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Eur Radiol (2009) 19: 391–399 DOI 10.1007/s00330-008-1148-x MAGN ETIC RE SONA NCE M. Fabel B. J. Wintersperger O. Dietrich M. Eichinger C. Fink M. Puderbach H.-U. Kauczor S. O. Schoenberg J. Biederer MRI of respiratory dynamics with 2D steady-state free-precession and 2D gradient echo sequences at 1.5 and 3 Tesla: an observer preference study Received: 6 March 2008 Revised: 14 July 2008 Accepted: 21 July 2008 Published online: 6 September 2008 # European Society of Radiology 2008 C. Fink . S. O. Schoenberg Department of Clinical Radiology, University Hospital HeidelbergMannheim, Mannheim, Germany M. Fabel (*) . J. Biederer Department of Diagnostic Radiology, University Hospital SchleswigHolstein, Campus Kiel, Arnold-Heller-Str. 9, 24105 Kiel, Germany e-mail: [email protected] Tel.: +49-431-5973154 Fax: +49-431-5973151 B. J. Wintersperger . O. Dietrich Department of Clinical Radiology, University Hospital Munich, Munich, Germany M. Eichinger . M. Puderbach Department of Oncological Diagnostics and Therapy, German Cancer Research Center, Heidelberg, Germany H.-U. Kauczor Department of Diagnostic Radiology, University Hospital Heidelberg, Heidelberg, Germany Abstract To compare the image quality of dynamic lung MRI with variations of steady-state freeprecession (SSFP) and gradient echo (GRE) cine techniques at 1.5 T and 3 T. Ventilated porcine lungs with simulated lesions inside a chest phantom and four healthy human subjects were assessed with SSFP (TR/TE= 2.9/1.22 ms; 3 ima/s) and GRE sequences (TR/TE=2.34/0.96 ms; 8 ima/s) as baseline at 1.5 and 3 T. Modified SSFPs were performed with Introduction In oncological patient care, magnetic resonance imaging (MRI) has been shown to be particularly sensitive for the detection of cerebral, abdominal and vertebral metastases [1–3]. Thus, comprehensive studies for whole-body MRI have been discussed and developed for screening and staging of metastatic cancer. Whole-body MRI may only be clinically established and fully accepted if the protocols include lung imaging at a reasonable image quality, especially since many malignancies come along with nine to ten images/s (parallel imaging factors 2 and 3). Image quality for representative structures and artifacts was ranked by three observers independently. At 1.5 T, standard SSFP achieved the best image quality with superior spatial resolution and signal, but equal temporal resolution to GRE. SSFP with improved temporal resolution was ranked second best. Further acceleration (PI factor 3) was of no benefit, but increased artifacts. At 3 T, GRE outranged SSFP imaging with high lesion signal intensity, while artifacts on SSFP images increased visibly. At 1.5 T, a modified SSFP with moderate parallel imaging (PI factor 2) was considered the best compromise of temporal and spatial resolution. At 3 T, GRE sequences remain the best choice for dynamic lung MRI. Keywords Lung . Dynamic MRI . Parallel imaging . Nodules . Infiltrates lung metastases [4, 5]. Since MRI of the lung was difficult for a long time, detection of either lung metastases or a primary malignancy of the lung, e.g., bronchial carcinoma, was considered a weak point of this strategy. Present protocols have filled this gap, and MRI of the lung has become a powerful tool for research and specific clinical applications [6, 7]. Within a few years, lung MRI was increasingly recognized as a radiation-free alternative to CT for specific clinical problems, e.g., in cases of evaluation for pneumonia [8–11], in particular for young patients as well as patients 392 with benign lung diseases [12, 13]. As an additional feature, dynamic lung MRI with real-time SSFP or GRE sequences is an easy and quick examination and was therefore readily accepted in clinical routine wherever it became available. Although computed tomography still