Introduction to digital imaging:
In contrast to conventional imaging digital imaging provides better letter after resolution as it employs the pixels or picture element on the image display. Therefore, temporal resolution, special resolution and contrast resolution are much better in comparison to conventional imaging. Further the digital imaging has advantage of recording the large number of images for a long time and a retrieve and derived image whenever required. It helps in better correlation between diagnosis and treatment of the disease. Digital radiography removed the requirement of the dark room. It was based on the digital/binary system, different from the conventional radiography which uses analog/continuous values.
Digital imaging includes:
History of digital imaging/digital radiography:
It was first installed in 1970s until the microprocessor and semiconductor memory systems were developed in 1980s and were designed to process the large amount of data generated from the system,
Pixel: The pixel is defined as the smallest unit of a digital image or graphic that can be displayed and represented on a digital display. The pixel is the logical unit in digital graphics.
The physics of the digital radiography uses the binary system (0, 1) in bits, pixels with 8 bit accuracy. Most of the digital system contains one pixel with 8 bit accuracy. So, one pixel has brightness values 0-255 decibel, a total of 26 (256) possible brightness values. Radiographic image contains block and white shades mostly. The FSR (film screen radiography) is replaced by digital radiography. FSR (Film screen radiography) film acts as both detector and the storage medium, while digital detectors are only used to generate the digital image, which is then stored in digital medium. The FSR (film screen radiography) is replaced by digital radiography
Advantages of digital imaging:
Digital imaging consist four separate steps:
Fluoroscopy and fluorography
Fluoro means: fluorescence material and scopy means: view (live images). It gives real time image.
Fluoroscopy is the process of reading real time image of the internal anatomical structure using a fluoroscopic screen made up of fluorescent material. The complete setup includes fluoroscopy screen, cassette holders and spot film device. There is an under couch tube lying below the X-ray couch that continuously emits radiation which penetrates the body of the patient then strike the fluoroscopy screen in front of the area of the interest. The fluoroscopy screen is made up of cesium iodide or zinc cadmium sulphate which is rare earth phosphor material. They absorb the energy of X-ray photons coming out of the body of the patient and convert them into light photons to display the live anatomy of the internal structures on the fluoroscopy screen.
The fluoroscopic unit consist of peddle switch which helps in continuous emission of X-rays from the tube. There is a cassette holder and spot film device to record the spot films of the designed area. Before fluoroscopy the radiography room has to be adopted for low lightening so that the image on the fluoroscopic screen is clearly visualized. Because fluoroscopy uses a continuous emission of high radiation exposure, radiation safety is a prime concern in fluoroscopy therefore techniques like intermittent fluoroscopy or on and off fluoroscopy is used for examinations that required prolonged fluoroscopic screening. For radiation safety of the operator, rubber flaps, lead apron, lead goggles, thyroid shields & lead gloves etc. are used.
Fluorography: It is the technique of the recording the real time image of area of interest during the fluoroscopy.
The X-ray image intensifier is used as a substitute of convectional fluoroscopy in special radiological procedures in modern scanning. It has several advantages over conventional fluoroscopy which are as under:
Image intensifier is made up of a vacuum tube having input phosphor plate and output phosphor plate. The input phosphor works as cathode and output phosphor as anode. It also has electron guns which focus the electron beam towards the output phosphor. The overall function of an image intensifier is to convert the energy of the incident electron photon into light photons of sufficient intensity to provide a visible real time image this occurs in several stages.
In the first stage incident ray photons are converted into light photons by the input phosphor. The input phosphor has a combination of cesium iodide (rare earth material) activated with sodium cesium iodide has h high atomic number because of which its absorption and conversion efficiency is very good. Further sodium acts as an activator to enhance the efficiency of input phosphor are converted as electrons by the help of a photo diode. As a result of this conversion a potential difference of 25-30Kv produced between the cathode and anode. Because of this high potential difference the electron beam is strongly attracted towards the anode and strike the output phosphor. There are several electronic guns placed at the inner side of image intensifier at various locations which help in replacing and focusing the electron beam towards the output phosphor plate.
Usually the output phosphor is made up of silver activated zinc-cadmium, sulphide. It absorbs the photo electrons coming out from the cathode and further converted into visible light photons which are displayed as a real time image on TV monitor with the help of different image transfer methods like DICOM transfer. Between the input and the output phosphor the image building capacity of the X-ray photons is magnified many times because they are multiplied several thousand times during the process of image intensifier highly sensitive to X-rays and show the overall radiation dose to the patient decreases many times.
Angiography: Angiography is the special radiological investigation dose to evaluate the vascular system of body with the help of intravenous injection of a positive contrast media.
