Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
ImPACT Technology Update No.1, 2nd Edition MDA 02024 £30 Multi-Slice CT Scanners January 2002 Introduction x-ray tube The first edition of this leaflet was issued in May 1999, when multi-slice CT was in its infancy. This second edition has been produced to include new scanner models and updated information on multi-slice scanner operation. The first 3rd generation multi-slice CT scanner, the Elscint CT Twin, was launched in 1992. The scanner had helical capabilities and the ability to acquire two transaxial slices simultaneously, using two parallel banks of detectors. Single detector bank In the second half of 1998, four manufacturers (GE, Siemens, Toshiba and Picker, now Philips) extended this concept by launching multi-slice CT scanners. All were 3rd generation helical scanners, with low voltage slip rings, capable of acquiring four CT slices in one x-ray tube rotation (see Fig. 1). Additional four-slice models have been introduced by Toshiba and GE. Some basic specifications of the systems currently available are shown in Table 1. 4 parallel detector banks Fig. 1. Single and multi-slice scanner concepts compared Note that Fig. 1 is simplified, the configuration shown could only offer a minimal choice of slice widths. In practice, between 8 and 34 detector banks are used in different groupings, but a maximum of 4 slices can be acquired simultaneously. GE recently introduced the eight-slice LightSpeed Ultra, and other manufacturers have announced 16 slice scanners that should be available by the end of 2002. These scanners are not discussed in this leaflet. Feature GE Lightspeed S [LS Plus] Philips Mx8000 Z-axis detector array length (mm) 20 20 20 32 32 Min. slice width (mm) 0.63 (x2) 0.5 (x2) 0.5 (x2) 0.5 (x4) 0.5 (x4) Max. slice width (mm) 10 (x2) 10 (x2) 10 (x2) 8 (x4) 8 (x4) Min. tube rotation time (sec/rev) 0.8 [0.5] 0.5 0.5 0.5 0.75 512 x 512 1024 x 1024 512 x 512 512 x 512 512 x 512 Generator output (kW) 53.2 60 60 60 36 / 48 Anode heat capacity (MHU) 6.3 6.5 5.3 7.5 4 Anode cooling rate (MHU/min) 0.84 0.73 0.73 1.4 1.4 Reconstruction matrix (max) Siemens Toshiba Toshiba Volume Zoom Aquilion Multi Asteion Multi Table 1. Comparison of Multi-slice CT Systems ImPACT Technology Update, 2nd Edition: Multi-Slice CT Scanners 1 GE Clinical Applications of Multi-Slice CT 16 x 1.25 The clinical advantages of multi-slice technology can be broadly divided into two categories. 20mm Their speed can be utilised for fast imaging of large volumes of tissue with wide slices. This is particularly useful in studies where patient motion is a limiting factor. With a four-slice system and a 0.5 second rotation, it is possible to acquire volume data up to 8 times faster than with a single slice, 1 second scanner. Applications in this area include trauma, thoracic, geriatric and paediatric examinations. Fast rotation times on multi-slice systems also expand the capabilities for ECG gated cardiac studies and cardiac calcification scoring. The other main advantage of multi-slice systems is their ability to acquire a large number of thin slices quickly, making routine abdomen acquisitions with 2-3 mm slice widths possible. The increased z-axis resolution provides high quality 3D visualisations, for applications such as CT angiography and virtual endoscopy. Multi-slice systems also have applications in CT fluoroscopy. By simultaneous imaging of a number of slices, real time image display for up to 4 slices is possible, resulting in improved localisation for interventional procedures. X-ray tube thermal loading for a given patient volume is lower for multi-slice than for single-slice systems because of the greater length covered per rotation. Volume coverage for a single helical run can therefore be increased. Another clinical advantage of multi-slice systems is the ability to reconstruct broad slices from a narrow acquisition width. This permits partial volume artefacts to be reduced without the increase in noise that would result from reconstructing narrow slices. Detector Arrays and Slice Width Although all the systems discussed in this brochure are capable of producing four slices in one x-ray tube rotation, the arrangement of detectors along the z-axis and the available slice widths vary between systems. Fig. 2 shows the three different detector array designs currently available. There is a large variation in the number and width of detector banks between the different designs. This affects: • • • • Minimum slice width available Number of slices at minimum width Range of slice widths available Maximum length imaged in one rotation Table 2 shows the possible combinations of slice width for each scanner, when scanning in sequential (standard axial) mode. 2 Philips and Siemens 5 2.5 1.5 1 1 1.5 2.5 5 20mm Toshiba 4 x 0.5 15 x 1 15 x 1 32mm z-axis Fig. 2. CT multi-slice system detector array designs (Distances given as effective size at isocentre) Another aspect that must be considered is the efficiency of the various detector array designs. Due to gaps between the detector banks, it could be predicted that a design employing a larger number of banks will be less efficient in terms of both dose and imaging. In practice, however, the gaps are relatively small and so other factors, such as irradiation beyond the imaged length, have a greater