Digital Imaging and DSA

Presented by :

Nisha Karna

M.Sc.MIT 1^st year

Roll.no 25

NISHA KARNA

Introduction

• Radiography is an art and science of recording images produced by X-rays on X-ray film.
• Till the past decade, the conventional film screen system was the most widely used and accepted method of recording images, wherein the film acts as an image receptor, display medium and a source of permanent record.
• Though these films are relatively inexpensive and provide high image quality, they have many limitations.

NISHA KARNA

Introduction

• Significant advancement in electronics and computer technology have resulted in film screen radiography  (FSR) systems being replaced by digital radiography (DR) systems.
• The process wherein digital detectors are used to capture information of an object is termed as digital radiography

NISHA KARNA

Advantages of digital radiography

• Digital images do not deteriorate physically or degrade chemically over time •
• They allow a true reproduction of quality from copy to copy and from generation to generation
• Digital images are flexible, allowing a variety of manipulation such as magnification, cropping, edge enhancement, compression, etc.
• Digital detectors allow implementation of a fully digital picture archiving and communication system (PACS) in which images are stored digitally and available anytime.
• The images can be distributed electronically by web based technology with no risk of losing clarity

NISHA KARNA

Evolution of digital radiography

Year Technique

1977. Introduction of DSA
1978. DSA first put into clinical use

1980 Use of storage phosphor image plates

1987. Amorphous selenium based image plates

1990 Charged couple device was introduced

1994. Selenium drum DR
1995. Amorphous silicon CSI flat panel detector

1997 Gd based FPD

2001. Gd based portable FPD

2001 Dynamic FPD fluoroscopy DSA

2007 Portable CR system

​

​

​

NISHA KARNA

The physics behind digital radiography

• Digital imaging uses binary system in which information is interpreted as bits.
• Most digital systems handle individual pixels with eight bit accuracy for displayed brightness

NISHA KARNA

NISHA KARNA

The physics behind digital radiography

• Digital imaging consist of four separate steps:

1. Image generation

2. Image processing

3. Image archiving

4. Presentation of the image

NISHA KARNA

Principle of digital radiography

• The digital detector is exposed to X-rays generated by a standard tube.
• The energy absorbed by the detector is transformed into electrical charges, which are then recorded, digitized and quantified into a gray scale.
• After sampling, post-processing software is required for organizing the raw data into a clinically meaningful image

NISHA KARNA

Digital radiography systems

• There are two basic types of digital radiography systems
• Computed radiography (CR) system use a storage Phosphor image plate enclosed in a light tight cassette.
• Direct digital radiography (DR) uses detectors that have a combined image capture and image read out process.
• DR systems can be further divided to direct and indirect conversion groups

NISHA KARNA

Digital radiography systems

NISHA KARNA

Computed Radiography

• Computed radiography (CR) systems use storage phosphor image plates with a separate image read out process
• The imaging plates are coated with europium activated barium fluorohalide (BaFX:Eu2+), the halide being bromide, iodide or a combination of both
• When the storage phosphor image plates (IP) are exposed to X-rays, the X-ray energy is absorbed and temporarily stored by the crystals by bringing electrons to higher energy levels

NISHA KARNA

Computed Radiography

• In the readout process, the image plate/detective layer after exposure to X-rays is taken to a CR reader where it is scanned by a high energy laser beam of a specific wavelength( 790nm)
• The stored energy is set free as higher energy blue light having a wavelength different from that of the red laser light.
• This light is collected by photodiodes/photomultipliers and signal converted digitally into electrical signal by an analog-to-digital converter(ADC) then amplified, digitized and used to form an image
• The imaging plate is ready for use again after exposure to white light

NISHA KARNA

Computed Radiography

NISHA KARNA

Computed Radiography

NISHA KARNA

Computed Radiography

NISHA KARNA

Computed Radiography

Advantages

• Being cassette based, CR systems can easily be integrated into existing radiographic equipment.
• Single CR systems can convert multiple radiography rooms to digital technology.
• They have a wide dynamic range leading to reduced rates of failed X-ray exposure.
• CR cassettes can be placed in any position thereby enabling flexibility for positioning for difficult views.
• In case of defect in the image plate, it can easily be replaced by the radiographer with no need for specialized equipment or service personnel

NISHA KARNA

Computed Radiography

Drawbacks

• It is a time consuming technique.
• Spatial resolution is lower than that of film screen radiography.
• Radiation dose required is same or more than film screen radiography

