A High-Resolution Computed Tomography (HRCT) scan is a specialized medical imaging technique. While standard CT scans provide broad, cross-sectional views, HRCT is designed to capture images with an exceptionally high level of detail. This technique allows clinicians to visualize minute structures and subtle abnormalities that would be blurred or invisible on a conventional scan. The primary purpose of this specialized protocol is to provide superior spatial resolution for precise diagnosis and monitoring.
Definition and Core Technology
A Computed Tomography (CT) scan uses a rotating X-ray beam and detectors to measure how tissues absorb radiation. Powerful computer processing then reconstructs these measurements into detailed, cross-sectional images, often referred to as “slices.” HRCT is not a different type of machine but rather a specific imaging protocol performed on a standard CT scanner.
The goal of this specialized protocol is to maximize spatial resolution, allowing doctors to look at the internal structure of organs with greater clarity. By adjusting the acquisition parameters, the scanner focuses on distinguishing between very small, adjacent structures. This technical adjustment makes the resulting images much sharper and more granular than those produced by routine scans.
Technical Differences from Standard CT
The “high-resolution” aspect is achieved by making specific adjustments to how the image data is acquired and processed. One primary technical difference is the slice thickness, which is dramatically reduced compared to a typical CT scan. HRCT slices are usually acquired at an extremely narrow width, often ranging from 0.625 millimeters to 1.5 millimeters, in contrast to the 3 to 5 millimeter slices used in a standard chest CT.
Another technical distinction lies in the image reconstruction algorithm, often called the kernel. HRCT employs a “sharp” or high-spatial-frequency reconstruction kernel that enhances the contrast between adjacent small structures. This algorithm optimizes the image for visualizing fine tissue textures, such as the delicate network of the lung tissue.
Primary Diagnostic Uses
HRCT is primarily considered the gold standard for evaluating diffuse lung diseases that affect the lung parenchyma, the functional tissue involved in gas exchange. The exceptional detail provided by the high resolution is necessary to identify subtle changes in the delicate air sacs (alveoli) and the tissue between them, known as the interstitium. This imaging technique is indispensable for diagnosing and characterizing interstitial lung diseases (ILDs), such as pulmonary fibrosis and hypersensitivity pneumonitis.
The scan allows radiologists to identify specific patterns that are hallmarks of these conditions. For instance, the presence of “honeycombing” (cysts clustered together) or “ground-glass opacities” (a hazy increase in lung density) helps differentiate between various types of ILDs and determine disease severity. HRCT is also highly effective in assessing small airway diseases, like bronchiectasis, where it can clearly show the abnormal widening of the bronchi.
Patient Experience and Safety Considerations
The actual procedure for an HRCT scan is quick and non-invasive, similar to a standard CT. The patient lies on a motorized table that moves through the doughnut-shaped scanner. For a chest HRCT, patients are instructed to hold their breath at full inspiration during the scan acquisition to ensure the lungs are fully expanded and fine structures are clearly visible.
When examining the lung parenchyma, HRCT is often performed without intravenous contrast dye. This is because the natural contrast between the air-filled lungs and the soft tissue is already high, offering no benefit from contrast material for the intended diagnosis. As with all CT scans, HRCT uses ionizing radiation, but the imaging protocol is optimized to deliver the lowest possible dose necessary to obtain diagnostic quality images.