Maintenance of equipment, image acquisition, and processing standards are quality control issues that fit into the concept of total quality management (TQM) or continuous quality improvement (CQI), as discussed in Chapter 11. The overall efficiency and effectiveness of imaging systems are evaluated beyond the mechanics of producing radiographic images. This chapter introduces the concept of whole system evaluation, considering image acquisition, processing, and evaluation issues as well as examination repeat analysis, communication issues, and system problem identification.

Quality Control Standards

The American College of Radiology requires compliance with standards of practice to assure quality in any imaging system. Three general areas define digital image quality: contrast, resolution, and noise. These areas must be monitored to avoid unnecessary repeat examinations and overexposure to patients and staff. There are a number of system tests that must be performed by service personnel and/or radiologic technologists and radiation physicists. With the increased sophistication of digital radiographic equipment, it is critical that these tests be performed in a consistent and thorough manner.

The American Association of Physicists in Medicine publishes the following documents based on work accomplished by task groups:

• Task Group 116—An Exposure Indicator for Digital Radiography

• Task Group 150—Acceptance Testing and Quality Control of Digital Imaging Units

• Task Group 10—Acceptance Testing and Quality Control of Photostimulable Storage Phosphor Imaging Systems

The following sections are in no way an exhaustive list of activities that should be performed. The manufacturer’s suggested list of systems tests should be performed as indicated in the equipment and service manuals.

Quality Control Schedules and Responsibilities

The radiologic technologist is the first line of defense in preventing, recognizing, and reporting quality control (QC) issues. Quality control is defined as a comprehensive set of activities designed to monitor and maintain a system or piece of equipment. The complicated and delicate nature of digital equipment necessitates frequent and consistent oversight to avoid image errors and unnecessary patient exposure. The following is a suggested schedule for proper digital system maintenance.

Technologist Responsibilities

Daily (Box 12-1)

• General system inspection including the following:

• Sensitometry

• Laser-generated sensitometry strip; film densities measurement

• Inspect images

• Dust particles, scratches, mechanical friction marks

• Network queue check

• Send images to picture archiving and communication system (PACS)

• Cleanliness of cassettes

• Are the cassettes free of dirt and debris on all surfaces? Dirt on the outside or inside of the cassette can result in an artifact in the image. The artifact could be an exposure artifact or processing artifact depending on when it occurs.

• Are barcode labels in good condition and able to be read? Labels in disrepair will compromise the connection of the imaging plate identification to the patient and exam identification information.

• Hinge and latch inspection

• Are hinges and/or latches in good condition? Broken latches or hinges can damage readers and will require a service call to get the reader in working order.

• Erasure of imaging plates

• Have plates been left unexposed for longer than 24 hours? The imaging plates that have not been erased recently have the potential to record exposure such as prolonged light exposure or scatter radiation. The safest procedure is to erase cassettes before use if unsure of the last erasure performed.

• Verification of digital interfaces and network transmission

• Is the reader communicating with the workstation? Are barcode readers working properly? Again, it is critical to maintain the link between the imaging plate and patient information.

• Inspect the laser printer for ink and paper. Make sure the printer is clean and the output bin is free of obstructions. If the printer can be used manually for copies, inspect the printer glass for dirt, fingerprints, etc., and clean according to the manufacturer’s specifications. Artifacts produced by dirt and fingerprints can appear and be interpreted as pathology, possibly resulting in false positive diagnoses.

Box 12-1

Summary of Radiologic Technologist Daily QC Duties

Inspect and clean cassettes

Inspect hinge and latch

Erase imaging plates

Verify digital interfaces and network transmission

Inspect laser printer

Weekly (Box 12-2)

• Clean and inspect receptors

• Clean cassettes as needed and inspect the image receptors for dirt or damage. Inspect the entire length of the connection cables for splits or exposure of wires. If breaks or wear have caused wire exposure, inform service personnel immediately. In addition to the danger of electrical shock to personnel, electrical shorts can cause failure of equipment and/or noise on the image.

• Equipment manufacturers should provide lists of appropriate system tests to be performed by the technologists. This type of testing may be performed by a designated QC technologist rather than by each individual technologist. All problems must be recorded and reported immediately. Examples of this type of test include the following:

• Image acquisition testing with phantoms

• Cassette integrity testing with special, standardized cassettes

• Clean the air intakes on the image plate reader.

• Air is used to cool the reader electronics. If dirt and debris are allowed to clog the intakes, the reader could sustain serious damage. In addition, debris entering the air intake could obscure the lens of the scanning laser or reader mirrors and produce artifacts.

• Clean the display screen, keyboard, and mouse.

• With multiple people using digital imaging systems, and because the digital imaging system makes use of touch screen technology, display screens get dirty very quickly. Multiple users leave multiple fingerprints that, because of the oil in the skin, attract and retain dust and dirt particles. From a visual standpoint, it is much easier to view images on a screen not obscured by streaks and smears. From a health standpoint, many hands on the same surfaces without proper cleaning can lead to increased transmission of illness. Care must be taken to properly clean and disinfect these surfaces according to manufacturers’ guidelines.

Box 12-2

Summary of Radiologic Technologist Weekly QC Duties

Clean and inspect receptors

Clean air intakes of imaging plate reader

Clean display screen

Clean computer keyboard and mouse

Monthly (Box 12-3)

• Reject analysis

• It is critical that repeat exposures are identified so that data concerning repeat reason, number of repeats, and the technologist responsible for the repeat can be analyzed. One issue is that whereas digital systems with integrated generator consoles record the milliampere-seconds (mAs) and kilovoltage peak (kVp) values directly on the film, nonintegrated systems do not. In cases where the department does not require manual technologist identification input, it is strongly recommended that a personal repeat rate log be kept by each technologist (see sample in Figure 12-1). This will allow the technologist to see possible trends in exposure, positioning, and procedural errors. For example, if the repeat reason is underexposure, the cause could be poor calibration of the automatic exposure control (AEC), or it could be poor imaging skills. Either way, identification of this trend will allow the technologist to improve imaging procedures and better protect the patient. This can be accomplished to a certain extent with a software program. Many vendors have software to automatically keep repeats in a folder for the QC technologists to review, eliminating the issue of technologists deleting repeat exposure images.

QC suboptimal images

Typically, a technologist is assigned the responsibility of coordinating analysis of images of suboptimal quality. This may be done in concert with a radiation physicist, the purpose being to identify equipment and technologist performance errors (see sample reject analysis form in Figure 12-2). This type of analysis helps determine the following:

Reject reasons

Reject reasons are fairly easy to identify in images that demonstrate the mAs and kVp values in the image and digital imaging and communications in medicine (DICOM) header. In images not displaying the technical factors, images that are out of the recommended exposure range can be identified, but unless the technical factors are manually input, there is no way to tell if the problem was caused by mAs or kVp errors. Without the use of side/position markers, there is no way to positively link an image with the performing technologist.

Positioning errors should be easy to identify. Again, without input as to performing technologist, it will be difficult to identify skills issues. It is strongly recommended that the department put a procedure in place that accomplishes this, not for punitive purposes, but for standardization of exposure practices. A major part of being a professional in the imaging sciences is having the integrity to accept responsibility for one’s own work. Positioning errors may also be the cause of incorrect processing, related to the position of the part on the imaging plate, collimation, or alignment resulting in poor images even if technique is correct. Vendors are developing software to minimize this, but software is no substitute for proper positioning, collimation, and alignment.

Anatomic side/position marker errors may be very difficult to identify. Because postexposure marking is easily done, technologists may not see the benefit of using personal ID markers. However, this may lead to serious, difficult-to-identify errors. In one case, a technologist performed a portable chest x-ray examination on an infant. Upon processing, she noticed that her personal marker was on the wrong side of the chest. Upon investigation, she discovered that she had marked the chest correctly, but the infant had dexrocardia that had never been identified during six previous exams in which the technologists had failed to use their personal ID markers. There is no substitute for using proper personal ID markers correctly. Incorrect or lack of use of side identification markers may result in legal issues if images are included in a court case. With no blocker for the technologist to use as an identifier of cassette and image orientation, there would be no way to prove proper marking without the technologist ID markers.

