DOI: 10.1148/radiol.2481070034
(Radiology 2008;248:264-271.)
© RSNA, 2008
Liver Imaging at 3.0 T: Diffusion-induced Black-Blood Echo-planar Imaging with Large Anatomic Volumetric Coverage as an Alternative for Specific Absorption Rate–intensive Echo-Train Spin-Echo Sequences: Feasibility Study1
Indra C. van den Bos, MD, PhD,
Shahid M. Hussain, MD, PhD 2,
Gabriel P. Krestin, MD, PhD, and
Piotr A. Wielopolski, PhD
1 From the Department of Radiology, Erasmus MC, University Medical Center–Rotterdam, Dr Molewaterplein 40, 3015 GD Rotterdam, the Netherlands. Received January 7, 2007; revision requested February 28; revision received May 1; accepted January 28, 2008; final version accepted January 31.
Address correspondence to I.C.v.d.B. (e-mail: vandenbos.ic{at}gmail.com).
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ABSTRACT
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Institutional Review Board approval and signed informed consent were obtained by all participants for an ongoing sequence optimization project at 3.0 T. The purpose of this study was to evaluate breath-hold diffusion-induced black-blood echo-planar imaging (BBEPI) as a potential alternative for specific absorption rate (SAR)-intensive spin-echo sequences, in particular, the fast spin-echo (FSE) sequences, at 3.0 T. Fourteen healthy volunteers (seven men, seven women; mean age ± standard deviation, 32.7 years ± 6.8) were imaged for this purpose. Liver coverage (20 cm, z-axis) was always performed in one 25-second breath hold. Imaging parameters were varied interactively with regard to echo time, diffusion b value, and voxel size. Images were evaluated and compared with fat-suppressed T2-weighted FSE images for image quality, liver delineation, geometric distortions, fat suppression, suppression of the blood signal, contrast-to-noise ratio (CNR), and signal-to-noise ratio (SNR). An optimized short- (25 msec) and long-echo (80 msec) BBEPI provided full anatomic, single breath-hold liver coverage (100 and 50 sections, respectively), with resulting voxel sizes of 3.3 x 2.7 x 2.0 mm and 3.3 x 2.7 x 4.0 mm, respectively. Repetition time was 6300 msec, matrix size was 160 x 192, and an acceleration factor of 2.00 was used. b Values of more than 20 sec/mm2 showed better suppression of the blood signal but b values of 10 sec/mm2 provided improved volume coverage and signal consistency. Compared with fat-suppressed T2-weighted FSE, the optimized BBEPI sequence provided (a) comparable image quality and liver delineation, (b) acceptable geometric distortions, (c) improved suppression of fat and blood signals, and (d) high CNR and SNR. BBEPI is feasible for fast, low-SAR, thin-section morphologic imaging of the entire liver in a single breath hold at 3.0 T.
© RSNA, 2008
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INTRODUCTION
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Magnetic resonance (MR) imaging is considered to be an imaging modality with high accuracy for diagnostic work-up of patients with suspected or proved focal liver lesions (1). Particularly, multishot echo-train fast spin-echo (FSE) sequences are important for lesion detection (1,2).
The transition of imaging protocols from 1.5 to 3.0 T is being evaluated by several authors (3–7). The introduction of 3.0-T MR imaging systems and multichannel reception coils capable of parallel imaging with high acceleration factors have shown promise for improved hepatobiliary imaging (4,8). The combination of a higher signal-to-noise ratio (SNR) than that of 1.5-T imaging (theoretically two times higher) (9), high-performance–gradient systems, and advanced software platforms allows, improved spatial resolution, which can be used to obtain one-half section thickness with identical coverage, or a fourfold speed increase in scanning time for identical resolution settings (3,8). Nonetheless, the fourfold increase in the specific absorption rate (SAR), as compared with 1.5 T, T1 lengthening, and slight shortening of T2 relaxation times necessitate a revision of the imaging parameters at 3.0 T. In addition, the higher SNR for 3.0-T imaging is usually hampered by a combination of factors, including B1 inhomogeneities and higher readout bandwidths (10,11), which limit the introduction of routine abdominal imaging at 3.0 T.
T1-weighted gradient-echo sequences at 3.0 T have taken full advantage of increased SNR, translating to thin-section three-dimensional imaging with higher in-plane resolution and improved imaging quality (8,12). However, T2-weighted FSE sequences at 3.0 T have been more challenging for two reasons. First, a fourfold increase in energy radiofrequency (RF) pulses is needed for proton excitation, resulting in increased RF heating and SAR levels (13). Higher SAR values limit the efficiency of sequences on the basis of FSE and steady-state free precession readouts, resulting in a lower number of sections per repetition time (TR), thus diminishing anatomic coverage. Second, B1 inhomogeneities and dielectric resonances are more pronounced at higher field strength, resulting in intensity modulations and focal shading from the resulting inhomogeneous RF excitation field.