provides the superior spatial resolution and a reasonable soft tissue contrast for lung imaging, specific clinical problems warrant the use of dynamic MRI (dMRI) for the assessment of respiratory mechanics. In particular for patients presenting with lung cancer, dMRI is a fast and reliable method to estimate tumor motion and chest wall or mediastinal adhesion and invasion [6, 7]. This information can be useful for surgery planning as well as high precision radiotherapy to protect the surrounding healthy lung tissue [14]. CT so far does not allow acquiring dynamic studies of respiratory motion on a routine basis. Low-pitch helical CT or equivalent 4D CT techniques acquire data from the shallow-breathing patient at rest. Retrospective gating corresponding to a selected respiration depth is used to reconstruct 3D data from multiple respiratory cycles. This CT technique does not allow examining respirationcorrelated motion of a tumor during variable conditions, e.g., forced in- and expiration[15]. In this respect, MRI is superior to 4D CT in determining tumor motion. With the implementation of parallel imaging on multi-channel MR systems and development of new reconstruction algorithms, MRI became faster and nowadays allows free-breathing, dynamic examination of the lung. With this background, recommended protocols for lung MRI have been discussed [15, 16]. One approach includes a short sequence of dynamic imaging at free breathing and at defined breathing maneuvers. Clinical usage of dynamic lung MRI at 1.5 T is well reported for detection of lung pathologies[10, 17, 18]. Therefore, dynamic sequences have been included into recommendations for standard MRI protocols of the lung with 1.5-T systems [6]. Initial results for non-dynamic lung MRI at 3 T are also described [19]. So far, the established FLASH 2D sequences provide a high temporal resolution (eight to nine acquisitions per second), but with an unsatisfying signal and parenchymal contrast. Steady-state free-precession (SSFP)-based protocols yield higher signal and contrast, but image quality might be degraded by artifacts, and the temporal resolution is inferior to 2D-FLASH [20]. The aim of this study was to compare the image quality of representative structures and sensitivity of artifacts in modified SSFP sequences with improved temporal resolution at 1.5 T and 3 T and to evaluate if 3 T offers advantages over 1.5 T and which of the modified sequences can be recommended. Since, to the best of our knowledge, the implementation of dynamic MRI at 3 T has not yet been evaluated, we compared the image quality of parallel imaging-accelerated SSFP techniques with a standard dynamic SSFP sequence at 1.5 T and 3 T. Materials and methods Magnetic resonance imaging Examinations were performed on two whole-body MR systems with field strengths of 1.5 T and 3 T (Magnetom Avanto/Trio, Siemens Medical Solutions, Erlangen, Germany). Both systems were equipped with a similar multichannel receiver system and identical gradient coil strength [45 mT/m (72 mT/m effective)]. Imaging was performed using a matrix coil system with 2 rings of 6 elements each (total 12 elements). Five different dynamic sequences were evaluated for lung MRI: An established true fast imaging with steadystate precession (SSFP, Siemens = TrueFISP) sequence with implemented parallel imaging at a factor of 2 (parallel imaging factor 2, in the following referred to with the acronym of the manufacturer as “PAT”) as recommended in the literature [20] served as the baseline and standard of reference (three images/s). Improvement of temporal resolution was achieved by reducing spatial resolution, using higher sequence bandwidth to lower TR (repetition time) and TE (echo time) and changes in the settings of parallel imaging algorithms. This resulted in two different modifications with either a temporal resolution of nine