What is a characteristic of Helical Computed Tomography (CT) scans except?

Characteristics of Helical CT Scanning

Thin collimation is NOT a characteristic of Helical CT scanning, as it is actually a key feature that enables improved image quality in helical CT technology.

Key Characteristics of Helical CT

Helical CT (also called spiral CT) has revolutionized medical imaging with several distinctive characteristics:

Motion Artifacts

• Motion artifacts are a characteristic feature of helical CT scanning 1
• Aortic motion artifacts are specifically mentioned as a limitation in CT aortography interpretation 1
• These artifacts relate to motion of structures (like the aortic wall) between the end of diastole and end of systole 1
• A 180° linear-interpolation algorithm can be used to reduce or remove these artifacts 1

Virtual Bronchoscopy

• Virtual bronchoscopy is a characteristic application of helical CT technology
• Helical CT enables volumetric data acquisition that allows for post-processing techniques like virtual bronchoscopy
• This application leverages the continuous volumetric acquisition capability of helical CT 1

Other Important Characteristics

Continuous Volumetric Acquisition

• Unlike conventional CT, helical CT provides continued volumetric acquisition as the patient moves through the gantry 2
• This enables scanning of large longitudinal volumes with high z-axis resolution 3

Rapid Scanning

• Dramatically shortened examination times compared to conventional CT 2
• Enables faster table speeds so patients can be scanned quicker 1
• Particularly important when patients are holding their breath with maximally inflated colons (for CT colonography) 1

Enhanced Vascular Imaging

• Improved visibility of vascular structures 2
• Better enhancement of parenchymal organs 2
• Capability for retrospective imaging and three-dimensional vascular studies 2

Why Thin Collimation is NOT a Characteristic Feature

Thin collimation is actually a parameter that can be adjusted in helical CT scanning:

• Slice thickness should be chosen to be at least half of the target polyp size to minimize partial volume averaging 1
• Narrower slice thicknesses potentially increase lesion conspicuity 1
• Submillimeter slice thicknesses are possible with 64-slice CT systems, but this results in larger datasets, increased image noise, and potentially increased radiation dose 1
• Numerous phantom experiments have demonstrated that polyps 6 mm or greater can be detected using slice thicknesses of 3 mm or less 1

Clinical Applications of Helical CT

CT Angiography

• Helical CT angiography provides a means to rapidly and noninvasively evaluate intracranial and extracranial vasculature 1
• The accuracy for detecting intracranial occlusions ranges between 92-100% sensitivity and 82-100% specificity 1
• CTA source images have shown similar sensitivity to MRI/DWI for detecting ischemic regions 1

CT Colonography

• Scanning in two positions (supine and prone) is mandatory to permit redistribution of colonic fluid and air 1
• Multidetector helical CT results in better colonic distention and fewer respiratory artifacts compared with single slice helical CT 1

Peripheral Vascular Disease

• Multidetector CT scanners enable rapid scanning of the entire arterial system 1
• Volumetric acquisition enables extensive image postprocessing, including multiplanar reformatted and maximum-intensity projection images 1
• Sensitivity and specificity of CTA for detection of stenoses >50% diameter are 90-100% 1

Common Pitfalls and Limitations

• Helical CT requires more attention to the relationship between contrast material administration and scanning timing 2
• Heavily calcified atheromatous disease can limit the ability to interpret CT images, especially in tibial arteries 1
• Factors unique to helical technology can produce artifacts that must be recognized when interpreting scans 2
• Radiation dose considerations are important, especially when using thinner slices or multiple acquisitions 1