What is the primary advantage of using a digital image receptor ir over film screen systems

Digital Cassette Systemss

Photostimulable Phosphor Systems.

Also known as computed radiography (CR) or cassette-based DR, photostimulable phosphor systems make use of the digital acquisition modality in which photostimulable storage phosphor (PSP) plates are used to produce radiographic images. PSP plates are also referred to as imaging plates (IPs). CR can be used in standard radiographic rooms just like film-screen systems; therefore, no special changes are needed to the x-ray rooms. The new equipment that is required includes the CR cassettes and phosphor plates, the CR readers, and the image-display workstations.

The storage phosphor plates are very similar in makeup to intensifying screens used in film-screen radiography. The biggest difference is that the storage phosphor plate can store a portion of the incident x-ray energy it traps within the material for later readout. The reader releases the stored energy in the form of light and converts it into an electrical signal, which is then digitized.

The CR cassette looks like the conventional radiography cassette. It consists of a durable, lightweight plastic material. The cassette is backed by a thin sheet of aluminum that absorbs x-rays. Instead of intensifying screens inside, there is antistatic material (usually felt) that protects against static electricity buildup, dust collection, and mechanical damage to the plate.

In CR, the radiographic image is recorded on a thin sheet of plastic known as the imaging plate. The IP consists of several layers, including the protective layer, phosphor layer, conductive layer, light-shield layer, support layer, and backing layer. The cassette also contains a barcode label or barcode sticker on the cassette or on the IP (viewed through a window in the cassette), which allows the technologist to match the image information with the patient-identifying barcode on the examination request.

With CR systems, no chemical processor or darkroom is necessary. Instead, after exposure the cassette is fed into a reader (Fig. 7-2) that removes the IP and scans it with a laser to release the stored electrons.

FIGURE 7-2 Computed radiography reader. (From Ballinger PW, Frank ED: Merrill’s atlas of radiographic positions and radiologic procedures, ed 10, St. Louis, 2003, Mosby.)

There is no change in how the patient is x-rayed compared with conventional film-screen radiography. The patient is positioned using appropriate positioning techniques, and the cassette is placed either on the tabletop or within the table or upright Bucky. Proper kVp, mAs, and distance must still be employed to produce a high-quality image. The biggest difference lies in how the exposure is recorded. In CR, the remnant beam interacts with electrons in the barium fluorohalide crystals contained within the IP. This interaction stimulates, or gives energy to, electrons in the crystals, allowing them to enter the conductive layer, where they are trapped in an area of the crystal known as the phosphor center. This trapped signal will remain for hours, even days, although deterioration begins almost immediately. In fact, the trapped signal is never completely lost. However, the residual trapped electrons are so few that they do not interfere with subsequent exposures.

kVp, mAs, and distance are selected for reasons similar to those for conventional film-screen radiography; kVp must be chosen for penetration and somewhat for the type and amount of contrast desired. Early on, manufacturers stated that the minimum kVp should be no less than 70. This is no longer true. The kVp values now range from approximately 45 to 120. It is not recommended that kVp values less than 45 or greater than 120 be used, because those values may be inconsistent and produce too little or too much excitation of the phosphors. Remember, the process of attenuation of the x-ray beam is exactly the same as in conventional film-screen radiography. It takes the same kVp to penetrate the abdomen with CR systems as it did with film-screen systems.

The mAs is selected according to the number of photons needed for a particular part. If there are too few photons, regardless of the kVp chosen, the result will be a lack of image receptor exposure to create sufficient phosphor stimulation. When insufficient light is emitted from the phosphors, it produces an image that is grainy, a condition known as quantum mottle or quantum noise (Fig. 7-3).

FIGURE 7-3 Image with quantum mottle.

Digital Cassette-less Systems

Digital cassette-less systems use various materials for detecting the x-ray signal. The detectors are permanently enclosed in a rigid protective housing. Both direct capture DR and indirect capture DR detectors are used with these systems.

Direct Capture.

In direct capture, x-ray photons are absorbed by the coating material and immediately converted into an electrical signal. The DR detector has a radiation-conversion material or scintillator, typically made of amorphous selenium (a-Se). This material absorbs x-rays and converts them to electrons, which are stored in the thin-film transistor (TFT) detectors (Fig. 7-4). The TFT is an array of small (approximately 100 to 200 µm) pixels. A pixel is a single picture element, and a matrix is a rectangular series of pixels. The resolution of digital images is determined by the individual size of each pixel. Each pixel contains a photodiode that absorbs the electrons and generates electrical charges. More than 1 million pixels can be read and converted to a composite digital image in less than 1 second.

