31 Future Technical Developments for CT of the Thorax
W. A. Kalender and M. Kachelriess
W. A. Kalender, PhD
Professor, Institute of Medical Physics, Universität Erlangen- Nürnberg, Krankenhausstrasse 12, 91054 Erlangen, Germany M. Kachelriess, PhD
Institute of Medical Physics, Universität Erlangen-Nürnberg, Krankenhausstrasse 12, 91054 Erlangen, Germany
CONTENTS
31.1 Introduction 461
31.1 Technical Developments 461 31.2 Image Quality and Dose 465 31.3 Conclusion 467
References 467
31.1 Introduction
What will the future bring? It is unnecessary to state that any prediction may prove to be wrong in the years to come, or, in the worst case, as soon as the book is printed. (This may be a reason why a wise editor transfers the respective task to make predictions to a potential author who is not wise enough to deny the task.) Predictions, or better yet, the discussion of future developments can be based on what technology and physics will provide, or it can be based on what the clinical demands are or will likely be. The question is: Will future developments be technology driven or application driven?
The initial development of spiral CT was application driven. The original project was initiated by the desire to detect pulmonary nodules reliably (Kalender et al.
1990; Vock et al. 1990). While it was easy to show early on that spiral CT allowed to provide the necessary performance in principle (Fig. 31.1a), it also became apparent immediately that the available technology, in particular the available X-ray power, was insuffi cient to image complete organs of large size, such as the lungs, fast enough and with high spatial resolution.
The general goal of imaging complete organs in time intervals shorter than a breath-hold period and at high isotropic resolution was not limited to the thorax, of course. In particular, all applications involving con-
trast medium administration and CT angiography in particular demanded faster scanning.
The necessary and successful strategy in the 1990s was to provide multi-row detectors so that several slices could be measured simultaneously. The state of the art which has been reached after 14 years of spiral CT and which is documented in this textbook is rep- resented by scanners which allow to acquire 16 slices of less than 1 mm each with a scanner rotation time of 0.5 s or less per 360° (Fig. 31.1b). It has been described in an excellent manner in the preceding chapters, both with respect to physics and technology and with respect to clinical applications. Isotropic sub-millime- ter spatial resolution and effective scan times as low as 100 ms are essential features which are routinely avail- able. Apparently, CT has reached a very high level of performance, and it may seem that the present multi- slice scanners already fulfi ll all clinical demands.
Is there still a need for further technical develop- ments in CT? Will future developments be driven by technology or by applications? Economic reason tells us that developments have to be application driven.
From the preceding chapters we learned, however, that only phase-selective imaging of the heart and the coronary arteries may demand further improve- ments in technology.
At the same time we observe many research activi- ties in the fi eld of experimental CT imaging with area detectors and circular or spiral cone-beam scan tra- jectories. The underlying mathematical concepts form a completely new fi eld of science (e.g., see Huesman 2002; Wang et al. 2000). Moreover, there are many promising and exciting results already. The CT imag- ing based on fl at-panel detector technology is a hot topic as is pointed out in Chap. 3; respective images show impressive anatomical detail of the order of 100–
300 µm (Fig. 31.2a). High-resolution imaging of biop-
sies, specimens, and small animals has become known
as “micro-CT” (Karolczak et al. 2001; Siewerdsen
and Jaffray 2000). Based on small focus or “micro-
focus” X-ray tubes and area detectors with small pixel
pitch, it allows, for example, to image anatomy in
experimental animals such as gene-manipulated mice
at levels of 10–100 µm (Fig. 31.2b). Although there are many drawbacks and limitations to be considered with respect to potential clinical use, these activities stimu- late appetite. Expectations are boosted further by mar- keting-driven hyperpositive statements from some of the manufacturers. In all, this may cause future devel- opments to be driven by advances in technology rather than by clinical demand.
In 2002 the question frequently asked was: “Will the slice race continue?” All manufacturers try to live up to
customer demands and expectations, and consequently they investigate solutions for detectors with higher numbers of slices or area detectors which allow cover- ing the organ or region of interest in a single view.
This fi nal chapter initially focuses on respective possible or likely developments in technology. It con- tinues with considerations of image quality and dose to point out which basic physics constraints have to be kept in mind. It comes back to application-related aspects in the Conclusion section.
