Definition/Introduction

Image quality can be defined as the attribute of the image that influences the clinician's certainty to perceive the appropriate diagnostic features from the image visually.[1][2]  Quality assurance or quality improvement is defined as the proactive actions to enhance the quality of care and services and cost-effectively remove the waste. In this article, we will discuss the fundamental concepts of digital radiographic image quality assurance. The most common digital radiographic detectors are computed radiography (CR) and digital radiography (DR). The important components of the radiographic image quality include contrast, dynamic range, spatial resolution, noise, and artifacts.[3] We will discuss these components briefly.

• Contrast: Radiographic contrast is a fractional difference in the signal or brightness between the structure of interest and its surroundings.[3] Contrast is generated by differential attenuation of X-rays by different tissues. Radiographic contrast is directly proportional to the atomic number, density, and tissue thickness.  For example, X-ray attenuation is least in air and higher in bone and between in soft tissues. In digital radiography, the contrast can be adjusted using image post-processing techniques where pixel values are changed to provide the expected range of contrast depending upon specific clinical requirements.[3]
• Dynamic range: The dynamic range is the range of various X-ray intensities that can be imaged by the detector.[3] The radiographic detectors which provide good contrast over a wide dynamic range are essential to obtain high-quality digital radiographs. The detectors with wide dynamic range will show very low or very high exposure values in an image, and viewers can view the range of different visible intensities. Although narrow latitude images show greater visible contrast, the extreme exposure intensities would appear too white or too black with no discernible contrast.
• Spatial resolution: Spatial resolution is the imaging system's ability to distinguish the adjacent structures separate from each other. A bar pattern containing alternate radio-dense bars and radiolucent spaces of equal width can be imaged to get the subjective measurement of spatial resolution in units of line pairs per millimeter. The modulation transfer function (MTF) is an objective measurement of the spatial resolution obtained by measuring the transfer of signal amplitude of various spatial frequencies from object to image.[3] MTF is the best way to measure spatial resolution. The factors affecting spatial resolution include magnification, X-ray focal spot size, detector resolution, patient motion, and image processing. A limiting system spatial resolution of 2.5 mm or higher is essential for digital radiographs.[3] In the CR system, scattering of the laser beam during image readout is the primary factor limiting the spatial resolution. In DR systems, the spread of light photons in converting X-rays photons to light and detector element (del) size are the most important determinants of the spatial resolution.[3]
• Noise: The radiographic noise is the random or structured variations within an image that do not correspond to X-ray attenuation variations of the object. The noise power spectrum is the best metric of noise that measures the spatial frequency content of the noise.[3] Quantum noise is primarily responsible for image noise, and the number of X-ray quanta used to form the image determines the quantum noise. Controlling exposure factors is the best way to reduce quantum noise.
• Signal to noise ratio (SNR): The signal to noise ratio is an important metric that combines the effects of contrast, resolution, and noise. Higher the signal and lower the noise, the better is the image quality. Images with high SNR allow the recognition of smaller and lower contrast structures. Detective quantum efficiency (DQE) is the best measure of signal to noise ratio (SNR) transfer efficiency of the imaging system.[3] The human detection ability improves with higher SNR.[2] The required radiation exposure is inversely proportional to DQE.[3]
• Artifacts: Artifacts contribute to poor image quality due to factors other than low resolution, noise, and SNR. These include unequal magnification, non-uniform image due to detector problems, bad detector elements, aliasing, and improper use of grids.

Issues of Concern

Factors affecting image quality.

