Cardiac spiral dual-source CT with high pitch: a feasibility study

Eur Radiol (2009) 19: 2357 2362 DOI 10.1007/s00330-009-1503-6 PHYSICS Dirk Ertel Michael M. Lell Frank Harig Thomas Flohr Bernhard Schmidt Willi A. Kalender Cardiac spiral dual-source CT with high pitch: a feasibility study Received: 17 February 2009 Revised: 27 April 2009 Accepted: 7 May 2009 Published online: 30 June 2009 # European Society of Radiology 2009 The authors would like to dedicate this paper to their esteemed colleague Werner A. Bautz, M.D., chair of the Department of Radiology of the University Hospitals of the University of Erlangen, who died unexpectedly on 18 December 2008. D. Ertel. W. A. Kalender (*) Institute of Medical Physics (IMP), University of Erlangen-Nürnberg, Henkestraße 91, 91052 Erlangen, Germany e-mail: willi.kalender@imp.unierlangen.de Tel.: +49-9131-8522310 Fax: +49-9131-8522824 M. M. Lell Department of Radiology, University of Erlangen-Nürnberg, Erlangen, Germany F. Harig Center of Cardiac Surgery, University of Erlangen-Nürnberg, Erlangen, Germany T. Flohr. B. Schmidt Siemens Healthcare, Forchheim, Germany Abstract Increase of pitch in spiral CT decreases data acquisition time; dual-source CT (DSCT) systems provide improved temporal resolution. We evaluated the combination of these two features. Measurements were performed using a commercial DSCT system equipped with prototype software allowing pitch factors from p= 0.35 to 3.0. We measured slice sensitivity profiles as a function of pitch to assess spatial resolution in the z- direction and the contrast of structures moved periodically to measure temporal resolution. Additionally we derived modulation transfer functions to provide objective parameters; both spatial and temporal resolution were essentially unchanged even at high pitch. CT of the cardiac region of three pigs was performed at p=3.0. In vivo CT images confirmed good image quality; direct comparison with standard low-pitch phase-correlated CT image datasets showed no significant difference. For a normalized z-axis acquisition of 12 cm, the corresponding effective dose value was 2.0 msv for the high-pitch CT protocol. We conclude that spiral DSCT imaging with a pitch of 3.0 can provide unimpaired image quality with respect to spatial and temporal resolution. Applications to cardiac and thoracic imaging with effective dose below 1 msv are possible. Keywords Dual-source CT. Cardiac imaging. Image quality. Dose. High pitch Introduction Cardiac spiral CT has been well established for nearly a decade now [1 3]. It provides phase-correlated imaging and represents the state of the art for cardiac CT angiography (CTA) today. Although high image quality and diagnostic performance are generally acknowledged, patient dose increases due to the need for low pitch values of typically between 0.2 and 0.4 [4, 5] and this is a point of concern. Increased pitch values (p) offer the advantage of decreasing both patient dose and the total data acquisition time by 1/p. It has to be investigated, however, whether disadvantages with respect to image quality may result. In particular, spatial and temporal resolution have to be assessed. Cardiac CT imaging is generally based on data covering a fan angle of 180 plus. The effective data acquisition time for acquiring the necessary data to reconstruct an image of a single slice is therefore directly proportional to the rotation time of the gantry; for single-source CT it is accordingly quoted as 1/2 the rotation time, for dual-source CT (DSCT) as 1/4 the rotation time. For our DSCT system with 330-ms rotation time, an effective CT data acquisition time of 82.5 ms results [5 7]. Total acquisition time for