Digital subtraction angiography (DSA): Digital subtraction angiography is a fluoroscopic imaging technique which is used in interventional radiology to clearly visualize the blood vessels of human body in a dense bony or soft tissue environment.
DSA was developed by Portuguese neurologist Egas Moniz in 1927 and the idea of image subtraction was given by Dutch radiologist Des Plantes in 1935.
DSA was frequently used during 1975 onwards after the sell dinger technique of femoral axis for angiography was dveloped in 1972, by Edwin Selldings.
In digital subtraction angiography images are produced using contrast medium and subtracting the pre contrast image or mask images from the subsequent images. DSA is acquisition of fluoroscopic images combined with injection of a positive contrast media and real time subtraction of pre contrast image and the most contrast image to perform angiography. Because of shading off or subtraction of the surrounding tissues/environment the process is known as digital subtraction angiography
Common indication for digital subtraction angiography:
Contraindication for digital subtraction angiography:
Patient preparation for angiographic procedures:
Contrast media: water soluble, iodinated, non-ionic contrast media like Iohexol 76% is used in 40ml volume and given intravenously through pressure injection.
Complication of angiographic procedure: It could be due to contrast media, due to anesthesia or due to technique.
Due to contrast media: General reaction like mild moderate or severe contrast media reactions.
Due to anesthesia: General anesthesia affects central nervous system (CNS)
Due to technique:
In convectional angiography images are acquires by exposing the area of interest with the image intensifier while injecting the contrast media into the blood vessels. However, the image obtained in angiography also includes over time structures and surrounding tissues apart from the contrast enhanced blood vessels. While the images are useful in determining the anatomical location of the blood vessels and gross variation in morphology these are not helpful in describing the blood vessels accurately due to the overwind and underlying structures, digital subtraction angiography is quite a common procedure. In this technique we first take a mask image of the interested area. The mask image is a simple radiography of the same area before the contrast media is injected into the blood vessels. The radiological device used to capture the image is a high definition image intensifier which than keeps on taking the images of the same area at a fixed rate of 1-6 frames/sec. each subsequent image of the area gets the original mask image subtracted out.
Mathematically speaking the incoming image is divided by the mask image. The radiologist will control as how much contrast media has to be injected, the rate of injection and the time for which the contrast media has to be injected.
Blood vessels of the smaller structures takes less time to be filled enhance the volume of contrast media is require less in comparison to large area of interest.
PACS: Picture archiving communication system
The picture archiving communication system is the substitute of traditional system of keeping the image record it provides economical storage, easy and quick access to a given series of examination and also causes convenience of the operator.
It also increases the overall efficiency of the department. Further the images stored in PACS can be transmitted to any other place through digital mode of transmission and therefore tele-diagnosis and tele medicine is possible.
The universal software used for digital transmission and storage of image in PACS is known as DICOM (digital imaging and communication in medicine). Non image data such as scanned documents, reports, prescriptions, etc. may be transmitted in industry standard formats like pdf (portable document format). Once the image is incorporated in PACS system. PACS has four important components such as:
PACS has following important uses:
Flat panel detectors: The flat panel detector system is an alternative conventional cassette used in diagnostic radiology which gives much more accurate diagnostic information better contrast and spatial resolution at a cheaper and economical cost. It is a widely used image recording device in digital radiography. The flat panel detectors are a type of solid-state x-ray, digital radiography device which are very much similar in function to the image sensor used in computed radiography, digital photography and videography.
The flat panel detectors can be used both in projection radiography and also as an alternating to image intensifier in diagnostic radiography. There are two major categories of flat panel detectors such as
Advantages of flat panel detectors over conventional radiography:
Xeroradiography is derived from the Greek word Xero means dry. Therefore, xeroradiography is the process of recording the image of internal anatomical structure without using any wet chemical processing unlike the manual system of development. Xeroradiography is a process in which picture of the body is recorded on a special type of paper known as the xeroradiographic film rather than normal silver halide emulsion x-ray film. In this technique a plate of selenium which is applied over a thin layer of aluminum oxide is uniformly charged by passing it in front of Scorton. Scorton is a corona discharge emitting material.
The corona discharge is a phenomenon in which a fluid, generally the air is ionized by photo conductor which is having a high electrical voltage or strongly charged.
When the selenium plate is strike with x-ray photons the charge on the plate is uniformly distributed or diffused out through out the selenium plate. The distribution of charge depends upon the intensity or proportion of x-ray photons striking the selenium plate this happens because of photo conduction the restaurant image or the imprint is than sent to the reusable paper plates. Since it doesn’t involve any photographic development stage or any kind of wet chemical processing. It is known as the dry imaging technique or xeroradiography. Xeroradiography requires more radiation dose in comparison to the manual radiography. Earlier it was used for mammography by the invasion of digital mammography.
Advantages of xeroradiography: