Digital Radiography Flashcards
What are the advantages of DR over SFR?
Improving image quality
Reducing patient radiation dose
Storage and management of images in digital form
Increasing patient throughput
Why is digitisation of SFRs not appropriate?
This approach has been found to considerably increase the workload of radiographic staff. Also, the resulting image is degraded below the (already limited) quality recorded on the original film. Film digitisation has, therefore, not been widely adopted as a gateway to DR and PACS.
What are the 3 generic stages of A DR detector?
X-rays are initially absorbed from the incident x-ray beam by the detector layer
The absorbed x-ray energy is then converted into a latent image (typically in the form of a pattern of electrical charge)
Finally, the latent image is read out as an (analogue) electronic signal prior to digitisation by the ADC
To be clinically effective, a DR image detector must have the following typical performance requirements (6):
Field coverage must be large enough to provide full anatomical coverage in all relevant radiographic examinations
The radiation sensitivity is high enough to ensure low dose operation
Each picture element (pixel) forming the image detector, must be sufficiently small to ensure adequate spatial resolution
The dynamic range must be large enough to cover the full range of radiation intensity both within a single exposure and across different examinations and patient sizes
Noise sources must be kept to a minimum to ensure optimum image quality
The image readout and display time must be sufficiently short to ensure rapid availability of images and efficient throughput of patients
Which of the following are characteristics of a DR system?
A. It is not compatible with an automatic exposure control
B. The latent image can take the form of a pattern of electrical charge
C. The DAC precedes the ADC
D. High x-ray sensitivity is needed to minimise patient radiation dose
A. Incorrect. Digital radiography systems can be operated under either manual or automatic control of the radiographic exposure factors.
B. Correct. The absorbed x-ray energy is converted into a latent image that is typically in the form of a pattern of electrical charge stored in the detector elements prior to being read out.
C. Incorrect. The latent image is read out as an analogue electronic signal that must be digitised by the ADC. The digital image may later be changed back to an analogue signal by a DAC, for example in order to be displayed on a PACS monitor.
D. Correct. The sensitivity of the detector is an important factor in determining the radiation dose required to produce a given image quality.
How is the pixel spacing derived from the analogue input?
In general, the formation of a pixelated image requires discrete sampling of an analogue signal such as a field of light photons or a pattern of stored electronic charge carriers. The pixel spacing or sampling interval 𝛥 is the distance between the centres of two adjacent pixels, measured at the entrance plane of the image detector.
What causes aliasing in DR?
the greater the fN the greater the spatial resolution of the acquired image. Sampling theory requires that the input signal which is sampled must not contain high spatial frequencies (equivalent to fine detail structures) which exceed fN - otherwise the digitised image will be subject to so-called aliasing artefact.
What is the fill factor of a system?
In solid-state designs of DR detector, an array of microcircuitry is integrated with the x-ray absorption layer. This makes it possible to directly read the image out of the detector in electronic form.
The readout microcircuitry comprises a high resolution array of active electronic components. Microcircuit design constraints inevitably mean that a fraction of the pixel area is occluded by the superstructure of the active component (plus associated switching and data lines). In principle this effect can reduce the sensitivity of the x-ray detector.
The degree to which the active portion of the pixel is occluded is described by the fill factor (FF) of the detector
How does bit depth change grayscale?
The greyscale value is expressed as a binary (or base-2) number. This can be compared with our normal counting system which uses the base-10 or decimal number system.
The greyscale resolution of a digital image is defined by the number of bits (binary units) of information per pixel where 1 bit of information per pixel corresponds to two possible greyscale values (0,1). Obviously 1 bit is suitable for depicting text, line drawings and graphs but is inadequate for representing a greyscale image.
10 bits - Corresponds to a choice of 2^10 or 1024 greyscale values {viz., 0 to 1023} - this is the minimum number of bits needed to represent a digital radiography image.
state of the art systems use 14bit
N bits Corresponds to 2^N possible greyscale values {viz., 0,1,2… to 2^N-1}.
How do you calculate the amount of data in an image?
N x M x G
N x M is the total number of pixels making up the image.
G is the associated greyscale information per pixel expressed in bytes.
Why is image compression used?
Over time, a radiology department utilising DR will generate prodigious amounts of image data.
The archive capacity requirements will, however, be eased if digital data compression is used.
Image compression, by up to a factor ~3:1 or so, is possible with no loss of image quality if a so-called reversible data compression algorithm is used.
More powerful so-called irreversible compression algorithms can be used to achieve compression factors of 10:1 or even greater.
It should be noted that as the degree of (irreversible) compression is increased the quality of the images will deteriorate, making them susceptible to artefacts and reduced spatial resolution.
What is x-ray luminescence?
The physical mechanism which describes the conversion of x-ray energy into light in a phosphor screen
What are the 2 subtypes of xray luminescence?
X-ray fluorescence
This describes the prompt (immediate) emission of light - this is the mechanism which underpins screen film radiography (SFR).