frames per second (GRAPPA PAT 2, TSENSE PAT 3) or a temporal resolution of ten frames per second (GRAPPA PAT 3). Detailed sequence parameters are specified in Table 1. These sequences were compared to a dynamic GRE technique with eight frames per second (fast low-angled shot gradient echo technique; FLASH). These protocols were transferred to the 3-T scanner, keeping imaging parameters as similar as possible. Minor necessary differences demanded by physical constraints (e.g., imaging time) are documented in Table 1. To overcome the limitations of phantom, for the experiments with simulated pulmonary lesions (nodules and infiltrates) in static conditions, four healthy subjects were examined additionally. Phantom experiments Five porcine heart-lung preparations were examined using a dedicated chest phantom as previously described [21]. A 6.5-mm tracheal tube was positioned into the trachea, and the lungs were inflated by evacuation of the artificial pleural space using a negative pressure of -20 to -30 hPa. For simulation of lung nodules, agarose gel (Agarose 5 g/l, BD, Franklin Lakes, NJ) was injected into one lung using a 50-ml syringe. Nodule volumes varied between 0.5 and 2.0 ml. For simulation of infiltrates, 50 ml of diluted 393 Table 1 Sequence parameters for lung MRI at 1.5 T and 3 T. Numbers in parentheses () indicate differing values at 3 T compared to 1.5 T Sequence FLASH 2D origin SSFP SSFP GRAPPA PAT2 SSFP GRAPPA PAT3 SSFP TSENSE PAT3 TR TE TA BW Flip angle FOV FOV phase Base resolution Phase resolution Slice thickness Time resolution (ms) (@3 T) (ms) s (@3 T) Hz/pixel (°) (mm) (%) (mm) (%) (mm) acquis./s 2.34 2.9 (2.9) 0.96 1.22 25 20 980 890 5 65 350 350 87.5 100 128 256 75 66 12 12 8 3 2.4 (2.7) 1.04 14 (16) 1,240 62 350 91.7 192 75 10 9 2.4 (2.7) 1.04 12 (14) 1,240 62 350 91.7 192 75 10 10 2.4 (2.7) 1.04 14 (16) 1,240 55 350 91.7 192 75 10 9 agarose gel was instilled into the bronchial system of the contralateral lung during reinflation. During the MRI, the lungs were ventilated at 8/min by a diaphragmatic pump; for details, see [22]. For the purpose of correlation and documentation of simulated lesions, helical CT of all phantoms was performed using a tube voltage of 120 kV and a tube current of 560 mA (64-slice CT; Somatom Sensation 64, Siemens Medical Solutions, Forchheim, Germany). Images were acquired with a slice collimation of 0.6- and 1-mm axial and coronal reconstructions with an increment of 0.5 mm using soft tissue (B30), and lung kernels (B50) were performed (Fig. 3). Subjects Fig. 1 a-e Dynamic lung MRI of the porcine lung phantom, simulated infiltrates in the left lower lobe at 1.5 T (upper line) and 3 T (lower line) using five different pulse sequences: a FLASH, b SSFP, c SSFP GRAPPA PAT2, d SSFP GRAPPA PAT3, e SSFP TSENSE (for sequence details, see Table 1) Since it appeared unacceptable for patients with severe lung disease to undergo the full protocol at both scanners, we decided to evaluate the sequences in four healthy nonsmoking subjects (median age: 32.5 years; age range: 30– 36 years) according to the guidelines of the local ethics committee. Informed written consent was obtained prior to the examinations. Scan direction was head first supine, arms positioned at the side of the body. No ECG-triggering or respiratory gating was used. All measurements were acquired in free breathing and during repeated deep in- and expiration as instructed by the operator of the MR system. 