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1
Department of Clinical Radiology, University of Munich, Klinikum Grosshadern,
Marchioninistr. 15, 81377 Munich, Germany.
2
Department of Medical Informatics, Biometry and Epidemiology, University of
Munich, Klinikum Grosshadern, 81377 Munich, Germany.
Received July 26, 2000;
accepted after revision December 15, 2000.
Address correspondence to U. J. Schoepf.
Abstract
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SUBJECTS AND METHODS. Seventy patients with suspected focal and diffuse lung disease were allocated into two groups for comparison. The first group (n = 35) underwent multislice helical CT of the chest with 1-mm collimation and a pitch of 6. From the raw data, 5-mm contiguous and 1.25-mm high-resolution CT sections were reconstructed. The second group (n = 35) underwent conventional single-slice helical CT and high-resolution CT. High-resolution CT sections and 5-mm scans were rated for overall image quality, spatial resolution, subjective signal-to-noise ratio, diagnostic value, depiction of bronchi and parenchyma, and motion and streak artifacts. The 5-mm scans were also rated for contrast resolution and depiction of the heart and vessels. Radiation dose was calculated.
RESULTS. We rated 5-mm multislice helical CT superior to 5-mm single-slice helical CT, having a significantly higher total score (p = 0.0001). No significant difference (p = 0.986) was found between multislice and single-slice high-resolution CT sections. Radiation dose was 5.55 mSv for multislice helical CT and 5.50 mSv for single-slice helical CT.
CONCLUSION. Contiguous chest scans of superior quality and high-resolution CT sections of equal image quality compared with single-slice helical CT can be obtained using multislice helical CT. Therefore, a comprehensive diagnosis is feasible in patients with suspected focal and diffuse lung disease by obtaining a single scan.
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Recently, multislice helical CT has been introduced into clinical practice [5]. The single detector bank of single-slice helical CT scanners has been replaced, for this technique, by multiple detector banks that can be combined during readout to acquire four slices simultaneously. The most prominent feature of this technology is increased speed. Compared with single-slice helical CT, the same volume can be covered more than four times faster with identical section thickness. Another option is to use thinner collimation to increase spatial resolution and reduce partial volume averaging. We evaluated this latter advantage. Our hypothesis was that a single, breath-held, thin-collimation multislice helical CT acquisition could generate a set of raw data that would provide all options for image reconstruction, allowing multiple diagnostic problems to be addressed with a single scan. This hypothesis was tested in patients with suspected chest disease with focal and diffuse components, in whom a comprehensive diagnosis was mandatory.
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Multislice Helical CT Protocol
Multislice helical CT was performed with the Somatom Volume Zoom scanner
(Siemens Medical Systems, Forchheim, Germany) with simultaneous acquisition of
four 1-mm slices. Patients were scanned caudocranially in one breath-hold.
One-millimeter collimation was used at a table feed of 6 mm/0.75 sec scanner
rotation (8 mm/sec) at 140 kV and 140 mAs. These parameters result in a pitch
(table feed per scanner rotation divided by collimation) of 6, equivalent to a
ptich of 1.5 in single-slice helical CT systems. From the raw data, 5-mm-thick
contiguous sections were reconstructed twice: once using a soft-tissue kernel
and a second time with a lung kernel. The same set of raw data was then used
to reconstruct 1.25-mm-thick sections at 10-mm intervals with a
high-resolution (bone) algorithm. In select patients, for whom multiplanar
reconstruction was desirable, the entire chest was reconstructed with 1.25-mm
overlapping sections and a high-resolution kernel (Figs.
2D,
4C, and
4D).