Clean imaging plates

Image plates should be removed from the cassette and inspected visually for dirt, hair, lint, scratches, or cracks. Weekly inspection is recommended, especially for departments with high throughput or frequent “dirty” case use. It is important that lint-free cotton gloves be worn to avoid further contaminating the imaging plate. A lint-free cloth such as a photographic lens cloth should be used to gently wipe debris off the imaging plate surface. A camel hair brush can be used but should be stored so that dust and dirt do not collect on it. If this is ineffective, cleaning solutions can be used. Use only cleaning solutions specifically recommended by the manufacturer and be sure to follow the Material Safety Data Sheet (MSDS) guidelines provided by the cleaning solution distributor. If the artifact cannot be removed, the imaging plate will have to be replaced.

Imaging plate disposal

Imaging plates contain a small amount of barium, which must be discarded according to state and U.S. Environmental Protection Agency (EPA) regulations.

Disposal must be handled by a licensed disposal company; no other means of disposal, such as putting it in trash cans, is acceptable. This type of disposal requires an EPA identification number, which is assigned by the state. Be sure to be familiar with disposal regulations.

Artifact identification

Monthly QC of images will help to identify recurring artifacts caused by debris on imaging plates, cassettes, laser lenses, and reader mirrors. Major artifacts should be noted at the time of processing and reported; however, smaller, less intrusive artifacts are sometimes missed and ignored, resulting in long-term problems.

Proper problem reporting procedures will provide a mechanism through which recurring quality trends will emerge. If, for example, several reports are received from a particular room that images are excessively noisy or too light, the room may need to be inspected for system interference or AEC recalibration. These procedures will also help service personnel determine what issues exist based on location and frequency.

FIGURE 12-2 Sample image analysis/reject form.

Box 12-3

Summary of Radiologic Technologist Monthly QC Duties

Equipment malfunction errors

Clean imaging plates

Artifact identification

Problem reporting

Service Personnel Responsibilities

Although specific responsibilities will vary from manufacturer to manufacturer and vendor to vendor, generally speaking service personnel have a duty to the consumer to ensure that equipment is being maintained properly. This is accomplished through a program of preventative maintenance that typically takes place semiannually. Preventative maintenance (PM) consists of a series of equipment tests that are performed by a service engineer. This engineer may be employed by the hospital or the equipment manufacturer.

Physicist Responsibilities for PSP Systems

Schedules for physicist review of photostimulable phosphor (PSP) digital imaging systems may vary depending on availability (see Box 12-4). One physicist may handle multiple medical facilities, visiting each one on a weekly or monthly basis. Others may be employed by only one facility and be much more active in determining review procedures. Typical responsibilities include the following:

• Semi-annual/annual

• Review of departmental images to:

• Reestablish baseline values

• Check exposure indicator accuracy with calibrated ion chamber

• Determine exposure trends

• Analyze repeat rates

• Review QC records

• Analyze service history

Box 12-4

Summary of Physicist Responsibilities

Semiannual/annual QC for PSP systems

Component inventory

Imaging plate dark noise and uniformity

Exposure indicator calibration accuracy

System linearity and autoranging response

Laser beam function

Limiting resolution and resolution uniformity

Noise and low-contrast resolution

Spatial distance accuracy

Erasure thoroughness

Aliasing/grid response

Imaging plate throughput

Acceptance criteria and quantitative relationships

Image processing: look-up table transforms and frequency enhancement

The standard QC tests for filtration, collimation, focal-spot size, kVp calibration, exposure timer accuracy, exposure linearity, exposure reproducibility, and protective apparel will remain the same, but the American Association of Physicists in Medicine has established a set of QC parameters to be followed for PSP systems in AAPM Report #93. This report details the tests and reports to be performed. Consult the AAPM website for the most up-to-date QC procedures for digital projection radiography systems.