394 Image analysis Table 3 Results of ranking by three independently observers (median/mode) for 3-T lung MRI, phantom Phantom studies as well as examinations of subjects were evaluated independently by three radiologists with different experience in reading chest MRI (3, 5 and 9 years MRI experience) using a double monitor PACS workstation (Chili Workstation, version 2.3, Heidelberg, Germany). Data sets were prepared and loaded before by a fourth, nonreading radiologist. The images were evaluated using parallel cine displays on the monitors and comparable window settings. The applied workstation allowed for up to eight parallel cine displays. Image evaluation was started when the viewer indicated real-time conditions for the cine display. The cine display could be halted, if necessary. The observers were blinded to sequence type, field strength, study time, subject name and phantom number. The series were displayed simultaneously, but with variable order on the double screen of the workstation. Criteria for evaluation were determined as followed: Phantom 3 T 1. Image quality andside-by-side comparison: Image ranking and comparison were done for all sequence variations acquired at both field strengths. Image quality was ranked in respect to (a) type and extent of artifacts (e.g., parallel acquisition, field heterogeneity and motion artifacts), (b) the delineation of anatomical structures (mediastinal structures, diaphragm and chest wall), (c) detection of lesions (lung nodules and infiltrates) within the chest phantom, and (d) overall impression of dynamic appearance using a five-point scale (best image quality = 1, worst = 5). For sensitivity of artifacts a three-point scale was used (0 = absence of artifacts; 1 = artifacts, diagnostic; 2 = severe artifacts, non-diagnostic). The readers were asked if the image quality was equal or if they preferred one to the other. 2. Signal intensity measurements of nodules and infiltrates: Measurements were performed for all sequences by an independent observer. Since parallel imaging was used for all sequences, the non-uniform distribution of noise Table 2 Results of ranking by three independently observers (median/mode) for 1.5-T lung MRI, phantom SSFP GRAPPA PAT3 2/2 4/4 5/5 4/4 2/2 2/2 SSFP TSENSE 1/1 3/3 3/2 3/3 2/2 2/2 obviated valid SNR measurements. Instead, the lesion contrast (CNODULES and CINFILTRATES) defined by the ratio of the signal intensity of the lesion and adjacent normal lung tissue was determined. For this purpose, circular regions of interest with an average number of 50 pixels were positioned in the lesions as well as the adjacent normal lung tissue[23]. Statistical evaluation For statistical evaluation, the median and mode of the rankings were calculated. As the ranking of the sequences resulted in ordinal data (1 = best quality, 5 = worst quality), the median and mode of the ranking were calculated; mode function represents the most frequently occurring value in a list of numbers. For the comparison of the sequences at different field strengths, we calculated the percentage for three categories: 1. Equal image quality 2. 1.5 Tesla preferred 3. 3 Tesla preferred. CT scans in the phantom studies were not further evaluated in detail, but served as a reference for the identification of nodules and infiltrates as a ground truth (Fig. 3). Table 4 Results of ranking by three independently observers (median/mode) for 1.5-T lung MRI, subjects Phantom 1.5 T FLASH SSFP SSFP 2D GRAPPA PAT2 Artifacts 0/0 1/1 1/1 Diaphragm 2/2 1/1 3/3 Mediastinum 2/2 1/1 3/3 Chest wall 2/2 1/1 3/3 Infiltrates 2/2 1/0 2/2 Nodules 1/1 1/2 2/2 FLASH SSFP SSFP 2D GRAPPA PAT2 Artifacts 0/0 1/1 2/2 Diaphragm 1/1 2/2 5/5 Mediastinum 2/1 1/1 4/4 Chest wall 1/1 2/2 4/4 Infiltrates 1/2 2/2 2/2 Nodules 1/1 2/2 2/2 SSFP GRAPPA PAT3 1/1 4/4 5/5 3/4 2/2 2/2 SSFP TSENSE 1/1 5/5 4/4 5/5 2/2 2/2 Subject 1.5 T FLASH SSFP SSFP 2D GRAPPA PAT2 Artifacts 0/0 0/0 1/1 Diaphragm 2/1 1,5/1 3/3 Mediastinum 3/2 1/1 3/3 Chest wall 2,5/2 1/1 3/3 SSFP GRAPPA PAT3 2/2 5/5 5/5 5/5 SSFP TSENSE 1/1 3/3 4/4 4/4 395 Table 5 Results of ranking by three independently observers (median/mode) for 3-T lung MRI, subjects Subject 3 T FLASH SSFP SSFP 2D GRAPPA PAT2 Artifacts 0,5/1 1/1 2/2 Diaphragm 2/1 1/1 3/3 Mediastinum 1/1 2/2 3,5/4 