NISHA KARNA

Direct Digital Radiography

• Digital radiography (DR) or direct digital radiography is a way of converting X-ray into electrical charges by means of a combined image capture and image readout process

​

​

NISHA KARNA

Direct Digital Radiography

• Direct conversion detectors have an X-ray photoconductor such as amorphous selenium that directly converts X-rays photons into an electric charge.
• Indirect conversion detectors have a two-step process for X-ray detection
• A scintillator -when X-rays strike the scintillator, the X-ray energy is converted to visible light.
• The visible light is then converted into an electric charge by means of photodetectors such as amorphous silicon photodioide arrays or CCDs (charged couple device)

NISHA KARNA

Direct Conversion

• Direct conversion type of detectors uses amorphous selenium as photoconductor that converts X-rays photons into electrical charges by setting electrons free.
• Selenium-based direct conversion DR systems are equipped with either

(a) a selenium drum or

(b) a flat panel detector.

NISHA KARNA

Selenium Drum-based System of Direct Conversion

• A rotating selenium drum with a positive electric surface charge is exposed to X-rays.
• During exposure, a charged pattern proportional to that of incident X-rays is generated on the drum surface and is recorded during rotation by an analog-to-digital converter
• A limitation of selenium drum detectors is that they are dedicated thorax stand systems with no mobility at all.

NISHA KARNA

Thin Film Transistor—Direct Conversion

• These arrays are constructed by adding an X-ray photoconductor as the top layer of the electronic thin-film transistor sandwich.
• Amorphous selenium is used as the photoconductor material
• It is sandwiched between two electrodes to which high voltage is applied.
• When this layer is exposed to X-rays, electrons and holes are produced
• The electronic charge is stored in capacitors and is read out sequentially

​

NISHA KARNA

Direct Conversion

NISHA KARNA

Thin Film Transistor—Direct Conversion

• These detectors have very high spatial resolution, moderate X-ray absorption efficiency (DQE) and an excellent fill factor
• Another advantage of amorphous selenium TFT array over selenium drum is that these detectors can be mounted on thorax stands and bucky tables

​

NISHA KARNA

Thin Film Transistor—Indirect Conversion

• Indirect conversion based on thin-film transistor (TFT) arrays are constructed by adding amorphous silicon photodiode circuitry and a scintillator as the top layers of the thin film transistor sandwich
• When X-rays strike the scintillator, visible light is emitted proportional to the incident X-ray energy.
• Visible light photon are then converted into an electric charge by the photodiode array, and the charge collected at each photodioide is converted into a digital value by using the underlying readout electronics

NISHA KARNA

Thin Film Transistor—Indirect Conversion

NISHA KARNA

Types of scintillators

• The scintillators used in indirect conversion detectors can be either structured or unstructured
• Unstructured scintillator: With use of unstructured scintillator, the visible light emitted by the material can spread to adjacent pixels, thereby reducing spatial resolution.
• Structured scintillators: To reduce the problem of scatter, some manufacturers now use a structured scintillator that consists of cesium iodide crystals that are grown on the detector

NISHA KARNA

Types of scintillators

NISHA KARNA

Types of scintillators

• Thallium-doped cesium iodide (CsI) is the most commonly used phosphor material.
• Another material that is used is gadolinium oxysulphide or Gadox (Gd2 O2S).
• CsI - based detectors are more efficient in X-ray absorption than gadox detectors and have better DQE

NISHA KARNA

Charge Coupled Devices

• Charged coupled devices (CCDs) is a light sensitive sensor for recording images that consists of an integrated circuit containing an array of linked or coupled capacitors
• Charged coupled devices can be used for radiography as part of either

(a) a lens coupled CCD system, or

(b) a slot-scan CCD system.

​

​

NISHA KARNA

Charge Coupled Devices

• The lens system is an array consisting of several CCD chips which form 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
• A drawback of the lens system is that it substantially reduces the number of photons that reach the CCD.

