Compare Direct Digital Radiography to Computed Radiography

Computed Radiography Detectors

Computed Radiography is a simplified term for, photostimulable phosphor (PSP)

detector systems. CR imaging plates are made of a mixture called Barium

Fluorohalide and more recently Cesium Bromide compounds, both doped with

Europium. (Bushberg, 2002).

An imaging plate is exposed to x-rays and they are absorbed by the photo-stimulable

phosphors. This absorbed x-ray energy is stored in the PSP and is read later by the

CR reader unit illustrated below (Figure 1,2). From here the laser light in the CR

reader stimulates the emission of stored x-ray energy and the visible light is released

from the plate. The light released from the plate is collected by a fibre optic light

guide and strikes a photomultiplier tube (PMT), where it produces an electronic

signal. (Figures 1,2) This electronic signal is digitized and stored as the image. To

entirely erase the plate ready for another examination the plate is exposed to a very

bright light source. (Bushberg, 2002)

Figure 1 (Korner, 2007) Figure 2 (Bushberg, 2002)

Direct Digital Detectors

Direct digital radiography requires a photoconductor that converts x-ray photons

into electrical charges by setting electrons free. The most commonly used

photoconductor element is selenium (z=34). The electrons released in the detector

layer are used to form the image directly. (Bushberg, 2002) A negative voltage is

applied to a thin metallic layer on the anterior surface of the detector. During

exposure x-rays are converted into electrical charges. The electrons excite and they

move through the selenium matrix by an electric field and are collected on the

detector elements. These detectors make use of the layer of selenium with a

corresponding underlying array of thin-film transistors (TFTs). The charge pattern is

recorded by the TFT array, which accumulates and stores the energy of the electrons.

(Figures 3,4)(Cowen, 2008)

With direct systems the electric field is applied to direct the electrons and in turn

eliminate blurring. Due to the ability to direct the path of electrons in direct flat

panel systems, the spatial resolution is limited only by the dimensions of the

detector elements, therefore improving spatial resolution. (Yaffe, 2007)

In summary CR systems use x-ray storage-phosphor image plates with a separate

image readout process and DDR is a way of converting x-rays into electrical charges

by means of a direct readout process.

Image quality

After exposure and readout, the raw data is processed to be displayed on computers

and placed in storage systems such as PACS, Web PACS or film. Processing software

can manipulate the image quality but it is the physical variations within the detector

systems that have the most influence on image quality.

Image quality is contributed by a wide variety of factors in an imaging system. The

quality can be affected by pixel/matrix size, scatter, dynamic range, spatial

resolution, attenuation coefficients and detector quantum efficiencies (DQE).

(Korner, 2007)

Dynamic range and Attenuation/Absorption Efficiency

The dynamic range describes the ratio between the highest and lowest x-ray

exposures a detector can accommodate that will contribute information to

meaningful image (Spahn, 2005). Both CR and DDR obtain wider and linear ranges

Figure 3 (Bushberg, 2002)

Figure 4 (Cowen, 2008)

(10000:1) compared to conventional film screen methods. Better dynamic range

allows enhanced image manipulation known as windowing, which is the adjustment

of contrast of an image after the image has been acquired (Figure 5). It also accepts a

wider variety of doses to be used in obtaining satisfactory images. This reduces the

risk of repeat exposures and enables high quality contrasts of soft tissue and bony

structures using one particular exposure. (Korner, 2007)

The particular chemicals used in CR and DDR effect the rate of photon absorption

and their varying K shell characteristics can enhance the diagnostic yield. CR

phosphor chemicals have a high absorption rate and a K-edge around 40keV (Figure

6). A high proportion of diagnostic photons are used in this energy range (Bushberg,

2002). Selenium in DDR has a low atomic number compared to CR phosphors and a

K-edge at 13keV this makes it good at absorbing low energy radiation such as in

mammography (Figure7)(Dendy, 1999). This can be an issue as the attenuation

coefficient in DDR is relatively low in examinations using higher photon energies (40-

Figure 5 (Korner, 2007)

130keV). DDR detectors overcome this by having high intrinsic detection efficiency

due to signals travelling in direct lines (Spahn, 2005).

Pixel and matrix size

Pixel and matrix size of the detector ultimately affects the spatial resolution of a

system. The maximum spatial resolution (Nyquist frequency, measured in cycles per

millimetre) is directly related to the pixel size and spacing. Therefore the smaller the

pixel size, or larger the matrix, the higher the Nyquist frequency. As seen in Table 1,

CR has a smaller pixel size and a larger matrix size giving a higher maximum spatial

frequency of up to 5lp/mm depending on cassette size. DDR is limited to one matrix

size and only has a maximum spatial frequency of 3.6lp/mm (Korner, 2007).

Figure 6 (Bushberg, 2002)

Figure 7 (Hubbell, 1996)

Spatial resolution

The modular transfer function gives the most comprehensive description of spatial

resolution of each detector. It describes the detectors efficiency of reproducing

sinusoidal variations in signal contrast at various spatial frequencies. Figure 8 below

demonstrates DDR having a superior MTF compared to CR systems (Cowen, 2008).