impact on performance. Scanners with detector banks that have greater z-axis coverage may potentially have problems with artefacts due to the geometry of the greater cone angle employed. GE Philips and Siemens 2 x 0.7 4 x 1.25 4 x 2.5 4 x 3.75 4x5 2 x 7.5 2 x 10 2 x 0.5 4x1 4 x 2.5 4x5 2x8 2 x 10 Toshiba 4 x 0.5 4x1 4x2 4x3 4x4 4x5 4x8 2 x 10 Table 2. Available slice widths in sequential mode Helical Pitch Definitions Currently, manufacturers of multi-slice systems are employing two different definitions of pitch. ImPACT use the terminology Pitchx (x-ray beam pitch) and Pitchd (detector pitch) to differentiate between the two: Pitch x = table travel per rotation x - ray beam collimation Pitch d = table travel per rotation detector acquisition width Detector acquisition width is defined at the isocentre. ImPACT Technology Update, 2nd Edition: Multi-Slice CT Scanners Therefore, Pitchd = Pitchx x No. of slices acquired simultaneously, and, on a single slice scanner, Pitchd = Pitchx Pitchx is determined solely by the x-ray collimation and table speed, whereas Pitchd will also depend on the number of slices acquired per rotation, as shown in the following examples: Example 1: Irradiated width: 20 mm, Table speed: 20 mm/rot, Detector acquisition width: 4 x 5 mm Results in: Pitchx = 1 Example 2: Pitchd = 4 Irradiated width: 20 mm, Table speed: 20 mm/rot, Detector acquisition width: 2 x 10 mm Results in: Pitchx = 1 Pitchd = 2 On a single slice system, example 1 would be similar in terms of dose and image quality to a 5 mm nominal slice with a table speed of 5 mm/rotation. This would result in a pitch value of 1, by either definition. Using Pitchd, the number of slices acquired per rotation must be known before any inference can be made about image quality or dose at the quoted pitch value. Philips use the first definition, Pitchx, whilst the other manufacturers are currently using the second definition, Pitchd. Philips have also introduced the nomenclature of Pitch Quad, Pitch Dual and Pitch Single, to indicate the number of slices acquired per rotation. In the examples given above, pitch would be quoted as: Pitch Quad = 1 for Example 1 and Pitch Dual = 1 for Example 2. It should be noted that the value of pitch does not change, because of their adoption of Pitchx. Helical Interpolation Algorithms To reconstruct an axial image from a helical data set, single-slice scanners have commonly used 180° linear interpolation algorithms. With this type of algorithm, the z-sensitivity profile (imaged slice width) for a helical scan with Pitchx = 1 is similar to that of an image acquired in sequential mode. Currently, on multi-slice scanners, the various manufacturers employ different approaches to helical interpolation. Some favour the approach commonly used on single slice scanners of interpolating over a fixed number of projection data points regardless of pitch. With this approach, as pitch is increased, the noise remains constant but the z-sensitivity increases. Others ImPACT Technology Update, 2nd Edition: Multi-Slice CT Scanners employ a z-filter interpolation method, where the interpolation is performed over a fixed z-axis distance. The latter approach results in a constant z-sensitivity over a range of pitches, but increased noise with pitch if the tube current is not altered. In both cases, non-linear weighting functions may be applied to the interpolated data. Image Quality In sequential mode, multi-slice systems should have largely the same image quality as equivalent single slice scanners. The efficiency of individual detector banks may vary however, resulting in different noise levels between the slices. The cone beam geometry may lead to unequal z-axis sensitivities for different slices. As explained in the previous section, the relationship of noise and slice width with pitch is not always the same on multi-slice as on single-slice scanners. For systems that employ z-filter interpolation to keep slice width constant with pitch, the image noise will increase with increasing pitch if the tube current is held constant. These systems will, however, automatically adjust the mA as pitch is changed, resulting in constant dose, noise and slice width. The level of interpolation artefact in helical mode shows an overall increasing trend with increasing pitch. However, the relationship is not as straightforward as on single-slice systems, with certain pitch values theoretically resulting in a reduced level of artefact. There are differences of opinion between the manufacturers on the subject of pitch optimisation. Some manufacturers recommend specific pitch values for “optimum image quality”, which in this context is thought to refer mainly to the level of helical interpolation artefacts. Toshiba recommend Pitchd values of 2.5, 3.0, 3.5 or 4.5, with four slices per rotation, for optimal image quality. Philips also recommends specific pitches, including Pitchx values of 0.875 and 1.25. On GE multi-slice systems, only two pitches are available. HQ (High Quality) and HS (High Speed) modes correspond to Pitchd = 3 and 6 respectively on their four slice models. Siemens claim there are no preferred pitch values on their multi-slice systems, with Pitchd settings of between 1 and 8 freely selectable. It is important to note that, in order to keep helical artefacts on a multi-slice scanner down to similar levels as those produced on a single-slice CT system, it may be necessary to use a Pitchx of less than 1. Thus, a four-slice scanner with the same rotation time as a single-slice scanner would not be able to obtain images of the same quality at four times the speed, but more typically at about three times the speed. It is possible, however, to use a lower tube current, so that, in spite of the partially overlapping helices, there would be no increase in average dose compared to a pitch of 1 for the same amount of image noise. 