl Flat Panel Detectors:Phosphors Conduction Band Forbidden Activator Gap Eg site Valence Band  Flat Panel Scintillator (CsI) CsI needles 5µm diameter X Photon Light Photons guided inside CsI needles (typ. each x-ray generates 3000 light Photons) For same resolution (MTF), the scintillator thickness can be high, leading to 20 µm excellent X-Ray absorption Pixel Pitch (i.e. 100µm) efficiency Scintillator  Scintillator & Reflector X-Ray Photons Reflector Light photons guided via CsI needle structure 10 needle width Flat Panel Detectors Flat Panel Detectors The need for a cassette and readout system is removed The flat plate is used and the signal sent directly to storage and to the monitor What is flat Panel Radiography?: a -Si 31 GE Revolution XQi Siemens Aristos TX  The Process X-Rays hit CsI Light hits a-Si Electrons and holes are drawn towards the positive bias and holes are stored in the charge storage wells beneath the electrodes When enough charge is stored a voltage runs across the a-Si and the electronics read this voltage  Readout The signals from the individual sensors are read out in sequence All the sensors in the first row are activated The signals are then led on parallel in the column direction to pre-amplifiers, amplifiers and to an ADC When the 1st row has been digitized, the 2nd row is activated Readout Flat Panel Detectors Flat panel detectors have a two dimensional array of imaging pixels. Each pixel is configured from a switching element and a storage element. Other lines are used to control the readout of the stored charge (latent image) from the array of pixels Other peripheral circuitry that amplifies, digitises and synchronises the readout. Pixel Architecture Scan Line Very High Photodiode Fill Factor Data Line Data readout done 1 row at a time PHOTODIODE/FET All columns read simultaneously ARCHITECTURE Flat Panel Detectors  Flat Panel Detectors Multiplexer Array made up of pixel electrodes. Amplifier Each pixel has a switch. Each row has a single Switching control control line to activate switch. Each column has a single signal line with a readout amplifier. This allows the imager to be read out one line at a time. Unlike CCDs there is no charge transfer from one pixel to the other.  Flat Panel Detectors Multiplexer Initialisation state ready to acquire image. Switching control All array switches are held off while exposure is made. Charge collected in pixel storage elements.  Flat Panel Detectors Multiplexer Switch on the first row are now switched on. Switching control Charge transferred down appropriate signal lines, digitised and stored via the multiplexer.  Flat Panel Detectors Multiplexer Switch on the first row are now switched on. Switching control Charge transferred down appropriate signal lines, digitised and stored via the multiplexer.  Flat Panel Detectors Multiplexer Switch on the first row are now switched on. Switching control Charge transferred down appropriate signal lines, digitised and stored via the multiplexer.  Flat Panel Detectors Multiplexer ADC Switch on the first row are now switched on. Switching control Charge transferred down appropriate signal lines, digitised and stored via the multiplexer.  Flat Panel Detectors Multiplexer Switches now returned to the off position. Switching control Next row similarly treated.  Flat Panel Detectors Multiplexer Switches now returned to the off position. Switching control Next row similarly treated. Switch is closed.  Flat Panel Detectors Multiplexer Switches now returned to the off position. Switching control Next row similarly treated. Switch is closed. Charge is read out.  Flat Panel Detectors Multiplexer Switches now returned to the off position. Switching control Next row similarly treated. Switch is closed. Charge is read out.  Flat Panel Detectors Multiplexer ADC Switches now returned to the off position. Switching control Next row similarly treated. Switch is closed. Charge is read out.  Flat Panel Detectors Multiplexer Switches now returned to the off position. Switching control Next row similarly treated.  Flat Panel Detectors Multiplexer Switches now returned to the off position. Switching control Next row similarly treated.  Flat Panel Detectors Multiplexer Switches now returned to the off position. Switching control Next row similarly treated.  Flat Panel Detectors Multiplexer ADC Switches now returned to the off position. Switching control Next row similarly treated.  Flat Panel Detectors Multiplexer All switches are now off ready to Switching control read out next image. Review Q Review Q  Clinical consideration with flat panel radiography Consideration Room lay out Choices – How many detectors Ceiling suspended per room telescopic – What type of Column suspended detector Ergonomics Table mounted Changing technique to Free detector adapt to the new technology. Ceiling suspended telescopic  Ceiling Suspended Telescopic Can accommodate almost every patient Can be used for erect chest to supine trauma radiography In the A& E department this system requires special trolleys OR transfer of All patients to the X-ray table. It is about $150,000 more expensive than a column suspended or table mounted unit. Comes with an amazing set of pre sets positions, at the touch of a button the tube and the detector can be be placed in the approximate position for horizontal beam neck of femur or horizontal beam lumbar spine. Offers the potential to have only one detector in each room. Thus reducing the cost of the overall DR installation. Ceiling suspended telescopic Ceiling suspended telescopic Ceiling suspended telescopic Ceiling suspended telescopic Picture of review station - #10 Picture of Workstation & Detector Pictures of detector & patients  Summary in these lessons , we Part one Described the different types of detectors used in DR discuss some advances of digital radiography. discussed the benefits of digital radiography. Part two: briefly looked at some image processing techniques used in DR. described the application of DR in improving the diagnostic capability of medical imaging