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Single-Slice Helical CT Protocol
Single-slice helical CT was performed using the Somatom Plus 4 scanner
(Siemens). First, patients were scanned caudocranially in one breath-hold with
5-mm collimation at a table feed of 7.5 mm/0.75 sec scanner rotation (10
mm/sec, pitch of 1.5) at 140 kV and 140 mAs. In addition to the helical chest
scan, axial high-resolution CT at 10-mm intervals was performed in each
patient using 1-mm collimation at 140 kV, 140 mAs, and 1.5-sec exposure. From
the single-slice helical CT raw data, 5-mm-thick contiguous sections were
reconstructed twice: once using a softtissue kernel and a second time with a
lung kernel. The axial high-resolution CT data were reconstructed with a
high-resolution algorithm.
Image Evaluation
The complete study of each patient was printed in 16-on-1 format without
text. Identical mediastinal and lung window settings were used for multislice
and single-slice helical CT studies. Single-slice and multislice helical CT
studies were then presented in random order to two experienced reviewers who
were unaware of the scanning technique. Studies were rated by consensus of the
two reviewers on a five-point scale from 1 (worst) to 5 (best). Criteria used
in rating included overall image quality; spatial and contrast resolution;
subjective signal-to-noise ratio; diagnostic value; depiction of the heart,
vessels, bronchial walls, and parenchyma; and motion and streak artifacts for
multislice and single-slice helical CT 5-mm contiguous soft-tissue and lung
sections. Multislice and single-slice high-resolution CT studies were rated
for overall image quality, spatial resolution, subjective signal-to-noise
ratio, diagnostic value, depiction of bronchial walls and the parenchyma, and
motion and streak artifacts.
Statistical Analysis
Statistical analyses were performed using SAS 6.12, JMP 3.1.6. (SAS
Institute, Cary, NC), and Microsoft Excel 98 (Microsoft, Redmond, WA). For
each of the individual criteria, the proportion of topnotch ratings (5 points)
was calculated along with the corresponding 95% confidence interval.
Straightforward summation of the ratings yielded a total score for each study.
Global assessment was based on Mann-Whitney tests for the comparison of these
total scores. Because this implied testing two hypotheses, the Bonferroni
adjustment was applied to adjust the alpha level: p values less than
0.025 were regarded as significant on a local level, thus preserving a global
alpha level of 0.05.
Radiation Dose
Effective radiation doses were determined according to Publication 60 of
the International Commission on Radiological Protection
[6] using Monte Carlo
calculations based on axis—dose conversion factors.
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Total score values for multislice helical CT 5-mm contiguous studies (median, 42; range, 39-55) were significantly higher than total scores for the corresponding single-slice helical CT studies (median, 39; range, 28-51; p = 0.0001), primarily because of the higher proportions of 5-point ratings for multislice helical CT studies for all criteria except streak artifacts.
When total score values of multislice (median, 32; range, 25-36) and single-slice (median, 32; range, 22-40) high-resolution CT studies were compared, no significant difference between the two techniques could be detected (p = 0.986). When differences for each image quality criterion were individually compared, the values for the two techniques remained comparable.
The scan parameters for multislice helical CT of the chest used in this study result in an effective equivalent radiation dose of 5.55 mSv in male patients. During conventional single-slice helical CT, the effective equivalent radiation dose to the patient is 4.25 mSv with the scanner settings used in this study. Twenty-four axial high-resolution CT sections with 1-mm collimation at 140 kV and 140 mAs amount to an effective equivalent dose of 1.25 mSv. Therefore, the radiation dose of multislice helical CT of the chest is equal to a total dose of 5.50 mSv needed to perform conventional single-slice helical CT and high-resolution CT.
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At the same time, the information that can be acquired in one breath-hold is multiplied. We used this increased information to address a host of diseases by acquiring a single scan and then adapting the postprocessing algorithms to the underlying disorder. Thus, with multislice helical CT, the focus in planning a CT study is shifting away from data acquisition, and postprocessing algorithms are becoming more important. Because all necessary information can be acquired in a single breath-hold, our approach increases both the ease of performing a CT examination of the chest and the patient's comfort during the procedure. If a comprehensive diagnosis in children, critically ill, or otherwise noncooperative patients is required, such patients no longer need be subjected to lengthy investigations with multiple breath-holds to match the scanning technique to the possible underlying disease. Procedure times are generally decreased; the current generation of multislice helical CT scanners offer reconstruction times of 0.5 sec per axial image. Even when reconstruction of the entire chest with 1-mm sections is desired, for performing multiplanar reconstruction or three-dimensional renderings, the total reconstruction time does not exceed 3-4 min.