Echo-planar imaging (EPI) has been evaluated as a potential fast imaging method for the evaluation of abdominal diseases (14,15). EPI can be applied for both morphologic imaging and more functional-oriented imaging, such as in the effective mapping of the apparent-diffusion coefficient (14–16). EPI has been advocated as a promising imaging sequence at 1.5 T (14,17) because of its acquisition speed and the higher contrast-to-noise ratio (CNR) for lesion detection. Despite several advantageous features, its clinical application in the abdominal region has been limited so far (15,18).
To our knowledge, no previous studies have reported on the implementation of a diffusion-induced, black-blood EPI (BBEPI) at 3.0 T with thin sections for large volumetric coverage with near-isotropic voxels of the entire liver in a single breath hold. Thus, the purpose of our study was to evaluate the feasibility of breath-hold diffusion-induced BBEPI with short and long echo time (TE) as a potential alternative for specific-absorption-rate (SAR)-intensive echo-train spin-echo sequences for liver imaging at 3.0 T.
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MATERIALS AND METHODS
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Volunteers and Imaging Systems
Our study had institutional review board permission as part of an ongoing sequence optimization project, and the volunteers provided informed consent. Fourteen healthy volunteers (seven men, seven women; mean age ± standard deviation, 32.7 years ± 6.8) with no previous history of liver problems nor recent heavy alcohol consumption (defined as >7 servings in the week before imaging) were imaged at 3.0 T (Signa Eclipse; GE Healthcare, Milwaukee, Wis) by using a whole-body coil for RF excitation and an eight-channel phased-array cardiac coil for signal reception. The system was equipped with high-performance gradients (40 mT/m, 266 msec rise time to peak maximum) and the option of using parallel imaging (array spatial sensitivity encoding technique) with a maximum acceleration factor of two for two-dimensional imaging.
MR Imaging Sequence
BBEPI (Fig 1) providing a trace diffusion-induced image (with the addition of three independent scans with diffusion sensitizing gradients in x-, y-, and z-axes) was chosen for volumetric liver imaging. The sequence was selected from an existing pulse sequence program on the system: pulse sequence family EPI, diffusion-weighted EPI, two-dimensional imaging mode. A 25-second breath hold at end expiration was considered for evaluation. The following imaging parameters were varied interactively to obtain full liver coverage in an optimization setting: section thickness (2, 3, and 4 mm), array spatial sensitivity encoding technique acceleration factor (none and two times), and diffusion b value (10, 20, 40, 50, 100, 250, and 500 sec/mm2). Matrix size was 160 x 192 (frequency and phase-encoding directions), field of view was 52 x 26 cm, rectangular field of view was 50%, and there was one signal acquired (internally collected as 0.5 signal acquired in the user interface). The resulting voxel size was 3.3 x 2.7 x 2.0–4.0 mm. A standard fat suppression RF pulse was used instead of a spectral-spatial water-excitation RF pulse. TR was kept constant at 6300 msec. TE was internally adjusted by the sequence program, but could be varied by using an "optimize TE" setting (resulting in TE variation of 20–35 msec, depending on the b value setting). For a low-TE setting (<35 msec), full liver coverage with 2.0-mm section thickness was obtained. The high-TE setting (80 msec) section thickness of 4.0 mm was chosen to obtain identical coverage, since the longer TE reduced by 40% the maximum number of sections that could be acquired with the low-TE setting. There was no intersection gap. To evaluate the extent of geometric distortions and signal loss around air-tissue interface regions (eg, liver-lung, air-filled bowel), three matrix sizes were collected: 64 x 192, 160 x 192, and 192 x 192. By using a phase-encoding direction of 192, the number of echoes is approximately 52 (total signal readout is 27 msec by using a frequency-encoding readout of 160 with ramp-sampling enabled).
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Figure 1a: Schematic presentation of (a) FSE and (b) EPI sequences, illustrating multiple 180° pulses after initial 90° pulse in FSE module, whereas in EPI, there is only one 180° pulse after initial 90° pulse. This results in decreased magnetization transfer contrast effects. TEeff = effective TE.
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Figure 1b: Schematic presentation of (a) FSE and (b) EPI sequences, illustrating multiple 180° pulses after initial 90° pulse in FSE module, whereas in EPI, there is only one 180° pulse after initial 90° pulse. This results in decreased magnetization transfer contrast effects. TEeff = effective TE.