Fig. 31.1a, b. Spiral CT has made tremendous progress within approximately one decade. a The fi rst scans of the thorax in 1989 were taken with single-row scanners at 10-mm collimation. b In 2001, 16-slice scanning at sub-millimeter collimation and with isotropic spatial resolution became available
a b
Fig. 31.2a, b. High-resolution cone-beam CT using fl at-panel detectors provides impressive anatomical detail and indicates the potential for further improvements in CT. a Image of a human hand at approximately 200-µm isotropic resolution. (Courtesy of Siemens Medical Solutions). b Image of a mouse knee at approximately 20-µm isotropic resolution
a b
31.1 Technical Developments
Technical developments can be divided into those related to scanner mechanics, X-ray system, detector, etc., the “hardware” as the fi rst category, and into image reconstruction, data handling, dedicated evaluation software, and other algorithms, with the “software”
as the second category. We discuss hardware-related aspects fi rst.
It is obvious that the rotation frequency of mechan- ical gantries can be increased further. There are many technical instruments of similar size and mass which rotate much faster; however, there are two problems related to CT: the number and the complexity of sub- systems such as integrated circuit boards, rotating X-ray tube anodes, etc., which would demand very specifi c adaptations to cope with the increasing cen- trifugal forces. This means increased or prohibitive cost and, in the worst case, susceptibility to failure.
Another important aspect is that it will be diffi cult to provide the necessary number of X-ray photons in shorter and shorter rotation times. Electron-beam CT is capable of performing scans within 100 ms or less; however, for high image quality and good low- contrast resolution single sweeps have to be added up resulting in high effective scan times. This was the reason why CT adopted multi-row detector technol- ogy and multi-slice scanning when the demand for higher-volume scan speed arose (Kalender 2003).
Further reduction of rotation times seems to be indi- cated only for cardiac applications, and this is techni- cally feasible. Rotation times of 0.2 or 0.3 s are the goal and would allow for effective scan times of below 100 ms using state-of-the-art algorithms for phase- selective cardiac reconstructions (Kachelriess and Kalender 1998; Kachelriess et al. 2000a, b). Fast
“conventional “scanners dedicated to cardiac applica- tions are likely to become available in the future.
To provide detectors beyond the current state of the art of 16 simultaneously acquired slices is cer- tainly possible, and they will become available in the near future. The design of multi-row detectors (an example is shown in Fig. 31.3a) can be extended; a prototype of a 256-row true CT detector (Fig. 31.3b) has already gone into operation. The related techni- cal challenge is, to a lesser extent, the detector itself, but rather the electronics involved and the cost associated with it. In any case, CT detectors in larger formats with performance characteristics equivalent to the present prime clinical systems will become available. Only the point in time and the cost are unknown.
In addition, other types of detectors, primarily fl at-panel detectors designed for conventional X- ray imaging, are under investigation. Amorphous silicon (aSi) fl at-panel detectors (FPD; Fig. 31.3c) and image-intensifi er systems (Fig. 31.3d) are in clinical use already for special applications such as for 3D angiography or intra-operative imaging of high-con- trast structures. Smaller-format solutions, such as charge-coupled devices (CCD) coupled optically to a phosphor screen, are in use for small-animal imaging (Fig. 31.3e) but do not easily lend themselves to an extension for clinical use comparable to the present standard CT image quality level; however, beyond the available sizes there is another essential problem associated with these “non-CT”-type detectors: their characteristics, mostly the dynamic range, dose effi - ciency, and temporal response will be limiting their performance with respect to low-contrast imaging.
To make up for low absorption effi ciency by higher exposure, and thereby higher patient dose, is not acceptable and will not be permitted due to regula- tions on the general use of CT. The defi nition and enforcement of reference dose levels for most stan- dard CT applications, for example, will prevent the introduction of detectors with reduced dose effi ciency (Die neue Röntgenverordnung 2002; European Commission’s Study Group 1998). Limitations in dynamic range and poor temporal resolution of pres- ent fl at-panel detectors additionally imply limitations with respect to image quality. With the hope that researchers and industrial developers prove us wrong, we predict that fl at-panel detector technology will not be able to replace dedicated CT-detector technology.
Such detectors, will, however, establish themselves for special applications such as intra-operative imaging.
With respect to other hardware components, it is easy to predict that CT will benefi t from the general technical development. Performance increases for computing power following Moore’s law are likely.
The same applies to other components such as data rendering and display technology. Improvements with respect to X-ray power, an old and mature tech- nology, will not be dramatic. Tremendous improve- ments have been provided since the introduction of spiral CT. But the physical and technical potential for further improvements is limited, and so is the moti- vation. The advent of multi-slice acquisition meant that the demand for long scan times diminished. It is not higher average power, but – if at all – higher peak power ratings that are demanded, in particular for cardiac CT with its short scan times.