Chest wall 1/1 2/2 4/4 SSFP GRAPPA PAT3 2/2 4/4 4/4 3/3 SSFP TSENSE 2/2 5/5 5/5 5/5 Results The comparison revealed different preferences for sequences in respect to different criteria at both field strengths and conditions (i.e., chest phantom vs. healthy subjects; Tables 2, 3, 4 and 5). Phantom experiments 1. Image quality and side-by-side comparison: a. Type and extent of image artifacts As expected, GRE showed the fewest artifacts at both field strengths followed by the baseline SSFP and the TSENSE SSFP (Tables 2, 3, 4 and 5). Fig. 2 a-e Dynamic lung MRI using porcine lung phantom, simulated lung nodules in the right upper lower lobe at 1.5 T (upper line) and 3 T (lower line) using five different pulse sequences: Modified SSFP sequences with GRAPPA PAT 2 and PAT 3 showed the most artifacts at both field strengths at all. However, artifacts were most prominent at 3 T, in up to 56% resulting in nondiagnostic image quality. All readers found more artifacts in images of the chest phantom compared to subjects. b. Delineation of anatomical structures For the delineation of mediastinal anatomy, the baseline SSFP sequence was preferred at both field strengths [median/mode (1/1), Tables 2 and 3]. In the phantom study the baseline SSFP sequence was rated best for delineation of the diaphragm at 1.5 T (1/1) (Table 2); GRE was preferred at 3 T (1/1) (Table 3). For the separation of the chest wall the baseline SSFP sequence achieved the best ranking at 1.5 T (1/1) (Table 2), (3 T: 2/2), (Table 3). c. Detection of lesions In detection of the simulated lesions GRE was rated best for the detection of nodules (1/1) at both field strengths (Tables 2 and 3). The baseline SSFP sequence was rated best in visualization of infiltrates at 1.5 T (1/1) (Table 2), whereas GRE was preferred at 3 T (1/2) (Table 3). The simulated lung abnormalities are demonstrated in Figs. 1 and 2, their correlation to the CT scan in Fig. 3. 2. Signal intensity measurements of nodules and infiltrates: a FLASH, b SSFP, c SSFP GRAPPA PAT2, d SSFP GRAPPA PAT3, e SSFP TSENSE) (for sequence details, see Table 1). 396 Fig. 3 a-b Correlation of lesions in CT and MR scans: a infiltrates (baseline SSFP, 3 T), b nodules (GRE, 3 T) Comparison of signal intensities at 1.5 and 3.0 T At 1.5 T, the lesion-to-background contrast in nodules ranged from 2.4 (2.04; 2.18) on the SSFP images with R=3 to 4.7 (3.16; 6.23) on the baseline SSFP sequence. The GRE sequence showed an intermediate lesion-to-background contrast of nodules 3.7 (2.52; 5.02) (Table 6). For the infiltrates, the lesion-to-background contrast was lowest on the SSFP images with TSENSE 2.1 (1.39; 2.83) and highest on the baseline SSFP sequence 4.3 (3.24; 5.29) with intermediate results for the GRE sequence, 2.21 (1.88; 2.54) (Table 6). At 3 T, the signal intensity measurements showed an increase in the lesion-to-background contrast of simulated lung nodules compared to 1.5 T for all sequences. This effect was most prominent on GRE images with an increase of 31% from 1.5 T to 3 T and least prominent on the modified SSFP sequence with a parallel factor of 3. The contrast of infiltrates on GRE increased by 21% from 1.5 T to 3 T (Table 6). In the side-by-side comparison, the GRE sequence was rated superior for 3 T in nearly 70%, whereas all other sequences were ranked superior at 1.5 T between 61 and 94% (Table 7). Healthy subjects 1. Image quality and side-by-side comparison: a. Type and extent of image artifacts GRE images showed the fewest artifacts at both field strengths (1.5 T: 0/0, 3 T: 0.5/1) (Tables 4 and 5) followed by the baseline SSFP and the TSENSE. Modified SSFP sequences with GRAPPA PAT2 and PAT3 showed the most artifacts at both field strengths at all (Tables 4 and 5) (Fig. 4). Fig. 4 a-e Dynamic lung MRI of a healthy subject at 1.5 T (upper line) and 3 T (lower line) using five different pulse