​

NISHA KARNA

Slot-scan CCD system

• Makes use of a special X-ray tube with a tungsten mode.
• The patient is scanned with a collimated fan-shaped beam, which is linked to a simultaneously moving CCD detector array having a match ing detector
• Slot-scan CCD systems are dedicated to chest radiography, mammography or dental radiography

NISHA KARNA

NISHA KARNA

Aspects of image quality

• The major factors effecting digital image quality are
• Pixels: The pixel is the foundation block of digital imaging.
• It is the smallest complete sample of an image
• Maximum achievable spatial resolution is defined by pixel size and spacing.
• Pixel size is a measure of resolution, wherein, smaller the pixel size (or larger the matrix), better the resolution

NISHA KARNA

Aspects of image quality

• Detector Size: The detector should be sufficiently large to capture the desired anatomic views and sufficiently compact for all clinical applications.
• Gray Scale: Gray scale refers to shades of gray available for reproducing faithfully subtle differences in densities in an accurate manner
• More the number of shades available, superior is the display
• For routine visual display, 256 intensities (shades of gray) are recorded on a nonlinear scale.

NISHA KARNA

Aspects of image quality

Spatial Resolution

• It describes the ability to distinguish fine spatial detail and to differentiate objects in an image
• As a rule of thumb:
• Increasing the sampling frequency helps to increase resolution
• Images composed with a greater number of pixels have a higher spatial resolution. However, images with higher resolution require large file sizes
• Spatial resolution in digital acquisition systems is expressed as pixel size, pixels/mm, or line pairs per millimeter (lp/mm)

NISHA KARNA

Aspects of image quality

Contrast Resolution and Dynamic Range

• Contrast resolution refers to the amount of gray scale differentiation that exists in an image.
• Digital detectors have high contrast resolution thus enabling thousands of shades of gray to be displayed
• In digital detectors, dynamic range is the range of X-ray exposure over which a meaningful image can be obtained

NISHA KARNA

Aspects of image quality

NISHA KARNA

Aspects of image quality

Modulation transfer function:

• Modular transfer function (MTF) is the capacity of the detector to transfer the modulation of the input signal at a given spatial frequency to its output.
• The ratio of the output and input modulation is known as the modulation transfer function.
• MTF varies with spatial frequency.
• Generally reduces progressively from 100 percent at low spatial frequencies towards zero at higher frequencies

NISHA KARNA

Radiation Exposure

• Digital detectors besides providing better image quality have the potential for substantially lowering the patient exposure
• DR systems offer a significantly higher potential for general exposure reduction because of their far superior quantum efficiency

NISHA KARNA

Newer Applications in CR

• Newer technological advances in CR have been introduced to overcome some of the limitations of CR system such as cassette handing, long readout time of PSP plates, low DQE and poor resolution.

1. Automated CR systems:

• Efficiency of CR systems has been recently improved by reducing the readout time and by removing the step of cassette handling.
• The readout time of PSP plate is reduced to less than 10 sec by line scan lasers and photodiode detectors in automated CR systems

NISHA KARNA

Newer Applications in CR

2. Newer structured phosphor for PSP plates:

• Newer phosphors like cesium bromide having a structured needle-shaped configuration of crystals have been recently introduced which reduce light diffusion.
• They are also more efficient with an increased DQE

NISHA KARNA

Newer Applications in CR

3. Mobile CR systems:

• To save labor, time and improve workflow of critically ill patients requiring bedside X-rays, portable, compact CR systems have been introduced in 2007.
• These systems have a mobile X-ray unit with an integrated CR reader which does away with the physical transport of the cassette to the CR reader, hence image is available in less than 25 seconds.

NISHA KARNA

Newer Applications in DR

• A number of technological advancements have taken place in digital radiography in hardware as well as software applications
• Tomosynthesis:
• In this technique multiple low dose exposures are given from various angles while the X-ray tube moves in an arc and the detector remains stationary.
• The images can be viewed singly or in a wire loop

NISHA KARNA

Newer Applications in DR

2. Dual energy imaging:

• Dual energy radiography methods are used to generate separate images of bone and soft tissue structures from two exposures made using different radiographic techniques,

3. Computer aided diagnosis (CAD):

• They 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 main advantage of CAD is that it alerts the radiologist to avoid overlooking diagnostically significant findings

NISHA KARNA

Newer Applications in DR

4. Automatic image stitching:

• This feature is useful when precise measurements in lengthy anatomical regions like the spine or lower limb are required.

5. Mobile DR:

• It consists of a 17 × 14 inch flat panel detector connected by a cable to a mobile X-ray system having a monitor.

