What is a Hounsfield unit in CT scanning?

A Hounsfield unit (HU), also called a CT number, is a standardized measurement of tissue density used in CT imaging. Water is defined as 0 HU, air as -1000 HU, and bone as approximately +1000 HU or higher. Each voxel in a CT image is assigned an HU value based on its X-ray attenuation, allowing different tissues to be distinguished and quantified.

What is the gantry of a CT scan?

The gantry is the circular, donut-shaped structure of a CT scanner that houses the X-ray tube, detector array, rotation mechanism, and data acquisition system. The patient lies on a table that moves through the gantry's central opening (aperture) while the X-ray tube and detectors rotate around them to acquire projection data from multiple angles.

What is helical CT scanning?

Helical (also called spiral) CT is a scanning technique where the X-ray tube rotates continuously while the patient table moves smoothly through the gantry. This creates a helical or spiral path of data acquisition rather than acquiring individual slice-by-slice images. Helical CT is faster, reduces motion artifacts, and allows volumetric data acquisition for 3D reconstruction.

What is pitch in CT scanning?

Pitch in CT is the ratio of table movement per rotation to the beam collimation width. Pitch = table travel per rotation / beam width. A pitch of 1.0 means no gaps or overlap. Low pitch ( 1) means gaps (faster scan, lower dose, but reduced longitudinal resolution).

What is window width and window level in CT?

Window width (WW) controls the range of Hounsfield units displayed as shades of gray — a wider window shows more tissue types with lower contrast, while a narrower window shows fewer tissue types with higher contrast. Window level (WL) sets the center of the window, determining which HU value is shown as middle gray. Common settings: lung (WW 1500, WL -500), soft tissue (WW 400, WL 40), bone (WW 2000, WL 500).

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CT Scan Physics: Hounsfield Units, Windowing & Helical CT

2026-05-29·CT·~1,300 words

Computed tomography is fundamentally different from conventional radiography. Instead of collapsing three-dimensional anatomy onto a flat detector, CT measures X-ray attenuation from hundreds of angles and mathematically reconstructs cross-sectional images. This article covers the essential physics every CT technologist needs to know.

The CT Gantry: What's Inside the Donut?

The gantry of a CT scanner is the circular structure that houses the imaging components. Understanding its anatomy is essential because every major CT physics concept ties back to how these components work together.

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X-Ray Tube

Mounted on a rotating slip-ring assembly. Rotates 360° (or more for helical) around the patient. Uses a high-power tube (30-100 kW) with advanced heat capacity for continuous operation.

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Detector Array

Arc of thousands of detector elements opposite the tube. Modern MDCT scanners have 64, 128, 256, or even 320 rows of detectors — each row captures data for a slice.

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Slip Ring

Brushed electrical contacts that allow continuous rotation of the gantry without tangling cables. This innovation enabled helical/spiral CT scanning in the late 1980s.

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Patient Table

Moves smoothly through the gantry aperture during helical scans. Table speed, combined with beam collimation, determines the pitch.

How a CT Image Is Made

The X-ray tube rotates around the patient, firing a fan-shaped beam through the body. Detectors on the opposite side measure how much radiation passes through. A single full rotation produces raw projection data. A computer then uses filtered back projection or iterative reconstruction to convert those projections into cross-sectional images.

Hounsfield Units: The Language of CT Numbers

Every pixel in a CT image represents a Hounsfield unit (HU), also called a CT number. This is a standardized scale of radiodensity that allows radiologists to identify tissues by number.

The Hounsfield Scale

−1000
Air

−500

−200
Fat

−100
Water

+200

+500

+1000
Bone

▸ The Hounsfield scale. Water is defined as 0 HU. Air = −1000 HU. Dense cortical bone = up to +3000 HU.

CT Numbers for Common Tissues

Tissue	Hounsfield Units (HU)
Air	−1000
Lung	−500 to −200
Fat	−100 to −50
Water	0
CSF / Fluid	0 to +15
White matter	+20 to +30
Gray matter	+30 to +40
Muscle	+40 to +60
Blood (acute)	+60 to +80
Contrast-enhanced tissue	+100 to +300
Cortical bone	+1000 to +3000
Metal (implants)	+3000+

The formula for calculating a CT number is:
HU = 1000 × (μ_tissue − μ_water) / μ_water
where μ is the linear attenuation coefficient of the tissue.

Window Width and Window Level

CT images contain far more grayscale information than the human eye can perceive — up to 4096 gray levels (12 bits). To make different tissues visible, we apply windowing.

Window Width (WW)

The total range of HU values displayed as shades of gray. Narrow window = high contrast (good for subtle differences). Wide window = low contrast (good for seeing many tissue types at once).

Window Level (WL)

The center HU value of the window. This is the value that maps to middle gray. Adjusting the level shifts which tissue appears brightest.

