A Computed Tomography (CT) scan is a specialized medical imaging procedure that uses X-ray technology and sophisticated computer processing to generate detailed, cross-sectional pictures of the inside of the body. Unlike a traditional X-ray, which produces a single flat image, the CT scan provides images in “slices,” allowing physicians to view internal organs, bones, and tissues layer by layer. Understanding your CT scan and the accompanying written report is an important step in taking an active role in your medical care. This guide provides the foundational principles to help you interpret the visual information and the technical language used by the radiologist.
How Cross-Sectional Images Are Created
A CT scan creates cross-sectional images using a rotating system of an X-ray source and electronic detectors housed within the circular opening, or gantry. As the patient moves through the gantry, the X-ray tube rotates rapidly, capturing thousands of individual X-ray measurements from a full 360-degree range.
These measurements capture how different tissues absorb, or attenuate, the X-ray beam. Dense structures like bone block more X-rays than soft tissues, while air blocks very few. Detectors measure the intensity of the X-rays that pass through the patient, and a powerful computer uses complex algorithms to reconstruct this data into a two-dimensional image slice.
Tissue density is translated onto the standardized Hounsfield Units (HU) scale, which determines the image’s shades of gray. The scale assigns pure water a value of zero HU and air a value of -1000 HU. Every point, or pixel, on the image is assigned an HU value corresponding to the density of the tissue it represents.
Principles of Image Interpretation
Interpreting a CT image requires understanding standard viewing conventions. The most common view is the axial, or transverse, plane, which shows the body as if it were sliced horizontally. Images are typically viewed as if the observer is standing at the patient’s feet and looking up toward the head.
This convention means the patient’s right side is displayed on the image’s left side, and the patient’s left side is on the right. While the axial plane is the primary view, the computer can reconstruct the data into coronal (front-to-back) and sagittal (side-to-side) planes for a complete three-dimensional understanding.
The visual language of a CT scan is defined by density, displayed using a spectrum of black, white, and shades of gray. Tissues are categorized using three main density terms. Hyperdense tissues appear bright white because they have high HU values, meaning they strongly attenuated the X-ray beam.
Bone, metal implants, and newly clotted blood (acute hemorrhage) are examples of hyperdense structures that appear white. Conversely, hypodense areas appear dark gray or black, representing low-density materials that allowed X-rays to pass through easily. Air in the lungs or bowel, as well as fat, are hypodense.
The intermediate shades of gray are isodense, generally referred to as soft tissue density, and include muscle, organs, and simple cysts. Abnormal findings often present as a density higher or lower than the surrounding tissue, such as a mass that is slightly hypodense compared to the adjacent organ. A break in the smooth white line of a bone, for instance, indicates a fracture.
Radiologists can digitally adjust the range of HU values displayed, a process known as “windowing,” to better visualize specific tissues. A “bone window” narrows the displayed range to focus on high-density values, making subtle fractures more apparent. A “soft tissue window” broadens the range to optimize the contrast between organs and muscle.
Understanding the Written Radiology Report
The written radiology report is a structured document designed to communicate findings and interpretation to the referring physician. The report begins with the Clinical Indication, which states the specific reason the scan was ordered, such as “rule out appendicitis” or “evaluate chronic back pain.”
Following this is the Technique section, which describes how the study was performed, noting details like the type of scanner used and whether a contrast agent was administered. Contrast agents temporarily make blood vessels or organs hyperdense and are used to enhance the visibility of certain structures or abnormalities.
The Findings section is the objective, detailed description of every structure examined in the scan, whether normal or abnormal. This technical narrative documents the size, location, and density of all observed anatomical features. Because this section is intended for a medical professional, it can be lengthy and dense with medical terminology.
The most clinically relevant section is the Impression or Conclusion, which provides the radiologist’s summarized interpretation and final diagnosis. This translates the technical findings into a concise clinical assessment. If a finding is not significant, the report may use jargon like “unremarkable” (nothing abnormal was seen) or “benign” (non-cancerous).
If the report mentions a “mass” or a “lesion,” it refers to an area of abnormal tissue identified by the radiologist. The impression states whether a finding is likely a tumor, an infection, or a non-serious issue like a cyst. The impression also includes recommendations for next steps, such as a follow-up scan or correlation with other lab tests.