Digital image quality and dose considerations: Rad Tech

Digital radiographic quality depends on several key concepts that affect how images appear and how much radiation reaches the image receptor, as well as the patient dose associated with the examination. The goal of the medical imaging is to obtain enough information to answer the clinical question while protecting the patient. As a radiologic technologist, understanding how these image quality concepts work together helps to balance image quality while keeping patient radiation dose as low as reasonably achievable, or ALARA for short.

When evaluating an image for diagnostic quality, spatial and contrast resolution are two of the most important characteristics to consider. Spatial resolution describes the ability to distinguish small, closely spaced high-contrast objects as separate objects. For example, mammography relies on high spatial resolution to show tiny breast microcalcifications as distinct details.

On the other hand, contrast resolution describes the ability to distinguish subtle differences in image brightness or grayscale between structures that attenuate X-rays by similar amounts. It helps make a subtle soft-tissue lesion visible against surrounding tissue.

Spatial frequency helps us understand spatial resolution by telling us how many line pairs can be distinguished within one millimeter. It's expressed as lp/mm where one line pair is one line plus the adjacent space. Spatial frequency tells us how many of these pairs fit within one millimeter and can still be seen separately. A high-spatial-frequency pattern contains fine, closely spaced detail, while a low-spatial-frequency pattern contains larger, more widely spaced detail.

Modulation transfer function, or MTF for short, describes how well an imaging system transfers object contrast to the final image across different spatial frequencies.

On an MTF curve, spatial frequency increases from left to right, and the MTF value shows how much of the object’s contrast is preserved. A curve that stays higher and extends farther to the right indicates better reproduction of detail. As spatial frequency increases and structures become smaller, the curve usually falls because less contrast is transferred to the image.

Contrast resolution is the ability to distinguish small differences in image brightness or grayscale. Dynamic range describes the number of gray shades that a digital system can record. A wide dynamic range gives digital imaging broad exposure latitude and allows the system to record thousands to tens of thousands of gray levels. However, the human visual system can distinguish only about 30 gray levels at one time. The wide latitude improves display flexibility, but it does not make every exposure acceptable, and it can make overexposure less obvious. This is where postprocessing becomes essential.