2358 coverage of the complete heart amounts typically to 6 10 s due to the low pitch. In consequence, data acquisition covers multiple cardiac cycles. More importantly, these acquisition schemes suffer from an increased patient dose. Adequate procedures such as modulating the x-ray current with respect to the cardiac phase (i.e. the so-called ECG pulsing) [8 10] and the adaptation of the pitch factor to the heart rate [5, 6] can reduce the applied dose. By applying these approaches to DSCT coronary angiography using a continuous spiral data acquisition, effective dose values were reduced down to typically 7 10 msv [11 14]. Moreover, an application of adaptive collimation to spiral coronary CTA can reduce effective dose further by approximately 10% [15]. Sequential cardiac data acquisition (prospective triggering) offers another alternative for reducing effective dose values down to typically 2 5 msv [16 18]; however, this entails the risk of misregistration between successive acquisitions due to breathing or other motion. Using spiral DSCT, complete sampling in the z-direction can be ensured for pitch factors up to 3.0 or slightly higher. The effective CT data acquisition time per single image is expected to be independent of the pitch value, and the same temporal and spatial resolution as in standard imaging are to be expected just the same. We here present a feasibility study for cardiac CT with increased pitch factors to test these hypotheses. Materials and methods Data acquisition We used a DSCT system (SOMATOM Definition, Siemens Healthcare, Forchheim, Germany) with 0.33-s rotation time and a collimation of 19.2 mm. Data were acquired with a thorax protocol at 120 kv using both tube detector systems. A dedicated software release allowed for data acquisition with pitch factors of up to 3. Standard image reconstruction was performed using a medium smooth reconstruction kernel (B30f) and an effective slice width of S eff =0.6 mm. For the in vivo study, data were acquired first using a standard low-pitch cardiac protocol. For this, retrospective phase-correlated image reconstruction was performed using the same kernel and effective slice width and a reconstruction phase identified by the CT system providing the least amount of motion artefacts. Phantom study For reliability assessment the slice sensitivity profile (SSP) was determined for different pitch factors. The phantom used consisted of water-equivalent material (cylinder with a diameter of 28 mm and a length of 111 mm) comprising a gold disk with thickness of 50 µm and diameter of 2 mm. Multiple SSPs were assessed and averaged for analysis to reduce noise. The spatial resolution in the z-direction was then characterized by calculating the modulation transfer function (MTF) and determining the frequency of the 10% and 50% MTF value. We validated the performance of the imaging system for pitch factors of 0.35, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0. For each pitch factor five consecutive measurements were performed. A 3D cardiac motion robot (QRM, Moehrendorf, Germany) was used to measure temporal resolution inplane for different pitch factors (Fig. 1). A highly attenuating rod was moved on a sinusoidal trajectory with 10-mm amplitude and a frequency of 40 beats per minute (bpm). Measurements were performed for pitch factors of 1.0, 1.5, 2.0, 2.5 and 3.0. The contrast of the moving rod against water was assessed and normalized to the contrast provided by a standard reconstruction of a stationary rod. This normalized contrast value is proportional to the temporal MTF value of the corresponding motion frequency [7]. If the same contrast value is achieved using different pitch factors, an equivalent temporal resolution can be assumed. Fig. 1 Cardiac motion robot (a) and attachments for the assessment of temporal resolution (b). The arrow indicates the highly attenuating rod used for this study

2359 Fig. 2 Assessment of the spatial resolution in the z-direction. Normalized SSPs for pitch factors of 1.0 (circles) and 3.0 (solid line) showing a monotonic decrease and symmetrical behaviour for both (a). The corresponding MTFs for pitches of 1.0 (circles) and 3.0 (solid line) indicate equivalent z-resolution for both pitch factors (b). Similar results were obtained using a bar pattern phantom; resolution in the z-direction of 12.5 lp/cm is shown for a pitch of 1.0 (c) and 3.0 (d) In vivo study Datasets of three pigs (German landrace pigs, Sus scrofa) were acquired to assess the effects of varying the pitch factor. The animals were sedated by repeated application of ketamine and midazolam via an infusion line into the ear vein. Animals breathed freely without any additional ventilation system. Data were acquired in spiral CT mode with a heart-rate-adapted low pitch factor as in standard cardiac CT with phase-correlated image reconstruction, and with a pitch factor of 3.0, which does not allow for retrospective phase selection. Both acquisition modes were performed with contrast media application (350 mg/ml iodine, Imeron 350, Bracco ALTANA Pharma GmbH, Konstanz, Germany) at an injection rate of 5 ml/s for a total volume of 50 ml followed by a saline flush of the same total volume and injection rate. The pigs were scheduled for cardiac CT examinations within a study regarding cardiac surgery which was approved by the local ethics committee. The dose index CTDI vol provided by the CT machine was used for dose assessment. The dose length product (DLP) was calculated for a normalized z-axis acquisition length of 12 cm. Using standard values for conversion, we estimated the effective dose value by using the corresponding conversion coefficient k=0.017 msv mgy 1 cm 1 for the chest [19]. Qualitative validation of motion artefacts was performed by visual assessment by an experienced radiologist. Coronary segments were rated with respect to a modified ACC/AHA classification [20] on a three-point score scale (3 = excellent, no artefacts; 2 = diagnostic, minor artefacts; 1 = nondiagnostic, major artefacts) for both datasets. Table 1 The 10% and 50% values (mean value ± standard deviation) of the modulation transfer function in the z-direction for different pitch factors Pitch 0.35 0.50 1.00 1.50 2.00 2.50 3.00 50% value 5.93±0.05 5.79±0.06 5.64±0.05 5.71±0.07 5.77±0.16 5.92±0.11 5.84±0.13 10% value 12.89±0.23 11.88±0.12 11.41±0.13 11.67±0.15 11.96±0.45 12.54±0.59 11.98±0.14

