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MiniCAT Provides Improved Images of the Head and Neck

J. Webster Stayman, Ph.D. (stayman@xorantech.com)
Vice President, Research and Development
Xoran Technologies, Inc.
309 North First Street
Ann Arbor, MI 48103

Author Description of Paper TH-D-L100J-3
Thursday, July 26, 2007, 11:36 AM
2007 AAPM Meeting, Minneapolis

The paper we will be presenting is on the spatial resolution properties of MiniCAT, a volume computed tomography (CT) scanner that is based on flat-panel technology.  In principle, volume computed tomography provides for the same kind of diagnostic images as traditional x-ray CT scanners; however, the data acquisition is somewhat different.  While a traditional CT collects data by rapidly spinning an x-ray source and detector around a patient many times - collecting only a few slices in a single rotation, volume CT collects many slices (hundreds or even thousands) in one rotation.  This form of acquisition allows for much more efficient scanning and can be used to reduce scan time, to reduce radiation dosage, and to relax design requirements on the CT scanner itself.

The MiniCAT is a volume CT scanner for head and neck imaging that has taken advantage of this collection efficiency.  The scanner is highly compact (fits in roughly a one square meter footprint), has modest power requirements (plugs into an ordinary wall outlet), is relatively low-cost, and has been optimized for low-radiation-dose head imaging.  These characteristics make the MiniCAT ideal for installation at the point-of-care, as opposed to a separate department or facility).  Typical traditional CT scanners are large and require dedicated rooms with high voltage lines, reinforced floors, etc.

Central to the MiniCAT design is a flat-panel x-ray detector.  Such detectors are based on the same kind of technology as flat-panel monitors for computers except instead of emitting visible-light photons; they are detecting x-ray photons.  These panels have the capacity for providing extraordinary spatial resolution (i.e., the ability to resolve small details in an image) largely due to the small size of their detector elements and this translates into excellent spatial resolution in the CT image.  Additionally, the spatial resolution tends to be isotropic (i.e., the same in all directions) due to the volume CT acquisition; whereas this is not always the case in traditional CT scanners.  Non-isotropic resolution means that fine details may only be visible when a patient is placed in the scanner with a particular orientation.  A three-dimensional isotropic image may be sliced in any fashion (i.e., oblique slices) and the ability to resolve details will be the same.

In the paper, we compare images from a traditional CT and the MiniCAT for imaging of the temporal bone - the portion of the skull that contains the ear canal, inner ear bones, cochlea, semicircular canals, etc. This portion of the human anatomy is notoriously difficult to image due to the very fine details and the alignment of some features with non-standard imaging planes.   For example, the width of one of the inner ear bones, the stapes, is about the length of the date on an American dime. 

In the image comparison, we find details (a facial nerve canal) that are clear in the MiniCAT scan and obscured in the traditional CT.  This is a clinical advantage to be able to resolve such important structures and may have an important impact on diagnosis and treatment of temporal bone diseases.  A technical analysis of the spatial resolution is also presented using standard techniques that shows the improved spatial resolution and high degree of resolution isotropy of MiniCAT.

Since the MiniCAT system is a dedicated head and neck scanner, it is unlikely to fully replace general-purpose full-body scanners; however, it is likely to play a very important role as a point-of-care scanner. Traditional CT scanners are expensive equipment, are often used continuously 24 hours a day, and require a relatively large dedicated room.  A MiniCAT system could be placed in relatively small room in an otolaryngology department (for example) that would put relief on the schedule for the full-body scanner.  This also improves patient workflow allowing for the CT scan during the same visit as the initial diagnosis and at the same location, eliminating the need for separate visits for scanning and follow-up. Bringing the CT scanner to the patient as opposed to vice versa, at the point of care, should be good for both physicians and patients.  That the MiniCAT system can do this with an improved ability to resolve fine features regardless of their orientation is an added advantage.

1The standard image planes are transaxial, coronal, and sagittal.  Given a 3D volume, one could choose any oblique slice in a kind of diagonal cut.  Some anatomical features are more obvious with an oblique slice in a certain orientation.