Basics of CT scan
Reading File
Finding Sources
Finding Sources
Searching PubMed
computed tomography radiation safety
Searching the Web
RadiologyInfo computed tomography CT scan patient safety basics
Reading File
Basics of a CT Scan
CT stands for computed tomography. It is also called a CAT scan. It uses rotating X-ray beams and computer processing to create detailed cross-sectional images, or “slices,” of the body. These slices can be reconstructed into 3D views. The FDA overview of CT compares the slices to sections of a loaf of bread.
How it works
- You lie on a narrow table.
- The table moves through a doughnut-shaped scanner.
- An X-ray tube rotates around the body and detectors measure how much radiation passes through tissues.
- A computer converts these measurements into images.
Different tissues appear differently:
- Air: black
- Fat: dark gray
- Soft tissue and fluid: gray
- Bone, metal, and contrast: white
What CT is used for
CT is fast and especially useful for:
- Head injury, stroke assessment, and brain bleeding
- Chest conditions such as lung disease, pulmonary embolism, or trauma
- Abdominal pain, appendicitis, kidney stones, and internal bleeding
- Bone fractures and complex joint injuries
- Cancer detection, staging, treatment planning, and follow-up
- CT angiography, which images blood vessels
Contrast-enhanced CT
Sometimes a contrast agent is used to make blood vessels, organs, inflammation, or tumors clearer.
- Usually it is iodinated contrast, injected into a vein.
- You may briefly feel warmth, a metallic taste, or a sensation of urinating. These are common, transient effects.
- Tell the team about prior contrast reactions, kidney disease, diabetes medicines, thyroid disease, pregnancy, or possible pregnancy.
- Contrast can rarely cause an allergic-like reaction or kidney injury in susceptible people. The RadiologyInfo contrast guidance explains its common uses and safety considerations.
Preparation and procedure
- Remove metal items such as jewelry and belts.
- Wear a gown if asked.
- For scans with IV contrast, fasting for a few hours may be requested.
- The actual image acquisition is usually only seconds to a few minutes, though total appointment time can be longer.
- Staying still is important. You may be asked to hold your breath briefly.
CT compared with other imaging
| Test | Uses radiation? | Best strengths |
|---|---|---|
| CT | Yes, X-rays | Fast, detailed anatomy, lungs, bones, trauma, bleeding |
| X-ray | Yes, lower dose | Quick assessment of bones and chest |
| Ultrasound | No | Pregnancy, gallbladder, fluid, many pelvic and soft-tissue examinations |
| MRI | No ionizing radiation | Brain, spinal cord, joints, and soft-tissue contrast; slower than CT |
Risks and safety
- CT uses ionizing radiation, so scans should be performed only when medically justified and optimized to use the lowest practical dose.
- The radiation dose varies by body part and scan protocol. Children are more sensitive to radiation, so pediatric protocols should be tailored to size.
- A CT scan does not leave radiation in your body afterward.
- If you could be pregnant, tell the radiology staff before the scan.
- In an appropriate clinical situation, the diagnostic benefit commonly outweighs the small radiation risk, as described in RadiologyInfo’s CT safety information.
Key takeaway: CT is a rapid, high-detail X-ray-based scan, particularly valuable in emergencies and for imaging lungs, bones, internal injuries, and many abdominal conditions.
HU
Reading File
Finding Sources
Searching the Web
Hounsfield units CT water air fat bone radiology reference values
Searching PubMed
Hounsfield units computed tomography
HU = Hounsfield Unit. It is the CT number that represents how much a tissue attenuates X-rays compared with water.
[
\text{HU} = 1000 \times \frac{\mu_{\text{tissue}}-\mu_{\text{water}}}{\mu_{\text{water}}}
]
where ( \mu ) is the X-ray attenuation coefficient.
