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Everything you need to know about Computed Tomography (CT) & CT Scanning


Chest: Angiography: Single Detector (SDCT) vs. Multidetector (MDCT) In CT Angiography

The latest versions of single detector CT scanners have provided the radiologist with what seemed to be unparalleled capabilities for vascular imaging. Subsecond scan times (.75 sec), coupled with long acquisition times (up to 60 sec), as well as near back to back spiral acquisition capabilities (for dual phase imaging) were a boom to the use of CT angiography. The ability to couple narrow collimation (typically 2-3 mm), close interscan spacing and an extended pitch (typically 2) helped to drive the development of CT angiography for application as diverse as carotid artery stenosis, evaluation of renal transplant donors and staging of pancreatic cancer.

However, the development of multidetector CT scanning over the past year has resulted in what amounts to as a ‘revolution’ in our imaging capabilities. In a simple side by side comparison MDCT provides what now are truly unparalleled capabilities (or at least until the next generation of MDCT scanners are introduced at RSNA 2001 or 2002) when compared to SDCT. A minimum of four times as large an area scanned per second, the ability to retrospectively select scan width, to routinely provide 1 mm collimation, as well as higher resolution with no ‘blooming’ of slice thickness are some of the key advantages of MDCT especially in regards to CT Angiography. The newest MDCT scanners allow routine cardiac gating as well as longer distances providing the capability for nearly unlimited scan volumes.

In the simplest of terms these capabilities have a two-fold impact on CT angiography:

1. All of the applications done with single detector CT scanners can be done better (i.e. AAA evaluation, renal donor evaluation)

2. Many new applications are being developed and introduced into clinical practice (i.e. evaluation of suspected mesenteric ischemia, coronary artery patency imaging that were not practical or possible with SDCT). A specific side by side comparison of the Siemens top of the line single detector scanner (Siemens Plus-4) and the top of the line multidetector scanner (Volume Zoom)are summarized in the following table. Similar comparison can be made for any other manufacturer (i.e. GE, Toshiba) with similar results.

 

Siemens Plus – 4 (SDCT)Siemens Volume Zoom (MDCT)
1 detector row8 detector rows
750 ms scans500 ms scans
1.33 slices/sec8 slices/sec
1 mm slice width0.5 mm slice width
50 second spiral study100 second spiral study
17 line pair resolution24 line pair resolution

Although at times technical specifications are seem more like automobile ads claiming that ‘you can go from 0 to 60 MPH in 7 seconds these specifications are best appreciated when looking at actual scan protocols. Although it is impossible to provide every protocol in this exhibit they can be found online at www.CTISUS.com. Several representative protocols are listed below to show the clear advantages of MDCT over SDCT.

 

SDCTFactorsMDCT
3 mmcollimation4 x 1 mm
3 mmslice thickness1.25 mm
1-2 mmreconstruction interval1 mm
2pitch6 - 8
6 mm/secvolume scanned12 mm/sec (minimum)
25 secscan delay25 sec

Renal Donor Evaluation – Arterial Phase

 

FactorsMDCT
collimation4 x 1 mm
slice thickness1.25 mm
reconstruction interval1 mm
pitch6 - 8
volume scanned12 mm/sec (minimum)
scan delay25 sec

The key numbers in the chart are that we routinely use 1 mm collimation and that total scan times are cut in half despite the use of the narrowed collimation. Both factors provide increased image detail and quality by allowed closer coupling of contrast injection and data acquisition for use of iodinated contrast (including use of lower volumes), decreased length of patient breathhold which essentially eliminates artifact due to patient respiratory motion and narrower collimation allowing for better resolution of smaller vessels without artifacts including "blooming" (up to a 27% increase in actual slice width with a pitch of 2 on a SDCT system).

MDCT also has several practical advantages in terms of the entire scanning process. Reconstruction of individual images is done at a rate of nearly 2.5 images per second as opposed to SDCT where reconstruction of data takes 6-10 seconds per slice. Images can be retrospectively reconstructed at varying slice widths so while images with 1.25 slice width at 1 mm intervals are ideal for 3D imaging, filming or local storage might suffice with 5 mm slice width at 5 mm intervals. MDCT does provide several new practical challenges including the volume of data that will need to be archived as well the need for 100-T lines for reasonable speeds of networking between the scanner and the workstation.

MDCT also provided what might have been the final and most compelling reason that volume interpretation is needed when volume imaging is done. When we generate a CT study of 200-500 individual images filming each and every image is both impractical and expensive. A single exam filmed 12 on 1 could fill up an entire alternator! Using a workstation is more logical except for the fact that scrolling through hundreds of images is equally impractical as it is prone to user fatigue and very large datasets do often slow down the display systems. Volume imagine with 3D display solves the problem withf real time display of the volume of data with the additional advantage of allowing the user to visualize and analyze the data in 3 dimensions. There is little doubt that imaging vascular anatomy is optimized in a volume perspective.

© 1999-2019 Elliot K. Fishman, MD, FACR. All rights reserved.