• Beam energy and kVp-The X-ray beam energy is an energy spectrum utilized in the formation of an image. It is directly proportional to the atomic number of the anode target, peak kilovoltage (kVp) of the X-ray generator, and amount of filtration in the beam.[3][4] Higher energy beams cause greater X-ray penetration, less degree of attenuation by the tissues, and more scatter radiation.[5] This results in lower contrast and lower dose. Conversely, lower energy beams cause higher contrast and require a higher dose as more photons will be needed to penetrate body tissues and form the image. For imaging of specific body parts, appropriate energy is selected to optimize the contrast and dose.
• Tube current-exposure time product (mAs) -Tube current determines the total photons impinging the patient to form an image. mAs is the product of tube current in milliamperes and exposure time in seconds. There is a linear relationship between mAs and patient dose. An increase in mAs leads to an increase in patient dose and reduction in noise. For a given exam, appropriate mAs should be selected to optimize the balance between noise and dose depending upon the clinical need. Exposure time can affect spatial resolution as long exposure times can increase the chances of patient motion leading to image blur.[5]
• Acquisition geometry-Image acquisition geometric factors affecting image quality include a source to image receptor distance, orientation, the amount of magnification, and size of the focal spot. The changes in source to image receptor distance result in variations in relative magnifications of anatomic structures in the image.
• Magnification-An increase in the air gap or patient to image receptor distance leads to an increase in magnification and a decrease in scatter radiation, resulting in improved image contrast and noise.[5] However, the radiation dose is increased as the patient is closer to the X-ray tube. As there is a fixed size of the focal spot, an increase in magnification can cause an increase in a blur.[5]
• The focal spot size-The size of the X-ray tube focal spot is inversely related to the spatial resolution. A decrease in focal spot size leads to improved spatial resolution.[5] However, an X-ray tube with a small focal spot has limited maximum output, leading to increased exposure time that can cause increased patient motion and motion blur.[5]
• Detector performance- The detector performance depends upon the resolution of the detector, detector element size, and SNR performance of the detector. Smaller the detector element size, the higher is the resolution. In an ideal scenario, the detector element size should be smaller than the smallest region of interest. The modulation transfer function (MTF) is the primary measure of detector resolution and not the detector element size.  A detector that maintains MTF value at greater spatial frequencies has a better resolution.
• Collimation- Collimation is defined as the confinement of the spatial extent of an X-ray beam that impinges upon the region of interest in the patient and detector. Effective collimation causes a decrease in scattered radiation that reaches the detector.[5] This leads to the improvement of image contrast and noise and increased SNR. It also causes less radiation exposure and a reduction in effective radiation dose to the patient.
• Anti-scatter grid- Anti-scatter grid improves image quality by decreasing the scattered radiation. However, it can also negatively affect image quality by attenuating the primary X-ray beam.[3][5]
• Image processing- After digital image acquisition, artificial adjustment of the contrast can be achieved using post-processing techniques to improve the visual perception, including histogram equalization, edge enhancement, grayscale processing, and noise reduction.[6] These techniques can be used to modify the effect of kVp on image contrast. If post-processing is not performed, digital images have low contrast between the different tissues. In digital radiography, pixel values are directly proportional to the exposure. The pixel values are changed after image acquisition to optimize the contrast depending upon the clinical scenario.

Clinical Significance

By adjusting kVp, decreasing mAs, and decreasing focal spot size, one can obtain high-quality digital radiographs with a lower radiation dose. Although a higher radiation dose leads to less noise and better image quality, one should be very cautious about the radiation dose to the patient. The radiographic systems should be optimized to obtain image quality that provides diagnostic accuracy at least possible radiation dose. The selection of radiographic projection affects the radiation dose. For example, in chest radiographs, anterior-posterior (AP) orientation has a higher radiation dose compared to the posterior-anterior (PA) view due to greater radiation exposure of breasts. In pediatric patients, the use of as low as reasonably achievable (ALARA) principle is essential during radiographic studies since children are more susceptible to the effects of ionizing radiation than adults.[7][8] The radiographic detectors with higher DQE provide superior SNR performance that enables radiation dose reduction without significantly affecting image quality, particularly in pediatric patients.[3][6][7]

There is a tendency to use more radiation dose in digital imaging to reduce image noise called 'dose creep.'[3] The utilization of a validated chart containing predetermined technical parameters based on the patient size helps avoid dose creep.[3] American Association of Physicists in Medicine Task Group 116 report is a great resource for recommended exposure indicator for digital radiography.[3]

The appropriate use of effective collimation and anti-scatter grid reduces the scattered radiation and improves image quality by reducing noise and improvement of SNR. The anti-scatter grid is most useful when the amount of scattered radiation is high, especially if the patient's thickness is greater than 10 cm.[3] However anti-scatter grid is not useful in smaller or pediatric patients or for smaller body part imaging.

For the troubleshooting of poor-quality radiographic images, the first step should be adjusting the post-processing parameters to see if the image can be reproduced with better image quality. One should optimize image acquisition and processing protocols to avoid repeat examination of the patients and unnecessary radiation exposure.

The optimal imaging protocols should be developed and established with the help of a medical physicist to obtain consistently high image quality at minimum possible radiation dose. The images should be properly compressed for transmission and storage without loss of significant clinical data. The appropriate image post-processing should be used to improve the image display. The imaging systems should comply with appropriate state and federal regulations. The imaging systems should minimize the incidence of poor-quality images and maximize clinical efficiency and continuous quality improvement.[3]

A meticulous quality assurance program is essential for consistently maintaining high-quality performance. The image quality should be monitored by doing acceptance testing to assure safety and image quality, periodical checkups and maintenance assessment, and thorough annual inspections under the guidance of the medical physicist.[9]

In summary, we discussed important components of the radiographic image quality and various factors affecting image quality. This knowledge is useful to obtain high-quality digital radiographs with the lowest possible radiation dose to improve the clinician's diagnostic accuracy.