Some software developments, e.g., new approaches
to image reconstruction and to computer-aided diag-
nosis, have been discussed in previous chapters. An important question for future developments is the question of whether reconstruction algorithms will be available that provide the established high level of image quality for scanners with slice numbers far beyond 16. Respective concepts for algorithms have been published and evaluated, and the necessary optimized implementations will certainly become available. An excellent review of the present con-
cepts is given in Chap. 1. The interested reader is also referred to specifi c literature (e.g., Wang et al. 2000;
Huesman 2002; Kalender 2003).
By now, many results have been obtained for cone- beam CT (CBCT) by simulations (Fig. 31.4a) and in small-animal imaging (Fig. 31.4b). They indicate that image quality at a level comparable to the present 16-slice scanners will be possible for clinical CBCT scanners at 256 slices and beyond. The question which
Fig. 31.3a–e. Area detectors for CT data acquisition. a Typical detector array used in clinical CT at present. It allows acquisi- tion of data for 16 slices of 0.75 mm simultaneously or alterna- tively 16 slices of 1.5-mm thickness each. b New concept for a 256-slice CT detector. (Courtesy of Toshiba Medical Systems).
c Radiographic fl at-panel detector integrated into a CT gantry.
(Courtesy of Siemens Medical Solutions). d Image intensifi er on a mobile C-arm used for intra-operative CT imaging of skeletal structures. (Courtesy of Siemens Medical Solutions).
e Experimental setup for micro-CT using a charge-coupled devices coupled optically to a phosphor screen
a
c
e b
d
remains relates to the necessary reconstruction times which are prohibitively long for some of the recon- struction approaches. Nevertheless, we can expect that solutions adequate for routine clinical use will be available for CBCT. The situation which arose with the introduction of spiral CT will persist, however: data acquisition will be faster than image reconstruction.
[We refer to multi-slice spiral CT (MSCT) as long as the assumption is acceptable that the different slice fans are strictly in parallel. This is the case up to four slices. Whenever the cone-beam geometry is taken into account explicitly during image reconstruction, we refer to CBCT.]
31.2 Image Quality and Dose
Image quality and dose in CT are very closely related. Most of the facts are well known. In particu- lar, the dependence of noise on dose for otherwise unchanged parameters of scanning and image recon-
struction holds true for conventional CT, for MSCT, and for the various forms of CBCT under discussion;
however, there are aspects, such as the relationship between spatial resolution, i.e., the size of the resolu- tion volume element, and dose, which are not general knowledge at present. A review of the situation has been given recently (Fuchs and Kalender 2003;
Kalender 2003) and is also addressed in Chap. 3.
We only briefl y restate the important fi ndings and comment specifi cally on the situation which results in increases in spatial resolution that can be provided with future CBCT systems (Fig. 31.2).
Patient dose in CT is one of the most important topics regarding the future as the general acceptance of CT is often linked to dose issues and potential associated risks. It will be essential in the future to provide the information about the actual patient dose values for any given CT examination. Such efforts have started recently (Kalender et al. 1999; Nagel 2000). Information will help to counteract some of the irrational fears. It is the conviction of the present authors that CT is in fact a low-dose modality. Using state-of-the-art approaches, as already described in
Fig. 31.4. Cone-beam CT reconstruction algorithms and image quality have been assessed with very promising results in a simulations and in b practical applications such as small animal imaging. Dose-effi cient algorithms are available which provide adequate image quality even for large slice numbers
a
b
chapter 3, it can be assured that the effective dose to the patient will be kept low, typically at 0.1–20 mSv, and defi nitely far below the 200-mSv value which is considered as the limit of the “low-dose region”
(Kalender 2003).
The necessary concepts for dose management are described in detail in Chap. 3. In general, patient dose increases linearly with the tube current–time product, the “mAs product,” for unmodulated tube current and for all other parameters kept unchanged;
noise varies inversely with the square root of the mAs product. This holds true for scanning in sequential and in spiral mode, and for single-slice, multi-slice, and cone-beam CT. The mAs product and thereby the noise level are chosen according to the diagnos- tic needs. Modulation of the tube current during each rotation and along the z-axis according to the attenuation allows reduction of dose and tube load effi ciently without a loss in image quality. The devel- opment of highest relevance for the future of CT is an automated exposure control (AEC) which will make sure that a given level of image quality which has to be defi ned or selected by the user will be provided in an automatic fashion (Fig. 31.5). Respective technical concepts and implementations are available by now as is pointed out in Chap. 3. These developments will become the state of the art in the future. They will ensure that, with image quality kept at a specifi ed level, patient dose levels will decrease.