Unlike CR the detection to signal production in DDR has very little lateral diffusion

due to intense channelled electric pathways and therefore produces a superior MTF.

The main deterrent that lowers MTF of DDR is the pixel sampling interval and the

physical size of the electrode collecting the signal. (Korner, 2007) In the readout

process of CR light is scattered before reaching the PMT. This contributes to a loss of

signal and hence has a lower MTF and consequently higher signal to noise ratio

(Bushberg, 2002). Newer dual sided CR phosphors, made from CsBr, have a

channelled crystal structure therefore less light scatter is evident compared to the

irregular crystal structure of single sided CR using BaFX (Cowen, 2007).

Figure 8 (Cowen, 2008)

Detector Quantum Efficiency (DQE)

Each detector system has specific rates of absorption of incident photons as

explained earlier with the K-edge absorption coefficients. It remains the detector

systems ability to convert the absorbed radiation into a resultant image whilst

maintaining a high signal to noise ratio. This is determined by the Detector Quantum

Efficiency (DQE)(Korner, 2007). DQE is a fundamental variable in relation to image

quality and is calculated by comparing the signal-to-noise ratio at the detector

output with that at the detector input and is measured as a function of spatial

frequency (Spahn, 2005). DQE is dependent on MTF, spatial frequency, radiation

exposure, and detector material. (Korner, 2007).

As seen in Figure 9, CR and DDR have a maximum DQE of 0.35 but it is DDR that

maintains a higher DQE as spatial frequency increases. (Table 1)(Cowen, 2008)

A higher DQE of an image system means they make better use of incident photons.

This means a smaller exposure is required for adequate signal to noise ratios and

hence an optimal image. Flat panel detectors have the ability to reduce radiation

dose, compared with CR for the same image quality. This is due to the better

quantum absorption and conversion efficiency associated with that technology

(Spahn, 2005)

Figure 9 (Cowen, 2008)

Comparison Table

Table 1: Physical characteristics of modern DR and CR image detector systems.

(Cowen, 2008)

Physical Parameter Dual-sided read CR Direct-conversion DR

X-ray absorber (Typical) 320 μm BaFH:Eu2+ (Turbid phosphor screen)

500 μm a-Se (Photoconductor layer)

Readout Mechanism Point-scan laser beam & dual-read assembly

a-Si:H Storage Capacitor & TFT Switch

Field coverage (cm2) (maximum)

43 × 43 35 × 43

Pixel sampling interval (μm)

100 139

Matrix array (maximum size)

4280 × 4280 2560 × 3072

Nyquist frequency fN (lp mm−1)

5 3.6

Dynamic range 104:1 104:1

Grey-scale resolution (bits)

12 14

Maximum data content per image (Mb) 36 16

Physical Parameter Dual-sided read CR Direct-conversion DR

Image availability (s) 60 30

DQE(0) 0.35 0.35

DQE(fN) <0.1 >0.1

Which System?

Both systems have their advantages and disadvantages for different clinical settings.

The main categories for making a decision are image quality, image acquisition time,

and portability.

The image quality as discovered is similar for both systems with DDR being slightly

superior. Image acquisition times are much quicker for DDR systems making

radiology departments more efficient which is important (Neitzel, 2005). This

increase of productivity is beneficial for patient care especially if they are in pain or

in a critical state e.g. intensive care patients. It is also beneficial for private

institutions with regard to increasing patient numbers hence creating a financial

advantage. CR has the advantage of being portable which allows for ease with

varying angles and receptor placement. This is particularly required for emergency

patients that are not mobile and difficult to manoeuvre.

If price was not a deciding factor I would like to incorporate both systems into a

department whether it was a major hospital or private practice.

I would definitely implement DDR for mammography units, because of selenium’s

high absorption efficiency at low exposures, as well as replacing wall mounted and

table Bucky systems as the image quality is superior and productivity is increased. CR

can then be primarily used for difficult procedures where cassettes need to be

placed under trauma beds or at difficult angles around a patient.

My departments are in remote rural settings where productivity is not an issue as

patient numbers are low. Training and assistance for a DDR system would be difficult

especially if problems arise. (Neitzel, 2005) Based solely on image quality I would

choose DDR but the need for a portable and flexible system is necessary in

emergency settings. Although portable DDR with WIFI is available, it hasn’t been

explored extensively in clinical situations and cassettes are fragile compared to the

robust CR image plates (Verma, 2008). Trial testing of mobile DDR equipment would

be necessary to find out its capabilities and make a decision to change completely to

a DDR system.

With the recent CR innovations of dual-sided image readout and channelled storage

phosphors, image processing times have decreased and DQE has improved by

approximately 50 to 100% compared with standard CR. (Cowen, 2007) Therefore CR

poses a challenge to DDR with respect to reducing patient dose and improving

productivity.

A decision based on my department would be to purchase a CR system.

If I was purchasing for larger workflow radiology departments a DDR system is ideal

as it provides better image quality and faster image acquisition times for efficiency.

References

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