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Fat-suppressed T2-weighted FSE (Fig 1) MR was performed with parameters as shown in Table 1.
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Table 1. Optimized Parameters for Diffusion-induced BBEPI with Short-TE, Long-TE, and Fat-suppressed T2-weighted FSE Imaging
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Image Analysis
Image quality was assessed both qualitatively (subjectively) and quantitatively on the optimized short-TE and long-TE BBEPI sequences. For the qualitative assessment, the following items were evaluated by two readers (I.C.v.d.B. and S.M.H., with 3 and 11 years experience in abdominal MR interpretation, respectively) in consensus: (a) overall image quality, (b) delineation of the liver (eg, how well the liver can be distinguished from the surrounding tissues), (c) tissue blurring, (d) fat suppression homogeneity, (e) suppression of the blood signal inside the hepatic vessels, and (f) geometric distortion at air-tissue interfaces by using the FSE sequence as the reference standard. These items were rated by using a scale from 1 to 5 (1 = unacceptable, 2 = poor, 3 = fair, 4 = good, and 5 = excellent). For quantitative measurements, operator-defined regions of interest of 100 mm2 were placed on liver and spleen images for measurement of mean signal intensity (SI), all performed by one of the authors (I.C.v.d.B.). Care was taken to exclude vessels or biliary structures. For analysis of the liver-to-spleen CNR (which can be used as an indication for liver-to-lesion contrast), region-of-interest measurements were performed in the spleen and liver at approximately 5 cm from the posterior coil array. The SI was normalized to the standard deviation of background noise of the liver SI sample and expressed as SNR. CNR was calculated as the difference in SI between lesion and liver, scaled to the standard deviation of background noise.
Statistical Analysis
Statistical parameters (mean, median, standard deviation, and range) were calculated by using software (SPSS, version 12.0.1; SPSS, Chicago, Ill). Significance levels of the data obtained in the qualitative and quantitative assessments were determined by using a nonparametric Wilcoxon signed rank test. A P value of less than .05 was considered to indicate a significant difference.
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RESULTS
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Sequence and Parameter Optimization
The selected short-TE and long-TE BBEPI sequences (Fig 2) were optimized for maximum section coverage (Table 1).
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Figure 2a: Optimal BBEPI settings in healthy volunteer. (a) Transverse short-TE BBEPI MR image (TR msec/TE msec, 6300/25; b value, 10 sec/mm2). (b) Transverse long-TE BBEPI MR image (TR/TE, 6300/80; b value, 10 sec/mm2).
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Figure 2b: Optimal BBEPI settings in healthy volunteer. (a) Transverse short-TE BBEPI MR image (TR msec/TE msec, 6300/25; b value, 10 sec/mm2). (b) Transverse long-TE BBEPI MR image (TR/TE, 6300/80; b value, 10 sec/mm2).
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Section coverage.—Within the 25-second breath hold, complete anatomic thin-section liver coverage was possible by using short-TE BBEPI (2-mm sections). Section thickness was doubled (4 mm) for long-TE BBEPI to increase SNR and to keep total liver coverage similar to short-TE BBEPI, as the longer TE selection resulted in an approximate 40% reduction in the maximum number of sections possible with short-TE BBEPI with the settings, as shown in Table 1.
b Value.—Section efficiency and image quality (sensitivity to bulk motion) were dependent on the selected b value. Increased b values resulted in lower section efficiency. The increased sensitivity to bulk motion with higher b values demonstrated better suppression of slow-flowing blood, especially in the region of the inferior caval vein, but with appearance of darker regions around the heart and bowel, especially with thin imaging sections (2.0 mm). Because SNR and CNR between liver and vascular structures decreased with longer TE, a b value of 10 sec/mm2 was taken as optimal, even though some remnant blood signal was sometimes observed in peripheral intrahepatic veins when thicker sections were selected.
Matrix size.—Variations in the number of frequency-encoding points that were used demonstrated an increased level of geometric distortions for higher frequency-encoding points, as well as increased signal loss (Fig 3). As previous experience demonstrated optimal image quality for measurements with less frequency points than did phase-encoding steps (15), the optimal imaging matrix was set to 160 x 192. A fixed high number of phase-encoding steps was chosen to improve the conspicuity of (often fluid-containing) liver lesions while not causing a significant increase in imaging duration.