The additional benefits of image acquisition with 1-mm collimation are manifold: If focal lesions such as lung nodules are found, targeted 1-mm reconstructions allow the detection of localized calcifications or parenchymal invasion that may have gone unnoticed on thicker slices. Also, not only lesion detection rates, but also conspicuity of small lesions can be increased by use of thin slices [10]. For this reason, both sensitivity and specificity in the diagnosis of pulmonary lesions are likely to be increased by thin-collimation multislice helical CT acquisitions. Finally, in a 1-mm multislice helical CT data set, all voxels are of roughly equal dimensions in the x-, y-, and z-axes. This isotropic data set can be rearranged in any plane with image quality similar to that in axial source images (Figs. 2A,2B,2C,2D and 4A,4B,4C,4D). The visualization and communication of complex anatomy are thus facilitated. Reconstruction of coronal and sagittal multiplanar reconstructions is an easy task that can be performed on most viewing platforms in no more than a few minutes of the technician's time.
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Several aspects of this study, and of multislice helical CT in general, need consideration. First, for obvious ethical reasons, we had to depend on two patient populations for our comparison. Although we matched these two populations closely and kept the technical parameters as comparable as reasonably possible, some bias in the scores cannot be excluded. In this study, the quality of 5-mm multislice helical CT images fused from a 1-mm acquisition was superior to the 5-mm single-slice helical CT acquisition with respect to most image quality criteria. This is likely attributable to the following effects: first, acquiring the helical data with narrow collimation of 4 x 1 mm effectively reduces partial volume effects and corresponding artifacts. The fusion of this data into 5-mm-thick sections restores low image noise. In addition, the slice-sensitivity profile of a 5-mm slice reconstructed from 1-mm data has sharper definition than that of a 5-mm single-slice helical CT acquisition [5, 7]. The better score values for the depiction of most organ systems in multislice helical CT sections in this study likely reflect these effects. With conventional single-slice helical CT, coverage of the entire chest with thin collimation requires increasing the pitch. Increasing pitch, in turn, increases the effective slice thickness, as compared with scans obtained without table motion, leading to loss in spatial resolution [11]. Therefore, high-resolution CT is usually performed in an axial, nonhelical mode [11]. The algorithms used in reconstruction of multislice helical CT data, in contrast to those used for single-slice helical CT, avoid slice broadening with increasing pitch [5, 7]. With multislice helical CT, 1-mm collimation can be used with high table speeds without paying the penalty of increased section thickness. As a result, it is possible to acquire high-resolution CT sections of equal image quality compared with axial single-slice CT.
We took advantage of the increased information that multislice helical CT can acquire in a single breath-hold to comprehensively diagnose focal and diffuse lung disease by performing a single scan on each patient. Although multislice helical CT increases our diagnostic capabilities, the massive amount of data generated by this technology puts significant strain on any image analysis and archiving system. Adapting the radiology environment to the demands generated by the introduction of everfaster scanning techniques is not a trivial task. New modalities for data transfer, data archiving, and image interpretation will have to be devised to make full use of the vast potential of multislice helical CT. Also, although multislice helical CT can be used in ways that result in a reduction of patient radiation dose, it has many applications for which the patient dose is likely to moderately increase [4]. This additional radiation dose has to be weighed against the benefits of long scanning ranges and high z-axis resolution.
However, the ability to scan large volumes with increasing speed and spatial resolution creates entirely new applications in CT, enabling fast and comprehensive diagnosis in our patients.
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