sequences: a FLASH, b SSFP, c SSFP GRAPPA PAT2, d SSFP GRAPPA PAT3, e SSFP TSENSE (for sequence details, see Table 1) 397 Table 6 Lesion-to-background contrast (nodules and infiltrates at 1.5 T and 3 T, mean, standard deviation, 95% confidence interval) Contrast nodule Sequence FLASH 2D Origin SSFP SSFP GRAPPA PAT2 SSFP GRAPPA PAT3 SSFP TSENSE Contrast infiltrates Sequence FLASH 2D Origin SSFP SSFP GRAPPA PAT2 SSFP GRAPPA PAT3 SSFP TSENSE 1.5 T 3T Mean SD 95% CI Mean SD 95% CI SNR increase(%) 3.77 4.70 2.96 2.44 2.56 1.5 T Mean 1.96 2.41 0.12 0.63 0.60 [2.52;5.02] [3.16;6.23] [2.88;3.03] [2.04;2.18] [2.84;2.94] 0.92 1.40 1.74 0.29 0.65 [4.90;6.10] [4.49;6.26] [2.99;5.20] [2.35;2.71] [2.41;3.23] 31 13 28 4 9 Std 95%CI 5.48 5.38 4.10 2.53 2.82 3T Mean SD 95% CI SNR increase(%) 2.21 4.27 2.64 2.26 2.11 0.51 1.62 1.42 0.35 1.13 [1.88:2.54] [3.24;5.29] [1.74;3.54] [2.03;2.48] [1.39;2.83] 2.79 4.52 2.37 2.79 1.98 1.06 3.15 1.32 1.52 0.28 [2.06;3.40] [2.52;6.51] [1.52;3.21] [1.82;3.76] [1.80;2.15] 21 6 -11 19 -6 Artifacts were most prominent at 3 T, in up to 60% resulting in non-diagnostic image quality (Fig. 4, lower line, especially 4d and 4e). Compared to the baseline SSFP and GRE, SSFP with a parallel imaging factor 2 was the preferred modification to the baseline SSFP sequence at both field strengths. GRAPPA PAT 3 and TSENSE PAT3 did not show any substantial benefit. b. Delineation of anatomical structures For the delineation of mediastinal anatomy in subject studies, the baseline SSFP was ranked best at 1.5 T (1/1) (Table 4), whereas GRE was rated best at 3 T (1/1) (Table 5). The baseline SSFP sequence achieved the best ranking for delineation of the diaphragm at both field strengths (1.5 T: 1.5/1; 3 T: 1/1) (Tables 4 and 5). For the separation of the chest wall, the baseline SSFP sequence achieved the best ranking at 1.5 T in the healthy subjects (1/1) (3 T: 2/2) (Tables 4 and 5). Discussion The comparison of the image quality and sensitivity of artifacts in modified SSFP sequences with improved temporal resolution in this study showed feasibilities of dynamic lung MRI at 3 T but implies further challenges. As known, (non-dynamic) lung MRI is already difficult at 1.5 T due to the low proton density and the high magnetic susceptibility of lung tissue. Multiple air-tissue interfaces result in large local magnetic gradients that cause intravoxel phase dispersion [24, 25]. This makes lung MRI even more challenging at 3 T. Nevertheless, recent studies have shown the feasibility of MRI at 3 T to detect diffuse lung disease [19, 26] and the increasing availability of 3-T MR systems in clinical routine (in some cases as alternative to 1.5 T) necessitates optimization of protocols for (dynamic) lung MRI (Table 8). In a previous study, five different pulse sequences were implemented at 1.5- and 3-T systems, and image quality was compared [27]. To comply with this development, the dynamic sequences evaluated in the present study as they were optimized for a common 1.5-T system were also transferred to a 3-T system. In different studies regarding cardiac cine MRI signal as well as the increase of artifacts at SSFP sequences are described [28, 29]. To the best of our knowledge, dynamic lung MRI sequences have not been evaluated at 3 T so far. Table 7 Comparison of ranking, subjects and percentages Table 8 Comparison of ranking, phantom studies and percentages Sequence Equal 1.5 T superior 3 T superior Sequence Equal 1.5 T superior 3 T superior FLASH 2D SSFP SSFP GRAPPA PAT2 SSFP GRAPPA PAT3 SSFP TSENSE 30.5% 27.7% 8.3% 19.4 5.5% 0% 61.1% 75.0% 63.8% 94.4% 69.4% 11.1% 16.6% 16.6% 0% FLASH 2D SSFP SSFP GRAPPA PAT2 SSFP GRAPPA PAT3 SSFP TSENSE 33.3% 20.0% 20.0% 17.7% 42.2% 6.6% 66.6% 66.6% 75.5% 37.7% 60.0% 13.3% 13.3% 6.6% 20.0% 398 As a baseline for the study, we used two dynamic lung MRI sequences proposed for 1.5-T systems (FLASH, SSFP). In