What are specifi c aspects relevant to CBCT? Irre- spective of the aforementioned considerations, dose will depend on the detector’s geometric and absorp-
tion effi ciency. The present CT detectors operate at high levels of typically 80% or better with respect to both parameters. The absorption effi ciency of fl at-panel detectors is signifi cantly lower, however, as has been pointed out already. With respect to geometric effi ciency, we have to consider that several of the scan and reconstruction approaches proposed at present do not make full use of all rays measured.
This problem is not well known and not transparent to the user. Often, data measured in the periphery of the area detector cone are not used in the recon- struction process. There are concepts and proven approaches by now, however, which allow full use of the data (Fig. 31.4a, right; Kachelriess and Kalen- der 2002); therefore, if the problem of absorption effi ciency is solved, detector and dose effi ciency will not pose a principle problem in the future.
There are constraints set by physics, however, which will remain unaltered and which set limits. Image noise increases with the fourth power of the spatial resolu- tion element, e.g., measured as the sampling distance (Fig. 31.6). Intuitively, this fact can be understood when looking at images reconstructed with different convolution kernels: image noise is reduced strongly when going from a sharp to a smooth convolution kernel. The mathematical background was elaborated in the early days of CT. It has been clarifi ed both theo- retically and experimentally again lately in view of the new situation with high-resolution fl at-panel detec- tors (Fuchs and Kalender 2003).
When going from a typical multi-row CT detec- tor offering approximately 0.5- to 0.8-mm sampling
Fig. 31.5. Automatic exposure control in CT.
In standard scanning with constant tube cur- rent (left) image quality varies with anatomical level. Modulation of the tube current with projec- tion angle and z-position allows to reduce mAs signifi cantly without a loss in image qual- ity (middle). Adaptive fi ltering which does not impair resolution leads to improved image quality at reduced mAs (right)
distance at the center of rotation to a fl at-panel array the sampling distance is reduced typically by factors of 3 to 5. It is exactly this feature which is marketed by manufacturers presently as the promise of “CT of the future”; “100-µm resolution” are quoted even in direct reference to cardiac applications. Clearly, we do not expect that this will be the future of clinical CT. Scan- ners operating at such levels of resolution can be set to use in research situations with small objects such as biopsies or small animals. This is the case already at present. Here the implication of the increase in spatial resolution on dose is tolerable as the object diameter and thereby attenuation decreases by orders of magni- tude. The scan of a mouse shown in Fig. 31.4b is asso- ciated with an effective dose of typically 200 mSv.
In clinical CT, such as, for example, in CT of the thorax, reduction of the sampling distance by a factor of 4 and a respective increase in spatial resolution would mean an increase in dose of 256 if we expect the same noise level as for the standard scan. This is not acceptable! We are in the comfortable situation presently that typical values of the effective dose in CT are of the order of or up to tenfold the natural background radiation levels (Kalender 2003). To postulate signifi cant increases in spatial resolu- tion with otherwise unchanged parameters would demand higher X-ray power and would incur signifi - cantly increased radiation levels well into the order of magnitude where even deterministic radiation effects can be expected. Confi rmation that this is not just a theoretical scenario can be obtained from small- animal imaging: according to reports from the U.S., mice have died due to scanning at very high resolu- tion in ineffi cient micro-CT scanners. The simple but fundamental message of Fig. 31.6 cannot be ignored.
If strongly increased spatial resolution is desired, strongly increased noise levels have to be accepted.
This means that low-contrast resolution or soft tissue discrimination will suffer or remain limited.
31.3 Conclusion
Computed tomography in general and, similarly, CT of the thorax, have reached a very high level of per- formance. The technology can be considered mature, the dose levels are acceptable, and most clinicians are content at present. There is no doubt that there will be further improvements: scan speed will be increased, the size of detector arrays will be enlarged, recon- struction approaches and speed will be improved, and all this will make CT scanning easier both for the radiology staff and for the patients. The rate of progress driven by technology will not be as over- whelming as it was in the past years. Patient dose considerations will set the limit in many respects.
Applications of CT in the thorax will be improved also. Dynamic CT, e.g., for the determination of ventilation and perfusion of the lung, myocardial perfusion measurements, and cardiac CT in general, are to be named. Moreover, cardiac CT appears to be the one and only candidate which may initiate sig- nifi cant application-driven developments. The use of detector arrays wide enough to cover the complete heart offers fundamentally new capabilities. Imaging of the complete heart will thereby be reduced from now typically 10–30 s to the order of 1 s or less.
Acknowledgements. The fi gures in this chapter, unless labeled otherwise in the legend, have been taken from the textbook, “Computed Tomography” (Kalender 2003), with permission of the publisher.
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