FIGURE 7-4 Direct capture digital radiography thin-film transistor detector. (From Carter C, Veale B: Digital radiography and PACS, St. Louis, 2010, Mosby.)

Indirect Capture.

Indirect capture detectors are similar to direct detectors in that they use TFT technology. Unlike direct capture, indirect capture is a two-step process: x-rays photons are first converted to light using a scintillator, and that light is then converted to an electric signal.

The scintillation layer in the IP is excited by x-ray photons, and the scintillator reacts by producing visible light. This visible light then strikes the amorphous silicon (a-Si), which conducts electrons down into the detector directly below the area where the light struck. There are two types of indirect conversion devices, the charged-coupled device (CCD) and the thin-film transistor array (TFT). The CCD uses a chip to convert light photons to electrical charge. The TFT array isolates each pixel element and reacts like a switch to send the electrical charges to the image processor. As with direct capture, more than 1 million pixels can be read and converted to a composite digital image in less than 1 second (Fig. 7-5).

FIGURE 7-5 Indirect capture digital radiography thin-film transistor array. (From Carter C, Veale B: Digital radiography and PACS, St. Louis, 2010, Mosby.)

Image Quality Factors

The acceptance characteristics of a diagnostic-quality image, termed image quality factors, fall into two main categories: (1) photographic qualities affecting the visibility of the image and (2) geometric qualities affecting the sharpness and accuracy of the image. There are four primary image quality factors. Two of these are photographic in nature: IR exposure/density and contrast. As an image quality factor, density was the term that was used to reflect the impact of IR exposure to the radiographic film. Radiographic density is defined as the overall blackening of film emulsion in response to this exposure. With digital systems, this important image quality factor has not changed but can be expressed simply as IR exposure because film is no longer the receptor of the image. In the digital environment, brightness is a monitor control function that can change the lightness and darkness of the image, but it is not related to IR exposure. Contrast is the visible difference between adjacent IR exposures or densities, or the ratio of black to white. The two geometric quality factors are recorded detail (also known as sharpness or resolution), the distinct representation of an object’s true borders or edges, and distortion, the misrepresentation of the true size or shape of an object.

A proper balance between the photographic and geometric properties of an image results in good image quality. The geometric properties allow the size, shape, and edges of the object of interest to be accurately represented, whereas the photographic properties allow these carefully reproduced characteristics to be seen.

By way of illustration, imagine that you are trying to take a snapshot of an ornately carved stone. You want every detail to be captured on film, so you take extra trouble to focus carefully. To make certain of success, you make three exposures, each at a different setting. When the film has been processed and printed, you examine your three photos. One photo is perfectly exposed, and you are able to see every important detail in the carving. The second is too dark, and any detail is hard to distinguish. The third is too light, and again, the details of the carving are impossible to see. Consider the two poorly exposed photographs. Just because a photograph is too dark or too light, does that mean that good detail sharpness is not present? These problems of overexposure and underexposure affect the visibility but not the sharpness of the detail.

You return to the carving, intending to use the proper exposure setting to get more photographs. This time, you forget to focus the camera properly, or you move while pressing the shutter. The resultant photograph has beautiful photographic properties but is fuzzy and blurred. This photograph can be said to possess good visibility but poor sharpness of detail. The desired image should have good levels of both characteristics (Fig. 7-6).

FIGURE 7-6 Different-quality photographs of a gravestone. A, Too dark. B, Too light. C, Out of focus. D, perfect.

When images are evaluated, sharpness and visibility of detail must be examined to assess overall quality. The photographic factors that control visibility of detail are considered first.

Photographic Qualities

Image Receptor Exposure/Density.

In conventional film/screen radiography, density has always been the result of IR exposure to the radiographic film. When film is replaced by a DR system, IR exposure becomes the critical factor affecting image quality and the radiographer must closely monitor the IR exposure values to ensure a good image. Radiographic density can be described technically as a comparison of the light incident on the film to the light transmitted through the film. If a digital image is printed to hard-copy film, the traditional term density can still be used.