addition to this, three modifications of the SSFP sequence using parallel acquisition techniques with PI factors of 2 and 3 at 1.5 and 3 T (GRAPPA and TSENSE) were included. The comparison of the two field strengths was facilitated by the fact that both scanners were equipped with similar gradient systems. The imaging parameters were kept as similar as possible. However, SAR limits and lengthening of T1 forced adapting the imaging parameters slightly (Table 1). In particular, it was difficult to maintain the same temporal resolution after transferring the sequence protocols. This implies that, already from the sequence setup, imaging at higher field strength appeared not to be beneficial. As expected, the baseline SSFP achieved the best image quality at 1.5 T. SSFP with GRAPPA PAT 2 achieved a significantly improved temporal resolution of nine acquisitions per second and was still ranked second best by overall image quality. Modifications with PAT3 (GRAPPA, TSENSE) were more prone to artifacts and were of no additional benefit. At 3 T, contrast and signal of GRE improved, while SSFP image quality suffered from artifacts. Furthermore, both modified SSFP sequences with a parallel factor of 3 showed severe artifacts, partially resulting in non-diagnostic images. By comparison, GRE showed the fewest artifacts at 3 T. Except for GRE, image quality at 1.5 T was rated superior to 3 T for all sequences. The evaluation of the phantom study revealed some general differences between phantom and subject data. In particular, more artifacts were seen in the phantom at 1.5 T. This may be explained by the structure of the phantom chest wall and its flanges, which include interfaces of different materials. Another factor was the in- and outflow of water into the diaphragmatic bubble inducing heavy flow artifacts. At 3 T the presence of artifacts on phantom studies was comparable with that found in subjects. Nevertheless, even with this given bias, the phantom experiments revealed useful information on how lung lesions were displayed. At 3 T, GRE was ranked best for conspicuity of simulated lung nodules in the phantom experiments. For delineation of the anatomical structures GRE as well as baseline SSFP achieved the best ratings at both field strengths, perhaps because of the increase of the signal-to-background ratio as well as the slightly higher spatial resolution of baseline SSFP. The SI measurements were consistent with the imaging parameters. In addition, the modified SSFP sequences offered a smoother fluid view of the respiration process, obviously due to the increased images per second as estimated by all readers. A first finding in the healthy subjects was an apparently higher signal intensity increase after transferring the protocol from 1.5 to 3 T. Other sequences showed this effect to a lesser extent. Due to the small number of subjects with variable parameters such as respiratory and cardiac motion, SI measurements were performed in the phantom study. Compared to imaging at 1.5 T, lesion-to-lung-tissue signal ratios at 3 T indicated a better lesion detection ability. This effect was more prominent for GRE than for the other sequences. The GRE sequence showed a significant increase of lesion signal intensity at 3 T (31%) with the fewest image artifacts. In particular SSFP with parallel imaging factors of 3 (GRAPPA PAT 3 and TSENSE PAT3) showed an even lower signal of the lesions at 3 T compared to 1.5 T. However, the difference between 1.5 and 3 T was only significant for the GRE sequence (p<0.001). In conclusion, the modified SSFP offered an equal temporal resolution with superior spatial resolution and signal compared to GRE at 1.5 T. However, image quality was lower compared to the original SSFP sequence. GRAPPA 3 and TSENSE PAT3 were of no additional benefit. 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