​

​

NISHA KARNA

Newer Applications in DR

6. Wireless FPDs:

• A wireless portable DR system can transfer image data wirelessly to the DR system. It does not have any cables and does not interfere with surrounding machines

7. Fluoroscopy and radiography:

• Real time digital imaging facilitates high quality radiography and fluoroscopy with up to 30 images/sec

NISHA KARNA

Exposure Indicator for Digital Radiography

• In contrast to FSR, where visual features such as under/ overexposure help in recognizing exposure errors, digital system lack in recognizing the same.
• As a result, the radiographer needs to monitor the exposure indicator (EI) associated with digital imaging system
• EI allow the radiographer to know the level of exposure the receptor has received and thereby determine if the correct exposure technique for the image was used

NISHA KARNA

Exposure Indicator for Digital Radiography

NISHA KARNA

​

Digital subtraction angiography

NISHA KARNA

Introduction

• Digital subtraction angiography (DSA) refers to a digital imaging technique for better visualization of contrast filled vessels
• Permits use of lower amount of contrast medium.
• Here, the pre-contrast image is digitally subtracted from post-contrast image to provide background suppression.
• This allows better visualization of low contrast vessels, but at a cost of increase in noise

NISHA KARNA

Types of Subtraction

• The types of subtraction techniques include
• Mask mode,
• Time interval,
• Dual energy and
• Hybrid subtractions.

NISHA KARNA

Mask-mode subtraction

• It is the most widely used process of DSA in which temporal subtraction is done.
• It involves initial acquisition of a frame of region of interest which is used to stabilize the exposure factors.
• Then a second image is taken and stored as mask image
• This mask image is subtracted from subsequently acquired images on pixel–by–pixel basis and show only contrast filled structures.

NISHA KARNA

Mask-mode subtraction

NISHA KARNA

Time interval difference subtraction

• Another mode of temporal subtraction where a consecutive previous frame is subtracted from current frame (e.g, frame 1 from frame 2, frame 2 from frame 3 and so on)
• This technique is very useful in cardiac imaging where there is rapid motion
• Images obtained can then be stacked to provide a composite image without motion.

NISHA KARNA

Time interval difference subtraction

NISHA KARNA

Dual energy subtraction

• Another technique in which the region of interest is exposed to higher kV (120 to 130 kV) and lower kV (70 kV) at very short interval (about 50 ms).
• Then, the higher kV image is subtracted from the lower kV image to produce an iodine and bone image (the soft tissue and gas shadows are eliminated).
• This technique has the advantage of elimination of motion artifacts.

NISHA KARNA

Dual energy subtraction

NISHA KARNA

Dual energy subtraction

• The limitations of this mode of subtraction include:
• Increased radiation dose,
• Reduced opacity of contrast opacified structures,
• Reduced SNR and
• Increased complexity of the equipment

​

NISHA KARNA

Hybrid subtraction

• Is a combination of dual energy and temporal subtraction

NISHA KARNA

Techniques used in DSA

• Road map technique
• Fluoroscopy fade technique
• Dynamic 3D road mapping technique

NISHA KARNA

Techniques used in DSA

Road map

• It is a technique in which static fluoroscopic image is subtracted from densely opacified vessel.
• Here, a short contrast run of the vessel is done under fluoroscopy to select the frame with maximum opacification of the vessel as the road map mask, which is stored in memory.
• Then subsequent live fluoroscopic image is subtracted from the road map mask for visualization of the vessel and the catheter/guide wire

NISHA KARNA

Road map

NISHA KARNA

Road map

NISHA KARNA

(a) DSA image of a wire obtained without road mapping. It is more difficult to maneuver the wire through the vessels when the path of the vessels cannot be visualized. (b) Image of the wire obtained with road mapping. Real-time images of movement of the wire overlaid on a static image of the vessels facilitate guiding the wire through vessel turns.

Fluoroscopy fade technique

​

• In which a reference DSA image is overlaid on the real time fluoroscopic image

​

NISHA KARNA

Dynamic 3D road mapping technique

​

• Is new development allows projection of 3D reconstructed vessel on live 2D fluoroscopic image.

NISHA KARNA

Peripheral DSA

• Can be performed with single contrast injection usin stepping technique.
• Two types:
• Stepping table technique
• Stepping gantry method

​

NISHA KARNA

Stepping table technique

• In stepping table technique, the table moves into three stations with X-ray tube and detector remaining fixed.