2360 Results Phantom study Figure 2a shows the normalized SSPs assessed at the isocentre for a pitch factor of 1.0 and 3.0. The profiles are monotonically decreasing and show a symmetrical behaviour. Resolution in the z-direction was measured by the MTF derived from the respective SSP; it remained essentially constant even for increased pitch values (Fig. 2b). This constant behaviour is also documented by the numerical 10% and 50% values of the MTF for different pitch factors in Table 1; the small variations of the values are mainly due to image noise. Additionally, Figs. 2c and d show a resolution phantom (bar pattern phantom) measured with a pitch factor of 1.0 and 3.0, respectively. Visual assessment of spatial resolution is in excellent agreement with the analysis of the MTF values; for both pitch factors a z-axis resolution of 12.5 lp/cm, equivalent to about 0.4 mm, was reached. In the tests of temporal resolution, the moving rod showed a constant contrast behaviour for different pitch factors. For p=1 the contrast value amounted to 95% (1,247 HU/1,310 HU) and for p=3 to 98% (1,289 HU /1,310 HU) of the initial contrast value of the stationary rod. The difference is not considered significant regarding a mean contrast value for all pitch factors of 98%±2%. Assessment for p=1.5, 2.0 and 2.5 showed a contrast value of 98% (1,282 HU/1,310 HU), 99% (1,301 HU/1,310 HU) and 100% (1,309 HU/1,310 HU), respectively. Fig. 3 Results of cardiac scans of a pig with a heart rate of 68 bpm at a heart-rate-adapted pitch of about 0.3 (a,c) and a pitch of 3.0 (b,d). Images in transaxial view (a,b) and curved MPRs (b,d) show identical quality (C 150/W 700)

2361 In vivo study During data acquisition the three pigs had a mean heart rate of 68 bpm, 101 bpm and 120 bpm for both examinations. The tube current used corresponded to an effective current time product of 140 mas and a CTDI vol of 9.92 mgy. For a normalized length of 12 cm a mean effective dose of 2.0 msv resulted. The images of the in vivo pig CT examinations showed good image quality at a pitch factor of 3.0. The mean image quality over all segments and pigs was 2.33 using the standard low-pitch phase-correlated image reconstruction and 2.13 for the data acquisition with a pitch factor of 3.0. For the high-pitch acquisition increased heart rate resulted in slightly reduced image quality of the distal branches compared with standard low-pitch acquisition; in three (pig with 101 bpm) and four (pig with 120 bpm) out of all 13 segments image quality rating was reduced by a single score point. At a medium heart rate (pig with 68 bpm) image quality showed an average rating of 3.00 and 2.85 points for standard low-pitch and high-pitch acquisitions, respectively. For both modes all branches were scored 3 points (excellent, no artefacts) except for two distal branches with a score of 2 (diagnostic, minor artifacts) for the high-pitch acquisition. Figure 3 shows transaxial and curved MPR (multiplanar reformation) images of the pig with medium heart rate for standard phase-correlated and for high-pitch acquisition, respectively. All images provide good image quality with respect to motion artefacts and allow the assessment of the RCA (right coronary artery) and the LCX (left circumflex). The curved MPRs indicate a well-resolved LCX for both acquisition modes. Even distal segments do not suffer from motion blurring. The images show no step artefacts or motion blurring in the z-direction. Discussion We performed a feasibility study of an innovative DSCT approach with a pitch factor of 3 applied to cardiac and thoracic imaging. Performance was assessed by phantom and in vivo studies. Spatial resolution in the z-direction was unaffected by pitch factors up to a value of 3, which is considered a very favourable result. Temporal resolution was found to be independent of pitch as was expected. Hence, high-pitch cardiac CT data acquisition can provide spatial and temporal resolution equivalent to the standard low-pitch phase-correlated imaging. The in vivo study of three pigs by CT using a pitch of 3 showed very good image quality with respect to motion artefacts and blurring. Effective dose was estimated as 2.0 msv. Direct comparison with low-pitch phase-correlated CT image datasets showed only very small, possibly nonsignificant detriments in image quality. A reason for this observation might be that data acquisition was not triggered by an external signal with respect to the cardiac phase, since this was not available for our prototype setup. Hence, data acquisition was not correlated to the heart cycle, i.e. RR interval, and prospective triggering with respect to the cardiac phase, i.e. reconstruction phase, was not possible. Acquisition within the in vivo study may have covered phase intervals with high cardiac motion and not the preferred diastolic phase. We expect that a triggering procedure with respect to the cardiac phase will improve image quality. Respective means will be available for patient studies. Conventional coronary angiography still is the state of the art in clinical routine with a mean effective dose of typically 5 6 msv [10, 21]. With the high-pitch data acquisition scheme presented here, coronary CT angiography can be performed on DSCT systems with dose values significantly lower than those of the conventional procedure. At the same time, data acquisition time is greatly reduced. With a table feed of 17.5 cm/s in our study the complete cardiac region of 12 cm was examined in less than 0.7 s. With shorter rotation time and collimation width increased to 40 mm, which have been announced by the manufacturer recently [22], data acquisition times could be reduced to below 0.3 s. Using optimized parameter settings, in particular lower mas and kv settings [23], effective patient dose values of 1 msv and below appear realistic. Conclusion DSCT imaging with pitch values of 3 and slightly higher provide unimpaired image quality with respect to spatial and temporal resolution. Total data acquisition time and patient dose in cardiac imaging can thereby be reduced significantly. References 1. 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