Key reference values
| Material/tissue | Approximate HU |
|---|---|
| Air | -1000 |
| Lung | -950 to -650 |
| Fat | -120 to -60 |
| Water | 0 |
| CSF | 0 to 15 |
| White matter | ~30 |
| Gray matter | ~40 |
| Blood | ~30 to 45 |
| Muscle | ~45 to 50 |
| Liver, noncontrast | ~45 to 65 |
| Cancellous bone | ~300 to 800 |
| Cortical bone | > +1000 |
| Metal | often > +3000 |
Interpretation
- Lower HU means less X-ray attenuation and appears darker, for example air and fat.
- Higher HU means greater attenuation and appears whiter, for example bone, calcification, acute blood, or iodinated contrast.
- HU values vary with scanner settings, patient factors, region measured, and whether IV contrast was given. Interpret them in context.
Why HU matters clinically
- Differentiate fat, fluid, soft tissue, calcification, and blood.
- Characterize lesions, such as an adrenal lesion or renal stone.
- Assess bone density opportunistically. For example, vertebral HU values have been correlated with low bone mineral density in the cited orthopedic text.
A CT image is displayed using window width and window level, which select a range of HU values to emphasize a tissue type:
- Lung window: low HU range
- Brain window: narrow range around soft-tissue HU
- Bone window: high HU range
Reference values are consistent with the NCBI HU review and Radiopaedia CT overview.
Explain CT window width and window level
Reading File
Finding Sources
Searching PubMed
computed tomography window width level
Searching the Web
CT window width window level definition radiology Hounsfield units
CT windowing
A CT scan contains a very wide range of densities, roughly from air at -1000 HU to dense bone or metal above +1000 HU. A display cannot show all these differences well at once, so windowing selects a range of HU values to display.
- Values below the selected range appear black
- Values above the range appear white
- Values within the range are displayed as shades of gray
Window width (WW)
Window width is the total range of HU displayed as gray shades.
[
\text{Lower limit} = WL - \frac{WW}{2}
]
[
\text{Upper limit} = WL + \frac{WW}{2}
]
Effect of changing WW
- Narrow WW: fewer HU values spread across the gray scale, so the image has higher contrast. Helpful for subtle differences in soft tissue.
- Wide WW: more HU values included, so the image has lower contrast but shows structures with widely different densities together.
Window level (WL)
Window level is the central HU value of the displayed range. It determines the brightness and which tissue density is emphasized.
- A lower WL centers the range on lower-density tissues such as lung or fat.
- A higher WL centers the range on denser structures such as bone or contrast-filled vessels.
Example
If:
[
WL = 40,\quad WW = 80
]
Then:
[
\text{Displayed gray range} = 0 \text{ to } 80\ HU
]
- Below 0 HU: black
- 0 to 80 HU: shades of gray
- Above 80 HU: white
This is a typical brain/soft-tissue window, well suited to differentiating gray matter, white matter, and CSF.
Common CT windows
| Window | Approximate WL / WW | Main use |
|---|---|---|
| Brain | 40 / 80 | Brain parenchyma |
| Stroke window | 35 / 35 | Subtle early ischemic change |
| Hemorrhage window | 70 / 170 | Acute intracranial blood |
| Lung | -600 to -700 / 1200 to 1500 | Lung parenchyma and airways |
| Mediastinal / soft tissue | 40 to 60 / 350 to 450 | Chest and abdominal soft tissues |
| Bone | 300 to 500 / 1500 to 2500 | Cortical bone and fractures |
Exact settings vary by scanner, institution, anatomy, and diagnostic question.
Easy memory aid
- Width = contrast: narrow width gives more contrast; wide width gives less.
- Level = brightness/tissue focus: it centers the displayed HU range around the tissue of interest.
For example, the radiology text describes a brain window of WL 40 and WW 80, whereas suspected intracranial hemorrhage may be assessed on a wider setting such as WL 70 and WW 170. A CT windowing reference explains that window and level map the large HU range to visible shades of gray.
This is a shared conversation. Sign in to Orris to start your own chat.