When a hard copy radiograph is viewed on a viewbox, the incident light is transmitted more easily through the light gray areas than through the darker areas. The darker areas that block the transmission of light are said to have greater radiographic density. Although it can be easily measured scientifically with a densitometer, density is often a subjective measurement, judged by the human eye. A radiograph must possess the proper density to present adequate visibility of detail to the viewer in the same way that a photograph should not be overexposed or underexposed to do justice to its subject. In many instances, a radiologist’s use of the term density refers to anatomic density and not to radiographic density. A report noting an increased density in the right lung field should be interpreted to mean that the lung tissue is denser than other tissues. The radiographic density in such an area would therefore be decreased because the denser tissue would absorb more of the x-ray beam than the tissue that is less dense. Many variables can affect IR exposure/density, including mAs, patient factors, kVp, distance, beam modification, grids, and image receptors (Table 7-1).

TABLE 7-1

Controlling and Influencing Factors of Image Receptor Exposure and Film Density

Controlling Factor	Influencing Factors
Milliampere-seconds	Patient factors
	Kilovolt peak
	Distance
	Beam modification
	Grids
	Image receptors

Milliampere-Seconds.

The greater the number of x-ray photons generated, the greater will be the resultant image receptor exposure and film density. Increasing the number of x-ray photons produced increases the exposure (in milliroentgens [mR]) in a directly proportional relationship, and this results in an overall increase in radiographic density. Milliampere-seconds (mAs) is the chief controlling factor of IR exposure/density and controls the number of electrons that flow from cathode to anode in the x-ray tube. This process in turn controls the number of x-ray photons produced. mAs is the product of mA and time. Any combination of mA and time producing equivalent mAs values should produce equivalent exposures and therefore densities. This process is known as mAs reciprocity.

mA×time=mAs

Examples

100 mA×110 sec=10 mAs

200 mA×120 sec=10 mAs

300 mA×130 sec=10 mAs

The mAs, mA, and time factors are all directly related to image receptor exposure and film density, as well as patient exposure. These effects also can be stated as follows:

• Increasing mAs increases image receptor exposure, film density, and patient exposure.

• Decreasing mAs decreases image receptor exposure, film density, and patient exposure.

The radiographs in Fig. 7-7 illustrate these effects.

Example

100 mA×110 sec=10 mAs=original exposure or film density A

200 mA×120 sec=10 mAs=maintains exposure or film density B

100 mA×15 sec=20 mAs=increases exposure or film density C

FIGURE 7-7 Radiographs show the influence of milliamperage and time on IR exposure or film density. A and B, Same milliampere-seconds with different milliamperage and time settings. C, Double the milliampere-seconds.

Patient Factors.

Various patient factors affect IR exposure. Patient size and thickness, the predominant atomic numbers of the materials (which may include contrast media intentionally introduced into the body), pathologic conditions, anomalies, temporarily compressed tissues, and a number of other techniques all change the subject density of the object being examined. As subject density increases, IR exposure decreases, and vice versa.

Kilovolt Peak.

In addition to the number of x-ray photons produced, the relative strength of the photons must be considered. An x-ray photon of very low energy would have difficulty passing through dense body tissue. Conversely, this same low-energy photon would pass easily through less dense tissue. This characteristic is referred to as the penetrating ability of an x-ray beam. Each average body part can be shown to best advantage by using an optimal kVp setting as a guideline.

The kVp setting determines the highest energy level, or the peak, possible for the photons within that beam. Most of the photons are, in fact, below the peak kilovoltage, covering a range from zero to peak value. The x-ray beam is described as polyenergetic or heterogeneous for this reason.

The relationship between kVp and exposure is not as simple as that of mAs. As kVp increases, image receptor exposure increases but not in direct proportion. The general rule of thumb to account for the change in image receptor exposure relative to change in kVp is called the 15% rule. Increasing kVp 15% will approximately double image receptor exposure. Decreasing kVp 15% will approximately halve image receptor exposure.

For example, imagine that the original kVp is 75. Of the original kVp (75), 15% equals 11.25 or approximately 11. If we want to double the image receptor exposure using the kVp, 11 kVp should be added to the original selection (75), which would result in 86 kVp. If we want to halve the image receptor exposure using the kVp, then 11 kVp should be subtracted from the original (75), which would then equal 64 kVp.

Using this rule to change kVp while maintaining the same image receptor exposure is also possible. This process is done by changing the mAs to compensate for the exposure change caused by the change in kVp. When this adjustment is made, the change in kVp does not change the quantity of the exposure, only the spectrum or energy of the photons. kVp can be changed while maintaining the same image receptor exposure as follows:

Increase kVp 15% and halve mAs.

Decrease kVp 15% and double mAs.

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