3 Whereas helical interpolation artefacts are an important issue for multi-slice scanners, especially in structures that change rapidly in the z-axis, patient movement artefacts are likely to be reduced because of the increased examination speeds. Dose The dose considerations for a multi-slice scanner are broadly similar to those of a single slice scanner. There are, however, some important differences. Dose utilisation in the z-axis tends to be somewhat poorer on multi-slice than on single-slice scanners. This is mainly because the x-ray beam width is generally slightly broader than the total imaged width, in order to achieve uniform irradiation over all the detector banks. Whereas on a good single-slice scanner the geometric efficiency may be close to 100% for all slice widths, on a multi-slice scanner it is more likely to be 80 – 90% for most slices, falling to around 70% for 1 mm and 50% for 0.5 mm slice widths. All helical scanning necessitates extra irradiation at each end of the helical run to obtain sufficient interpolation data to reconstruct the required volume. On multi-slice scanners, four helices are acquired simultaneously, so the extra irradiated length is likely to be longer, and the resultant dose length product slightly greater, than on a single-slice scanner. On a single-slice scanner, tube current is normally held constant for changes in pitch, since there is no variation in noise with pitch. Thus, patient dose falls as pitch increases. On a multi-slice scanner using z-filter interpolation, noise increases with pitch. Manufacturers overcome this effect by incorporating an automatic rise in tube current as the pitch increases, so that patient dose will remain constant for all pitches. When comparing doses quoted by different scanner manufacturers, it is important to consider whether the mA quoted is the actual tube current or an “effective mA” value, which is the tube current divided by Pitchx, also sometimes called “mA per slice”. CTDI can be measured using the same methodology as for single-slice systems. The dose measured should be divided by the total irradiated width, e.g. 20 mm for a scanner acquiring 4 x 5 mm slices. Even though the irradiated slice width (up to 32 mm) is quite large relative to the length of the standard 10 cm CT chamber, the resultant underestimate in dose has been found to be only 2% at most. Speed and Volume Coverage All the 4-slice systems have sub-second rotation times available. On the GE LightSpeed Plus, Philips, Siemens and Toshiba Aquilion systems, the minimum 360° scan time is 0.5 second, while on the GE LightSpeed S and Toshiba Asteion systems it is 0.7 - 0.8 second. The maximum volume coverage possible in one helical run will be determined by a combination of the tube characteristics and the maximum length covered per rotation. The Toshiba tube has the highest anode thermal capacity and the highest anode cooling rate (Table 1). The Toshiba scanner is also capable of imaging 32 mm of the patient in a single rotation compared with 20 mm for the other systems. The product of the increased z-axis coverage and the faster rotation can offer scanning times that are in the region of 4 to 8 times faster than for a single slice scanner. Manufacturers have quoted higher potential speed gains than this, but it is considered unlikely that these will occur in routine clinical practice. Indeed, to obtain image quality comparable to single-slice scanning, it is more likely that scanning times would be 3 to 6 times faster. Software, Upgrades and Ease of Use Multi-slice scanner software continues to undergo major developments. User interfaces are being refined as the manufacturers get more clinical feedback, and different clinical applications are coming to light. Some manufacturers offer multi-slice capabilities as optional upgrades to existing single-slice scanners. For centres with one of these systems, the most cost efficient route to multi-slice scanning will probably be achieved through this path. Multi-slice scanners offer increased options in scan protocol selection, such as collimated slice width, imaged slice width, pitch and interpolation algorithm. Drawing the correct compromise between flexibility of settings and pre-defined combinations of these parameters will be important in both staff training and clinical ease of use ImPACT, St. George's Hospital, Tooting, London SW17 0QT Tel. 020 8725 3366 Fax. 020 8725 3969 e-mail: [email protected] website: www.impactscan.org ImPACT is the UK's national CT evaluation centre, providing publications, information and advice on all aspects of CT scanning. Funded by the Medical Devices Agency, it is part of a comprehensive medical imaging device evaluation programme. Crown Copyright, 2002 4 ImPACT Technology Update, 2nd Edition: Multi-Slice CT Scanners