​

NISHA KARNA

Stepping gantry method

• In stepping gantry method, the tube-detector/II moves keeping the table fixed
• In both methods, pre contrast images are acquired at different stations and stored as mask image.
• Subsequently, matching post contrast injection images are taken at same positions while chasing the bolus of contrast and subtracted from the corresponding mask image to produce clear image of peripheral arteries

NISHA KARNA

Benefits

• SignifIcantly reduces use of contrast
• Reduces examination time

NISHA KARNA

• An important advancement in digital fluoroscopy is rotational angiography where the X-ray tube – detector system rotates through usually 90° to 180° while acquiring continuous images
• Mask images are initially acquired at 0.8° to 2° gaps followed by post contrast rotation and acquisition at same positions.
• Then DSA images are produced at each angle and reconstructed to produce 3D angiograms

NISHA KARNA

Post-processing in DSA

Mask pixel shift

• Is a software modification feature used when smaller patient motion occurs after the mask image is acquired.
• By shifting the pixels of the mask image, reregistering of the mask with post contrast image is possible thus obviating motion artifacts
• This technique may be manual or automatic

NISHA KARNA

Mask pixel shift

NISHA KARNA

Post-processing in DSA

Remask

• Is a similar feature where another mask image is selected, which is temporally closer to the contrast image.
• This is useful when patient motion occurs prior to contrast image, but after initiation of the acquisition.

NISHA KARNA

Post-processing in DSA

Image summation

• is a property in which two or more frames of a DSA acquisition are summed into a single image
• is beneficial when rapid acquisition opacifies part of a vessel in each frame and summation adds up the frames to produce a single image showing the entire vessel.

NISHA KARNA

Image summation

NISHA KARNA

Image summation

NISHA KARNA

(a) Early-phase image from abdominal aortography. (b) Image obtained with image summation of early- and late-phase images shows single bilateral renal arteries and external iliac arteries.

Post-processing in DSA

Stacking

• Is a similar technique used in carbon dioxide angiography where the fragmented boluses of the gas are added to produce a complete picture of the opacified structure.

NISHA KARNA

Post-processing in DSA

Landmarking

• Is a feature in which lesser intensity (10- 20%, but is manually adjustable) of original image is added to the subtracted image
• Provides anatomical landmarks in subtracted images, useful in subsequent intervention.

NISHA KARNA

Landmarking

NISHA KARNA

Noise smoothening

• The technique operates by reducing the statistical fluctuations in each pixel by averaging the pixel with its closest neighbours, the first pixel is smoothened by averaging nine nearest neighbours.
• The final image is blurred image of the original

NISHA KARNA

Noise smoothening

NISHA KARNA

Edge enhancement

• Substracting noise smoothened image from the original image results in edge enhancement

NISHA KARNA

Edge enhancement

NISHA KARNA

Edge enhancement

NISHA KARNA

Last image hold

• Last image hold uses the digital information from the last frame (that is stored on the computer) and continuously provides this to the video system so that the display monitor continuously shows the patient’s anatomy even after the x rays have been turned off
•  Dramatically decrease the ionizing radiation exposure to the patient and medical imaging staff.

NISHA KARNA

Gray-Scale Processing�

•  Refers to adjustment of the displayed contrast and brightness of the digital image
• The goal is to map (associate) pixel values from our image data set to some shade of gray on our display monitor so that visualization of the anatomy of interest is optimized

​

NISHA KARNA

Temporal Frame Averaging�

• Temporal frame averaging is used to decrease displayed image noise
•  Is accomplished by continuously displaying an image that is created by averaging the current frame with one or more previous frames of digital fluoroscopic image data

​

NISHA KARNA

Dose reduction technique in DSA

• Intermittent fluoroscopy is the most important due to the fact that the radiologist can control fluoroscopy time and can judiciously use it intermittently to reduce total patient and personnel dose
• Last image hold technique helps in reduction of dose
• Use of reference image, in DSA is helpful in significantly reducing radiation dose and contrast medium
• Pulsed fluoroscopy provides substantial decrease in patient radiation exposure.
• Dose spreading is a method to reduce skin dose when lengthy interventional procedures are performed

​

​

NISHA KARNA

Conclusion

• The future of radiography is digital.
• And it is imperative that technologist/radiologists be familiar with the technical principles, criteria for image quality and radiation exposure issues with the various digital radiography systems that are currently available along with the technological advancements that are taking place.

NISHA KARNA

Thank you

NISHA KARNA