Digital imaging

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:

  • Computed radiography (CR)
  • Direct radiography (DR)

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,

Basic Physics:

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:

  1. Better image quality
  2. Better storage quality
  3. Images are safe physically and chemically
  4. Retrieving facility
  5. PACS (picture archiving communication system)
  6. Less radiation exposure
  7. No use of film in DR (direct radiography) system

Disadvantages:

  1. Exposure errors i.e. under/over expose
  2. Noise inherent in images
  3. Different densities in chest X-ray

Digital imaging consist four separate steps:

  1. Image generation
  2. Image processing
  3. Image archiving (PACS)
  4. Presentation of the image

Fluoroscopy and fluorography

Fluoroscopy:

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.

Image intensifier:

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:

  1. Marginally decreased overall radiation dose to the patient because of high conversion efficiency of the phosphor material used because of high atomic number and mass attenuation coefficient.
  2. As a result of decreased radiation dose longer examinations can be done without high risking of radiation dose.
  3. Image intensifier has a temporary to permanent image storage system like magnetic tapes to store the different images of an examination series. As a result there is no requirement of keeping the manual record, filing the images in the patient file or manual retrieving. It drastically reduces human effort.
  4. Image intensifier gives enhanced resolution and better image quality it doesn’t blur the images.
  5. Image intensifier has the advantage of easy excess to patient record and automatic retrieving system because of its storage capacity of images of previous and current examination can be compared for better diagnostic information.
  6. Image intensifier also provides reduced radiation dose to the operator as the X-ray photons falling the input phosphor are concentric and not scattered.

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:

  1. Arterial-venous malformation (AVM)
  2. Arterial-venous fistula (AVF)
  3. Varicose vein (veins with impaired venous return)
  4. Congenital anomalies (abnormalities related to birth)
  5. Renal stenosis
  6. Thrombosis (blood clotting inside artery)
  7. Aneurism and embolism
  8. Renal impairment due to VATER anamolis

Contraindication for digital subtraction angiography:

  1. Sensitivity to contrast media reactions
  2. Increased CT, BT and PTB (prothrombin time)
  3. Local sepsis
  4. Renal failure or senior cardiac disease

Patient preparation for angiographic procedures:

  1. Informed concern of the patient
  2. 6-8 hours of oral fasting
  3. Xylocaine test should be performed
  4. Remove all the radio opaque material from the area of interest
  5. The puncture side should be prepared as for surgery

Accessories required:

  1. Seldinger needle (it is a long 26 gauze needle with stylet and thick plastic cover)
  2. Xylocanine 2% as local anesthesia
  3. Teflon coated guide wire
  4. Catheter
  5. Spirited swap, cotton benzoid tincture
  6. X-ray unit with image intensifying facility
  7. Emergency drug
  8. Mackintosh towel, bandages, antiseptic solution, 20 & 50 ml syringes
  9. Contrast media

Contrast media: water soluble, iodinated, non-ionic contrast media like Iohexol 76% is used in 40ml volume and given intravenously through pressure injection.

Technique:

  1. The patient will come to the radiology department as per the appointment and preparation and after changing the hospital gown and removing the radio opaque materials, will lie supine on X-ray table.
  2. Make sure that all the diagnostic tests perform for the procedure are in normal value.
  3. After confirming that the puncture site is surgically prepared
  4. Fix the femoral artery on the place
  5. Give a small stab incision to remove the tissue layers over the femoral artery after injecting 2% xylocaine local anesthesia over the femoral artery
  6. Make a small hole in the femoral artery and inject the stylet into the femoral artery using a bevel angle of 45°
  7. If the needle is injected into femoral artery a give way feeling will be felt and blood will rush into the needle hub, if not the surgical needle should be withdrawn back and reinserted with changed angle or moving it slightly left/right/up and down. Once the needle is into femoral artery fix it with adhesive tape.
  8. Now gently remove the needle stylet and clean the blood
  9. Now only the hard plastic cover remains in-situ
  10. A metallic guard wire is gently inserted through this plastic cover and forwarded to the desired location under the control of image intensifier
  11. Now the outer plastic cover is also removed and a catheter is threaded over the guide wire and forwarded towards side of examination
  12. Now the guide wire is also removed leaving only the catheter is connected with contrast coated syringe and contrast media is gently inserted at a pre determine rate using the pressure injector.
  13. Now the imaging of the area is done and finally catheter is removed from the puncture side.
  14. The puncture side is sutures and sealed with benzoid tincture to avoid entry of any external infection into the blood vessels.
  15. The patient should be observed for any changes in vital sign (BP, temperature, pulse and dilation of pupils). If everything is normal discharge the patient with an instruction to consume plenty of oral fluid/water so that the contrast media circulating inside the body with blood is eliminated out through kidney at a rapid rate.

