Chest: Technical Aspects of Chest Multidetector CT
Technical Aspects
Overview
Multidetector CT (MDCT) is far more than an upgrade of single detector CT (SDCT), and in many instances provides technology and capabilities that were never even considered possible a few short years ago. In this review, we will look at some of the core concepts and principles of MDCT with a special focus on the differences between MDCT and SDCT especially as it relates to thoracic CT scanning. Although most of the major scanner manufacturers offer at least one version of MDCT there are definite differences in technology and capability between the various vendors. For example, the detector array used, one of the key features on any scanner differs between GE and Siemens, the two largest scan vendors. GE (and Toshiba) offer a fixed array design while Siemens (and Marconi) offer an adaptive array design. Although each manufacturer claims superiority, we will try to define the specific advantages of one or the other techniques. Please note that at Hopkins we use a Siemens VolumeZoom scanner and our knowledge of the system and our hands-on experience with it are far more extensive that any information we could gather on any other scanner.
With a fixed array detector systems, the detector size is constant at 1.25 mm and there are 16 elements (20mm total) (Figure 1). Elements can be added together in groups of 2, 3 or 4 prior to sampling resulting in four 2.5 mm channels, four 3.75 mm channels or four 5 mm channels. One potential problem of the fixed detector array are that the multiple joints between detector rows reducing the efficiency of the detectors which can result in poor signal. Additionally, the outer edges of the detector produce a shadow effect further deteriorating image quality. Another limitation of the fixed array system such as the GE LightSpeed scanner is that typically only 2 different pitches are used; pitch 3 (HQ or high quality mode) and pitch 6 (HS or high speed mode).
With scanners that use "Adaptive Array Detectors" such as the Siemens VolumeZoom the detector array consists of a series of 8 detectors set in pairs from the middle out of 1 mm, 1.5 mm, 2.5 mm and 5 mm for a total of 20 mm across (Figure 2). These detectors are then used to create either sets of 4 detectors of 1 mm, 2.5 mm or 5 mm thickness. The pitch can be chosen with equal quality anywhere from 1 to 8. This provides for maximum flexibility when designing study protocols. For thoracic applications we routinely use the 2.5 mm thick detectors and for special applications the 1.0 mm thick detectors. One way of looking at MDCT is as the 4 "C"s:
Continuously rotating tube/detector system
Continuous radiation
Continuous data acquisition
Continuous table feed
Although this list can in great part be true in a single detector system, the differences between MDCT and SDCT are crucial and can be broken up into several categories. They include:
Pitch
Scan slice profile
Noise
Radiation dose
Collimation
Let us now look at each of these individually in detail:
Pitch
With the newest versions of SDCT a pitch of up to 3 can be obtained. The advantage of a higher pitch is that a specific volume can be scanned in a shorter period of time which is critical in such applications as pediatric CT, CT angiography, pulmonary embolism evaluation and virtual imaging. The radiation dose for that volume will be decreased accordingly as long as the mA is not changed. However, these advantages come at a price as the slice width profile widens with increasing pitch. The slice is widened by up to 27% with a pitch of 2. With MDCT the pitch can be selected anywhere between 1 and 8. Despite the increased pitch the slice width is kept constant with MDCT because of the reconstruction algorithm is in fact independent of pitch. This algorithm is the Adaptive Axial Algorithm. One of the confusing differences among the various manufacturers is the definition of pitch depending on whether a SDCT or MDCT is being discussed. In single slice CT the definition is straightforward:
Pitch = table movement per rotation/slice collimation.
So for a typical 1 second rotation scanner a pitch of 2 means the table traveled 10 mm with a 5 mm slice width or collimation. With multislice CT it is more complicated as collimation and width may be different because a single collimator can produce several different scan widths. For multislice CT the definition is:
Pitch = table movement per rotation/single slice collimation. With a 0.5 sec scanner there are 2 rotations per second. So if the table travels 12 mm in a second and a 1 mm collimator is used then the pitch would be 6 (6 mm/ 1mm).
Slice Width and Collimation
With SDCT the true width or the "effective slice thickness" of the reconstructed image is influenced by pitch and the reconstruction algorithm used (wide vs. slim). With a pitch of 2 and a slim algorithm the "effective slice thickness" may increase by 27%. With MDCT and the use of the "Adaptive Axial Interpolation" both the scan width and collimation are correct without any blooming. With SDCT the slice collimation and the slice width are the same. This parameter is selected prior to the study and can not be modified in the reconstruction sequence or in post processing. With MDCT there is increased flexibility. The user selects in advance the detector collimation of the study. On our scanner the typical selections are for 4 detectors and are 1 mm, 2.5 mm and 5mm. What is important however is that each detector can provide several true slice widths. They are listed in table 1 but a closer look at the 1 mm collimation shows that scans can be reconstructed anywhere from 1.0 mm to 5 mm to 10 mm. This allows the user in the same study to have images with high resolution but increased noise (1 mm slice thickness) and images with standard resolution but less noise (5 mm slice thickness). In practice this provides unparalleled capabilities including obtaining true volume datasets for CT Angiography and standard 5mm scan slices for looking at the heart and mediastinum.
Radiation Dose
With a pitch of 8 to 1 one would at first glance think that MDCT should significantly decrease radiation dose for patients. This is based on the realization that with SCDT that the applied dose decreases linearly with increasing pitch. In practice MDCT can provide low dose studies (i.e lung cancer screening) but as set up routinely the applied dose is independent of pitch. There is no decrease for increasing the pitch as the user selects the mAs for the desired slice width and the tube current will adapt to maintain image quality, independent of pitch. Although this feature seems like a good idea it is important to monitor exactly what dose an individual patient receives.
Image Noise
In both SDCT and MDCT, the image noise is dependent on a number of factors which may or may be controlled by the assigned study protocol. They are:
mAs
kV
Kernel
Slice thickness
Patient size
Collimation
Image display
MDCT can help limit noise by modifying the mAs and slice thickness as needed for each individual case. Because of the number of slice widths that can be selected from a single collimation one is able to for example get the necessary volume data for a 3D CT angiogram while at the same time get noise-free images for looking at the liver. The advanced processing algorithms and faster computers allow between 2 and 3 images to be reconstructed per second depending on the scanner available.
Detector Collimation
The collimation used by the scanner has a number of fixed selection (Figure 3) choices which can then be expanded as needed. Although there are many choices that can be made retrospectively it is important to be aware that the final choices are limited by the collimation.
Conclusion
This is a brief look at some of the factors which are critical to understanding and using MDCT. Please recognize that each of the MDCT scanners is different and please contact your product specialists to make sure you are getting the most out of your scanner.