Oral and Maxillofacial Radiology Lecture 3 - Digital Radiography PDF

Oral and Maxillofacial Radiology -II Lecture 3 DIGITAL RADIOGRAPHY Digital radiography is a filmless imaging system. It is a method of capturing a radiographic image using sensor, breaking it into electronic data, and presenting and storing the im...

Oral and Maxillofacial Radiology -II Lecture 3 DIGITAL RADIOGRAPHY Digital radiography is a filmless imaging system. It is a method of capturing a radiographic image using sensor, breaking it into electronic data, and presenting and storing the image using a computer. Differences between conventional film and digital image: Conventional Film (ANALOG) Digital image In conventional film the In digital image, the images differences in size and are numeric and discrete in distribution of black metallic terms of spatial distribution silver results in continuous of pixels and in terms of density spectrum different shades of gray of each pixel BASIC TERMINOLOGY Digital imaging: It is a filmless imaging system in which an image is captured using a sensor, converting it into electronic data, processed, stored and displayed using a computer and related imaging software. Digital image: An image composed of pixels that can be stored in a computer. Digitize: In digital imaging, to convert an image into a digital form that, in turn, can be processed by a computer. Pixel: A discrete unit of information. In digital electronic images, information is contained in, and presented as, discrete units of information. It is also termed as picture element. Sensor: In digital imaging, a receptor that is used to capture an intraoral or extraoral image. It contains light sensitive elements arranged in regular grid of rows and columns and a two-dimensional grid of elements corresponding to pixel. ANALOG TO DIGITAL CONVERSION Absorption of X- rays generate small voltage at the sensor corresponding to the pixel. Analog to digital conversion takes place in two steps: 1. Sampling: A small range of voltage values are grouped together as a single value. 2. Quantization: Every sampled signal is assigned a value. These values are stored in computer and represent an image. These values range from 0 to 255 where 0 represents black and 255 represents white. All the values in between 0 and 255 represent different shades of grey Digital image receptors are of many different technologies and come in many different sizes and shapes. There are different names in use to identify these receptors in medicine and dentistry. They can also be distinguished from one another as: 1. Direct – device or sensor have the ability to generate image in computer without any other external device 2. Indirect – in which there is a scanner which can scan the latent image on a receptor and that scanner generates the image on the computer The most useful distinction is that between two main technologies: (1) solid-state technology and (2) photostimulable phosphor (PSP) technology. In medicine, the use of solid-state detectors is referred to as digital radiography. In dentistry, intraoral solid-state detectors are often called sensors. The other main technology, PSP, consists of a phosphor-coated plate in which a latent image is formed after x-ray exposure. The latent image is converted to a digital image by a scanning device through stimulation by laser light. This technology is sometimes referred to as storage phosphor on the basis of the notion that the image information is temporarily stored within the phosphor. Other times the term image plates is used to differentiate them from film and solid-state detectors. The use of PSP plates in medical radiology is referred to as computed radiography. SOLID STATE DETECTORS They collect the charge generated by x-rays in a solid semi-conducting material. Images are formed rapidly after exposure. Pixel size varies from 20-70 micrometers. Charge Coupled Device, CCD Introduced to dentistry in 1987, and was the first digital image receptor used for digital intr-oral imaging. Consists of a silicon wafer for recording image. These are more sensitive to light than x-rays. To increase the x-ray absorption, there is a scintillating layer made of Gadolinium oxybromide or Cesium iodide. Working- After exposure with x-rays, electron-hole pairs are generated in the silicon wafer and form “charge-packets”. Each charge packet corresponds to a pixel. Voltage from each pixel are assigned a numeric value corresponding to a gray scale level. The charge pattern from the individual pixels in the matrix represents latent image. Complementary Metal Oxide semi-conductors, CMOS These were produced for digital cameras and computer chips and the technology is less expensive than CCD Detectors is Silicon but different from CCDs in which they are processed. In CMOS, each pixel is directly connected to a transistor. After exposure of the detector, small voltage is generated after absorption of x-rays. The voltage in each transistor is read separately, by a frame grabber, and stored and displayed as a digital gray value. PHOTOSTIMULABLE PHOSPHOR PSP plates absorb and store energy from x rays and release this energy as light (phosphorescence) when stimulated by another light of an appropriate wavelength. The PSP material used for radiographic imaging is “europium doped” barium fluorohalide. Fiberoptics conduct light from the PSP plate to a photomultiplier tube. The photomultiplier tube converts light into electrical energy. The variations in voltage output from the photomultiplier tube correspond to variations in stimulated light intensity from the latent image. The voltage signal is quantified by an analog-to-digital converter and stored and displayed as a digital image. Advantages and disadvantages of digital radiography Advantages: No need for chemical processing, thus avoiding the hazards associated with handling chemical solutions. Superior gray scale resolution Increased speed of image viewing Easy storage of patient information and records  Easy transfer of images electronically. Image enhancement and manipulation. Increased efficiency at work place Disadvantages: Initial set up cost is expensive Sensor size and thickness Large pixels result in poor resolution and structures may not be represented accurately Diagnostic image quality screens/monitors are required for optimal viewing. Infection control Wear and tear Images need to be backed up to a separate storage area regularly. Loss of image quality and resolution on hard copy print-outs when using thermal, laser or ink-jet printers. Legal Issues