Standard CT Window Presets

Preset	Window Width	Window Level	Best For
Lung	1500	−500	Pulmonary parenchyma, airways
Soft Tissue (Abdomen)	400	+40	Liver, spleen, kidneys, pancreas
Brain	80	+35	Differentiating gray/white matter
Bone	2000	+500	Osseous structures, fractures
Subdural / Brain Blood	150	+40	Acute hemorrhage detection
Angiography	600	+200	Contrast-filled vessels

Example: For a soft tissue window (WW 400, WL 40), any HU value below −160 (40 − 200) appears pure black, and any value above +240 (40 + 200) appears pure white. The 400 HU range between them is distributed across 256 shades of gray.

Helical (Spiral) CT Scanning

Before the 1990s, CT scanners used axial (step-and-shoot) mode: rotate, stop, table advances, repeat. Helical (spiral) CT changed everything by enabling continuous rotation with simultaneous table motion.

How Helical CT Works

The X-ray tube rotates continuously (no stopping between slices)
The patient table moves smoothly through the gantry during rotation
Data is acquired in a helical path around the patient
Mathematical interpolation reconstructs true axial slices from the helical data

Advantages of Helical CT

Speed — a chest CT can be acquired in a single 10-20 second breath-hold
No respiratory misregistration — eliminating gaps between slices due to breathing
Volumetric data — enables 3D reconstructions, MPR (multiplanar reformatting), and CT angiography
Reduced motion artifacts — faster acquisition = less patient motion
Lower contrast dose — optimal timing for contrast bolus tracking

CT Pitch: The Critical Parameter

Pitch is the ratio of table movement per rotation to the total beam collimation width.

Formula: Pitch = Table travel per rotation (mm) / Beam width (mm)

Pitch Value	Type	Effect
< 1.0	Overlapping	Better image quality, higher patient dose, longer scan time
1.0	Contiguous	No overlap or gaps, balanced dose and quality
> 1.0	Gapped	Faster scan, lower dose, reduced longitudinal resolution

Clinical Pitch Guidance

Low pitch (0.5-0.8): Cardiac CT, brain perfusion — where maximum image quality is needed, and dose is a secondary concern.

Standard pitch (0.9-1.1): Routine chest, abdomen, and pelvis — the default for most protocols.

High pitch (>1.2): Trauma scans, uncooperative patients, pediatric low-dose — prioritize speed and dose reduction.

CT Dose Metrics

CT delivers higher radiation doses than conventional radiography, so dose monitoring is critical. Three key metrics:

CTDI_vol

CT Dose Index (mGy)

The average dose within the scanned volume. Standardized measurement using acrylic phantoms (16 cm for head, 32 cm for body). Accounts for pitch.

DLP

Dose-Length Product (mGy·cm)

CTDI_vol × scan length. Used to estimate total energy delivered. Converted to effective dose using region-specific conversion factors (k-factors).

SSDE

Size-Specific Dose Estimate

CTDI_vol adjusted for patient size (water-equivalent diameter). More accurate than CTDI_vol for individual patient dose, especially in pediatrics.

Image Reconstruction: From Projections to Pictures

Raw CT data (sinograms) must be reconstructed into viewable images. Two main methods:

Filtered Back Projection (FBP)

The traditional method used since the 1970s
Applies a mathematical filter (kernel) to projection data, then back-projects across the image matrix
Different kernels: soft (smooth, low noise, lower spatial resolution) vs bone/edge (sharp, higher noise, better detail)
Fast computation, well-understood physics

Iterative Reconstruction (IR)

Starts with an initial estimate, repeatedly compares it to measured data, and refines
Produces comparable image quality at 30-60% lower dose compared to FBP
All major vendors have their own algorithms: ASiR (GE), iDose (Philips), SAFIRE (Siemens), AIDR (Canon)
Model-based IR (MBIR) uses statistical and system models for even greater dose reduction

Key Takeaways for the Registry

Hounsfield units quantify tissue density: water = 0, air = −1000, bone = +1000+
Window width controls contrast; window level sets the center HU value
Helical CT enables fast volumetric scanning with continuous tube rotation and table movement
Pitch = table travel per rotation / beam width; lower pitch = higher quality, higher dose
CTDI_vol and DLP are the standard dose reporting metrics
Iterative reconstruction reduces dose by 30-60% vs filtered back projection

Want to compare CT with other modalities? Read CT vs MRI: When to Use Which or explore our CT Scan modality overview.

About the author: This guide was prepared by the Radiography 101 Clinical Team, referencing Christensen's Physics of Diagnostic Radiology (4th ed.), CT Physics: The Basics of Computed Tomography, and current ARRT CT exam standards. Content is reviewed for clinical accuracy.