Helical Versus Conventional CT

The advent of helical CT Scanning has greatly increased the potential of CT imaging of the thorax. Short acquisition times, utilizing a combination of slip ring technology, patient translation, movement reduction algorithms, higher powered X ray tubes with rapid heat dissipation and more efficient X ray detector, allow the entire thorax to be imaged in matter of seconds. Most importantly this usually means within a single breath hold.

Ct imaging of the thorax may necessity demonstration of the bony thoracic cage and the soft tissue structure within , in their entirety, or it may focus on a particular organ or pathology. One or all of the following areas may included:

• Pleural cavity
• Mediastinum
• Cardiovascular structures
• Bony cage ribs and sternum

All of these structures move on respiration , there fore single breath-hold scanning techniques have revolutional CT Thorax scanning. This and other advantages of helical scanning of the thorax are listed below:

• The data is continuous, because a whole volume is scanned
• Small areas pathology are unlikely to be missed; this can happen with conventional CT Scanning if the slices are not contiguous
• The whole volume can scanned in one brath-hold, so there is no misregistration of data due to respiratory movement
• Contrast enhancement is more uniform through the study so the amount used may be reduced, optimised and captured to perform CT angiography or multiphase studies
• It can be faster for the patient; this has excellent indications for pediatric , geriatric and trauma scanning and for increasing workloads
• Imaging manipulation, such as multiplanar (MP) and three dimensional (3D) reformatting of images, shaded surface display (SSDs) and maximum intensity projection (MIPs), has expanded and improved because the continuous data acquisition allows overlapping slices abnd reconstruction at any point within the volume of raw data. This has helped to make CT angiography a reality
• Advanced 3D post processing techniques are now being developed to allow a simulated view from inside a lumen or anatomical space, indicating that indicated intraluminal angiography
•  And simulated bronchoscopy may be examination of the future
• A radiation dose reduction is indicated because the likelihood of repeat scans due to patient movement is reduced. Also the scans are undertaken at a lower mAs values because of the height tube output required for spiral CT scanning: this is a direct reduction of radiation dose.

It can seen that the benefits to the patient and imaging department of the fast volume data acquisition are manifold. There are of course some disadvantage which are generally outweighed by the advantages and which are reducing with rapidly improving CT technology . They do however , warrant a mention