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)

Symptoms:

  1. Dizziness
  2. Change in vital signs
  3. Convulsion
  4. Vomiting / nausea
  5. Vision blurring

Due to technique:

  1. Bleeding and pain at the puncture site
  2. Hematoma due to improper vein puncture
  3. Thrombosis
  4. Retention of catheter or guide wire
  5. Perforation of walls of the artery due to tip of catheter
  6. Introduction to external infection into the body if aseptic conditions are not followed

After care:

  1. Observe the patient for an hour and note the vital signs. In case of any significant finding the patient should be kept in medical supervision.
  2. Ask the patient to have plenty of oral fluid so that the contrast media in the blood stream gets filtered out from the body as soon as possible.
  3. Provide mild analgesics to control pain. Also, prophylactic dose of antibodies may be required to cover the possibility of external infections.
  4. Seal the puncture site properly with benzoid tincture.

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:

  1. The imaging modality such as a plane x-ray film, CT scanner or MRI unit.
  2. A secured network connection for transmission of patient information.
  3. Work station where interpretation and reviewing of images and archiving of the images can be done.
  4. A storage device capable of storing sufficient number of images of a given examination series and retrieve the images and reports whenever require.

PACS has following important uses:

  1. Hard copy replacement
  2. Remote access
  3. Efficient and effortless storage along with easy retrieving
  4. It helps in radiology work flow management and increase the overall efficiency of the department.
  5. It gives a reasonable economical storage facility

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

  1. Direct flat panel detectors: The direct flat panel detectors are made up of a photo conductor named as amorphous and convert the incident x-ray photons directly into electric charge. When the x-ray photons strike upon a layer of amorphous selenium it generates an electron hole pair due to photoelectric effect by the interaction of x-rays with matter. A high voltage potential is applied to the depth of amorphous selenium layer which attracts the electron-hole pair towards the corresponding electrode. Therefore, the electricity or current generated is directly proportional to the intensity of radiation beam striking the selenium layers. The trapped-in energy or the radiation signal is than read out by using the thin film transistor applied just below the amorphous selenium layer and the data is that digitally transferred to the computer monitor to display the image of internal anatomical structures. The direct flat panel detectors give much better image resolution because it doesn’t involve any scintillation material therefore producing the blurring of the image. Further the indirect flat panel detectors use small pixel size with the help of the TFT technology and therefore can provide high spatial resolution.
  2. Indirect flat panel detectors: The indirect flat panel detectors contains a layer of scintillation material either gadolinium oxy sulphide or cesium iodide the scintillation layer absorb the energy of x-ray photons and convert them into light photons. Directed behind the scintillation layer there is an amorphous silicon detect array which is manufactures and installed using a technique very much similar to that used in production of LCD (liquid crystal display). Like in TFT or LCD millions of small sized pixels ranging from a size of 0.2mm-0.3mm are employed on a thin film transistor on the amorphous silicon mounted on s glass subtract. When the x-ray beam coming out of the body of the patient strike with the amorphous silicon layer, their energy is converted into right signals which is again converted into electrical signals by a photo diode connected with individual pixels and these electrical signals are than read out by the thin film transistor system after the electrical signals are amplified. The read-out signals are that digitally transmitted through wire transmission to the image processor which with the help of Fourier transformer convert them into digital signals to produce a viewable image ion the monitor.

Advantages of flat panel detectors over conventional radiography:

  1. Unlike conventional radiography there is no need to post the cassette from one place to another and therefore the transition time is saves which increases the overall efficiency of department.
  2. Negligible chances of damaging the image recording system like cassette breakage, leakage of light or damaging of intensifying screen etc. as compared to conventional radiography and computed radiography.
  3. Much cost effective as it doesn’t involve chemical processing also it in error free and therefore films are not spoiled in repeating the examination.
  4. It gives excellent tissue details and image contrast resolution as compared to the convectional radiography.

Xeroradiography:

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:

  1. Since there is no chemical processing steps involve it reduces the cost and also eliminate out the error and fault common during chemical processing.
  2. The paper plate that is being used in xeroradiography is recyclable.
  3. Xeroradiography gives better diagnostic information of internal trabecular pattern of the bones therefore is a diagnostic milestone in ruling out the disease like osteomyelitis (infection of muscles/or skin effecting bones)