X-ray phosphorescence
This describes interactions where the emission of light is delayed over a timescale of many minutes, hours and days.
WHat radiographic technique uses phosphoresence?
X-ray phosphorescence is the physical mechanism exploited in the digital x-ray imaging technique known as computed radiography (CR).
WHat are the components of a CR imaging plate?
A CR IP comprises a thin layer of storage phosphor crystals embedded in a polymer binder.
The x-ray sensitive phosphor layer is coated upon a plastic substrate for mechanical support.
The top surface of the IP is protected from abrasion by a layer of toughened plastic.stals embedded in a polymer binder.
WHat is a common storage phosphor in CR?
The one commonly used in CR IPs comprises barium fluorohalide crystals activated with divalent europium ions i.e. BaFX:Eu2+.
X stands for a specific mix of halogen atoms selected from bromine, iodine and possibly chlorine atoms.
What is the difference between Standard CR IPs and High resolution IPs?
A standard IP comprises a thicker layer of phosphor crystals of larger size and includes a light reflection layer to increase sensitivity.
A standard IP typically absorbs 40% of the x-ray photons incident upon it.
A high-resolution IP comprises a thinner layer of finer phosphor crystals and usually does not include a light reflection layer.
High-resolution IPs are reserved for examinations demanding high spatial resolution.
High-resolution IPs have lower fractional x-ray absorption efficiency and therefore demand a higher x-ray dose than ST IPs.
Where is the IP housed during the film exposure?
During the exposure of the patient the IP is housed in a light-tight plastic or metal container known as a cassette.
Image plate cassettes have a similar construction to those used in conventional radiography, having a radiolucent front face (often constructed from carbon fibre) plus a lead backing layer.
How is a latent image formed in CR?
The absorption of an x-ray photon in a phosphor crystal gives rise to a high energy photoelectron which subsequently ionises a large number of atoms along its track releasing thousands of electrons.
In CR, the latent image comprises a pattern of electrons which have become temporarily trapped at specific sites throughout the layer of phosphor crystals. These electron traps correspond to dislocations in the crystal structure which are created during the IP manufacturing process.
The pattern of trapped electrons mirrors the intensity distribution of the x-ray beam originally incident upon the IP. On average, each x-ray photon absorbed in the IP gives rise to over a hundred trapped electrons.
Under normal conditions, electrons remain trapped for some time until they spontaneously relax back to their ground state releasing their stored energy as light photons
How is the image readout in CR?
the IP is scanned with a laser beam operating in the red part of the optical spectrum. The laser beam scans the IP with a raster pattern (corresponding to a set of parallel stripes) enabling the whole CR image to be read out in a systematic way.
Laser stimulation of trapped electrons causes them to immediately relax back to their ground state releasing their stored energy as light photons but in a structured way so an image can be constructed.
The stimulated emission occurs in the blue part of the optical spectrum. Laser stimulated emission is a so-called ‘destructive readout mechanism’ as it eliminates the latent image in the process.
What energy bands are there within a phosphor?
The phosphor used in an IP is an electrical insulator and therefore has an energy band diagram comprising:
A set of filled valence states (corresponding to states where electrons are tightly bound to atoms) plus
An empty conduction band (corresponding to states where electrons would be free to move through the material)
The valence and conduction bands are separated by a large energy gap representing states which electrons are forbidden (by laws of physics) from occupying.
In a CR IP however, situated within the energy gap close to the bottom of the conduction band are special electron states which are empty and correspond to the electron traps
WHat is the typical diameter of the laser in CR readout?
typically 0.1 mm
WHat happens to the IP after laser scanning?
the IP is transported past a bank of high intensity lamps which erases any residual CR signal
WHat happens to the stimulated emission signal in CR?
The signal photons have to be collected with high efficiency using a carefully designed light-guide.
Signal photons are conveyed via the light-guide to a high sensitivity light sensor known as a photomultiplier (PM) tube. A blue filter is mounted over the PM tube to exclude extraneous light.
As the laser scans the IP the PM tube produces a time series electrical current. This electrical current is signal processed before transfer to the analogue-to-digital converter (ADC). The ADC converts the electrical current signal to a stream of digital data which is transferred to the system computer.
A CR system exploits which of the following physical processes?
Laser stimulated phosphorescence
What does the detector dose indicator provide?
provides the operator with an indication of the mean dose incident on the image plate (IP). Unfortunately, there is not yet a standard definition of DDI. The relationship between DDI and IP dose varies depending on the manufacturer – for example, DDI may be inversely related to dose, or a function of the log of the dose.
In practice, interpretation of the DDI is often complicated by the fact that its value also depends on certain factors. Nevertheless, the DDI represents an invaluable quality control tool.
How does the curve for CR differ from SFR?
The intensity response of CR is totally linear, with no low or high dose saturation effects.
The corresponding dose latitude, often referred to as the dynamic range, of CR extends over a very wide range of detector entrance dose levels, ~ 104:1.
The CR response can be compared with the familiar (non-linear) S-shaped characteristic curve of radiographic film
