Computed Tomography

Overview

Computed Tomography (CT) machines offer detailed, rapid, and three-dimensional views of internal anatomy. Since their introduction in the 1970s, CT scans have become indispensable for diagnosing trauma, infections, cancer, vascular disease, and much more.

Unlike traditional X-rays that produce 2D images, CT scanners acquire multiple axial (cross-sectional) slices using rotating X-ray emitters. The absorption and transmission patterns of these multi-angle X-ray cross-sections are are then reconstructed into 3D images. These images can be reformatted into coronal (front-facing) and sagittal (side-facing) views without additional radiation exposure.

How CT Scans Work

Video Overview: How a CT scan sees inside of you in 3D:

https://www.youtube.com/watch?v=H_dReVZ6zlM&ab_channel=CuriousDoc

The patient lies on a motorized table that moves into the gantry—the circular, donut-shaped part of the scanner.
An X-ray tube rotates around the patient, emitting narrow beams at various angles.
Detectors opposite the X-ray tube capture the transmitted X-rays.
A computer processes these signals to reconstruct axial (cross-sectional) images.
The system can digitally reformat images into other planes (e.g., coronal, sagittal).
Images are displayed with the patient’s right side on the left of the screen, consistent with radiographic convention.

Understanding Hounsfield Units (HU): The Basis of CT Contrast

CT images use Hounsfield Units to quantify tissue density, based on how much X-rays are absorbed:

Substance	HU Value
Fat	-120 to -90
Water (reference)	0
Soft Tissue/Muscle	+40 to +60
Bone	+400 to +1000+
Metal	+1000 and above
Air	-1000

This scale allows precise differentiation of tissues. However, to visualize specific structures effectively, radiologists adjust window and level settings.

Understanding Window and Level: Adjusting Contrast to See What Matters

Video Overview: Window Width and Window Level:

https://www.youtube.com/watch?v=A5aqQwipju8&ab_channel=simply_rad

A raw CT image contains information from a wide range of tissue densities. To improve image interpretation, radiologists often adjust the window and level settings:

Level (also called "center"): The midpoint HU value of the tissues you want to focus on. Sets the center of the grayscale. It adjusts which tissue type is visually "neutral" (mid-gray).
Window (also called "width"): Width (WW, also called "width"): The range of HUs displayed around the level. Defines the range of HU values assigned shades of gray. A narrow WW spreads the grayscale over a small HU range (making small differences visible); a wide WW compresses it over a large range (small differences in HU values are less visible, but more is shown).

For instance, if a Window Width is 400 HU and a Level is 40, then the grayscale will cover tissues of HU between -160 HU and 240 HU. Tissues below the window range (-160) are displayed as black, whereas those above the window range (240) are displayed as white. The CT still retains the actual HU values in the raw scan data, even if it's not visible on your screen.

Analogy:

Think of Level as the “brightness” setting centered on a specific tissue, and Window as the “contrast” range, how wide or narrow the spectrum of densities you want to visualize.

A narrow window increases contrast, useful for subtle soft tissue differences (e.g., brain).
A wide window decreases contrast but allows viewing of structures with very different densities (e.g., lungs or bone).

Standard Window/Level Presets for Clinical CT Interpretation

Anatomy	Level (HU)	Window Width (HU)	Use
Bone	300	2000	Highlights dense cortical bone and fractures
Brain	35	80	Differentiates gray and white matter
Liver	50	200	Optimized for parenchymal evaluation
Lung	-700	1500	Differentiates airways, vessels, parenchyma
PE (Pulmonary Embolism)	50	351	Vascular detail of pulmonary arteries
Abdomen	40	400	Visualizes bowel, fat planes, organs

Example:

A Brain Window with Level = 35 and Window = 80 means the image is centered around tissues with HU ~35 (soft tissue), displaying HU values from -5 to +75 (35 ± 40). This increases contrast between gray and white matter.
A Bone Window with Level = 300 and Window = 2000 covers a range from -700 to +1300 HU, showing both air and dense cortical bone, sacrificing soft tissue detail.

Radiologists can also manually adjust the window and level to tailor the image to the pathology they're investigating.

Descriptive Terms in CT Interpretation

Hyperdense: Brighter than surrounding tissue (e.g., blood, calcium)
Hypodense: Darker than surrounding tissue (e.g., edema, cyst, fat)
Isodense: Same density as surrounding tissue, making it harder to distinguish
Enhancement: Increase in brightness after IV contrast administration (e.g., tumors, infection)
Mass effect: Compression or displacement of surrounding tissues due to a lesion or swelling
Midline shift: Displacement of brain structures across the midline due to mass or hemorrhage
Ground-glass opacity: Hazy appearance in the lungs that partially obscures vessels but does not fully consolidate (e.g., inflammation, edema)
Consolidation: Region of lung tissue filled with liquid instead of air, appearing white (e.g., pneumonia)
Calcification: Dense white areas caused by calcium deposition
Fat stranding: Streaky or fuzzy appearance of fat, often seen near inflamed organs (e.g., appendicitis, diverticulitis)

Contrast vs. Non-Contrast CT

Contrast agents are hyperdense liquids, typically iodine-based, administered intravenously to enhance the visibility of blood vessels, organs, and pathological structures in CT imaging. Iodinated contrast absorbs X-rays more strongly than surrounding tissues, appearing hyperdense (brighter) on the scan. This enhanced visibility allows clinicians to assess vascular anatomy, detect tumors, evaluate organ perfusion, and identify sites of inflammation or infection. Contrast is especially crucial when tissue differences aren't easily distinguishable on non-contrast scans. It is important to note that the iodine-based contrasts are nephrotoxic and should be used with caution in patients with kidney disease.

Read more about when to order a contrast-enhanced CT here:

https://www.aafp.org/pubs/afp/issues/2013/0901/p312.html

Radiation Dose and Safety Considerations

CT delivers more radiation than a plain X-ray, but several measures are in place to reduce exposure:

Dose optimization: CT technologists use protocols to ensure the lowest dose that still yields diagnostic-quality images—adhering to the ALARA (As Low As Reasonably Achievable) principle.
Scan range control: Only the necessary anatomy is scanned, minimizing total irradiated volume.
Reconstruction flexibility: Original data can be reformatted or windowed differently without re-scanning.
Contrast precautions: Iodinated contrast may affect kidneys; use non-contrast protocols for at-risk patients, and assess renal function (e.g., eGFR) before administration.

References 1.Burbridge, B., & Mah, E. Computed tomography (CT). LibreTexts. https://med.libretexts.org/Bookshelves/Allied_Health/Undergraduate_Diagnostic_Imaging_Fundamentals_%28Burbridge_and_Mah%29/03%3A_Principles_of_Imaging_Techniques/3.06%3A_Computed_Tomography_%28CT%29 2.Radiology Cafe. CT overview. https://www.radiologycafe.com/radiology-basics/imaging-modalities/ct-overview/ 3.Radiopaedia. Windowing (CT). https://radiopaedia.org/articles/windowing-ct 4.St. Vincent’s University Hospital. Radiological descriptive terms. http://www.svuhradiology.ie/diagnostic-imaging/radiological-descriptive-terms/ 5.Centers for Disease Control and Prevention. ALARA: As Low As Reasonably Achievable. https://www.cdc.gov/radiation-health/safety/alara.html