Technical Requirements

Progress made in the clinical implementation of CT colonography would not have been possible without significant advances that have been made in CT imaging technology over the past 10 years. Avail-

Department of Radiology, Mater Misericordiae University Hospital, Eccles Street, Dublin 7, Ireland ability and ease of access to this technology is crucial for any CT colonography service to allow rapid acquisition, processing and reading of CT colonography datasets.

Technologically, there are three basic components to a CT colonography examination: 1) multislice CT hardware for image acquisition, 2) software and associated platforms for post processing and reading of data sets and 3) adequate transfer networks between the hardware and software components with appropriate image data storage facilities.

While techniques for image post processing and rendering have a major impact on how the final image is viewed, the spatial quality of the dataset will fundamentally be determined by the initial CT acquisition parameters. Although much of the early work on CT colonography was performed on single row helical scanners (Fenlon et al. 1999), multidetector CT (MDCT) is now the accepted standard for current CT colonography research protocols and for performing clinical examinations in everyday practice. MDCT allows acquisition of a single breath-hold thin section CT examination of the entire colon in relatively short scan times. A typical acquisition takes 12-15 s using a 16-slice MDCT with a decrease in both respiratory artefacts and improved colonic distension compared with single slice acquisition (Hara et al. 2001). Image artefacts and misregistration secondary to motion and breathing at single slice CT scanning have been shown to increase both diagnostic errors and evaluation times (Fletcher et al. 1999). These artefacts are virtually eliminated using MDCT acquisition. Any department purchasing a new CT scanner to include an expansion of their service to include CT colonography should choose an MDCT.

Once the CT data is acquired, images should be reconstructed according to a standard protocol and automatically transferred to a reading workstation for review. There are numerous options available with regard to CT colonography workstations. Appropriate software is available from both the leading CT manufacturers and specific CT colonography software vendors. Such workstations allow datasets to be read in a variety of formats, most commonly 2D images with multiplanar reconstructions (MPR) and 3D endoluminal views for problem solving. Furthermore, software programmes are now capable of generating automated 3D reconstructions of the colonic mucosa from the acquired datasets with an average time for reconstruction in the order of 5-8 min for a complete 3D fly-through. The relative merits of each method will be discussed in a subsequent chapter.

Consideration must also be given as to how CTC datasets will be archived and how datasets may be retrieved to facilitate comparison with previous CT colonography studies. The volume of data generated for each CT colonography examination precludes hardcopy printing of all acquired images. Using a 16-slice MDCT with 3-mm slice acquisition and 1.5- mm slice overlap, a standard study with 40 cm of Z-axis coverage in both supine and prone positions will typically comprise over 600 slices. At the standard 512 512 bits of resolution each study will require over 500 MB of memory for storage. A single patient's examination, therefore, will occupy almost an entire conventional compact disc (CD) which has a memory capacity of 700 MB. An alternative to CD for archiving is DVD. While relatively inexpensive, use of DVD requires purchase of a DVD reader as most commercially available workstations come with only an integrated CD reader. The actual hard drive memory capacities of workstations vary (14,000 MB in our department) which if used for CT colonography alone can accommodate only 20 cases. In reality, these workstations are also used for 3D reconstruction of other studies including vascular and orthopaedic examinations, resulting in a real need for external storage.

Without an integrated PACS system effective high volume CT colonography that allows rapid image retrieval and comparison with previous studies is extremely difficult. Many issues relating to memory storage and networking infrastructure are simply resolved with the implementation of PACS. PACS offers numerous potential advantages including the viewing of studies in remote locations, a decrease in the number of lost datasets, and the large capacity image storage. Its greatest advantage is that it facilitates the rapid retrieval of previous studies which is a significant advantage in the setting of a screening program. Until recent years many CT workstations did not have a PACS compatible interface. Furthermore many radiology departments do not yet have PACS. When choosing a workstation for CT colonog raphy interpretation, careful consideration should be given to its PACS compatibility.

The network interface between the CT workstation and reading workstations must be seamless, transfer of datasets must be fast and automatic, and datasets must be available for reading without any loss of diagnostic information. Transferring datasets of this size places a considerable demand on any network whether it involves transmission from CT workstations to reading workstations or from the primary reading stations to remote reader locations. The connecting network cable must be at least a category 5 UTP connection with switches producing speeds up to 100 Mb/s. Speed of transfer will be compromised if the network is of insufficient size and if high volumes of data are being transferred simultaneously.

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