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Figure 3a: Short-TE BBEPI MR images (TR/TE, 6300/25 msec; b value, 10 sec/mm2; section thickness, 2 mm) show effect of matrix size on signal loss, geometric distortion, and resolution. a–c Show effect of decreasing frequency points (from left to right), illustrating increased geometric distortions and signal loss in regions of increased magnetic susceptibility for higher frequency-encoding points (a and d), with approximately threefold interecho spacing, as compared with c and f.
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Figure 3b: Short-TE BBEPI MR images (TR/TE, 6300/25 msec; b value, 10 sec/mm2; section thickness, 2 mm) show effect of matrix size on signal loss, geometric distortion, and resolution. a–c Show effect of decreasing frequency points (from left to right), illustrating increased geometric distortions and signal loss in regions of increased magnetic susceptibility for higher frequency-encoding points (a and d), with approximately threefold interecho spacing, as compared with c and f.
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Figure 3c: Short-TE BBEPI MR images (TR/TE, 6300/25 msec; b value, 10 sec/mm2; section thickness, 2 mm) show effect of matrix size on signal loss, geometric distortion, and resolution. a–c Show effect of decreasing frequency points (from left to right), illustrating increased geometric distortions and signal loss in regions of increased magnetic susceptibility for higher frequency-encoding points (a and d), with approximately threefold interecho spacing, as compared with c and f.
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Figure 3d: Short-TE BBEPI MR images (TR/TE, 6300/25 msec; b value, 10 sec/mm2; section thickness, 2 mm) show effect of matrix size on signal loss, geometric distortion, and resolution. a–c Show effect of decreasing frequency points (from left to right), illustrating increased geometric distortions and signal loss in regions of increased magnetic susceptibility for higher frequency-encoding points (a and d), with approximately threefold interecho spacing, as compared with c and f.
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Figure 3e: Short-TE BBEPI MR images (TR/TE, 6300/25 msec; b value, 10 sec/mm2; section thickness, 2 mm) show effect of matrix size on signal loss, geometric distortion, and resolution. a–c Show effect of decreasing frequency points (from left to right), illustrating increased geometric distortions and signal loss in regions of increased magnetic susceptibility for higher frequency-encoding points (a and d), with approximately threefold interecho spacing, as compared with c and f.
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Figure 3f: Short-TE BBEPI MR images (TR/TE, 6300/25 msec; b value, 10 sec/mm2; section thickness, 2 mm) show effect of matrix size on signal loss, geometric distortion, and resolution. a–c Show effect of decreasing frequency points (from left to right), illustrating increased geometric distortions and signal loss in regions of increased magnetic susceptibility for higher frequency-encoding points (a and d), with approximately threefold interecho spacing, as compared with c and f.
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Image Quality
Qualitative assessment.—There was no significant difference in overall image quality, delineation of liver, or tissue blurring in short-TE BBEPI, long-TE BBEPI, and fat-suppressed T2-weighted FSE MR (Table 2).
The quality of the fat suppression and suppression of blood signal in hepatic vessels was significantly greater in BBEPI than in FSE sequences (P < .05).
The extent of geometric distortions at air-tissue interfaces was assessed as 3.4 in short-TE BBEPI, as 3.2 in long-TE BBEPI, and as five in fat-suppressed T2-weighted FSE (P < .05), with the worst found in liver-lung and liver-bowel areas (Fig 4).
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Figure 4a: FSE and EPI MR image comparison shows geometric distortion. (a, b) Transverse fat-suppressed T2-weighted FSE (TR/TE, 5000/80; section thickness, 3.5 mm) breath-hold images. (c, d) Corresponding transverse short-TE BBEPI (TR/TE, 6300/25; section thickness, 2 mm; b value, 10 sec/mm2) breath-hold images show locations of geometric distortion. Air from small bowel near liver (solid arrows) induces geometric distortion and signal loss in short-TE BBEPI image. In this volunteer, geometric distortions close to liver-lung interface (dotted arrows) were largest of all volunteers evaluated (worst image).
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Figure 4b: FSE and EPI MR image comparison shows geometric distortion. (a, b) Transverse fat-suppressed T2-weighted FSE (TR/TE, 5000/80; section thickness, 3.5 mm) breath-hold images. (c, d) Corresponding transverse short-TE BBEPI (TR/TE, 6300/25; section thickness, 2 mm; b value, 10 sec/mm2) breath-hold images show locations of geometric distortion. Air from small bowel near liver (solid arrows) induces geometric distortion and signal loss in short-TE BBEPI image. In this volunteer, geometric distortions close to liver-lung interface (dotted arrows) were largest of all volunteers evaluated (worst image).
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Figure 4c: FSE and EPI MR image comparison shows geometric distortion. (a, b) Transverse fat-suppressed T2-weighted FSE (TR/TE, 5000/80; section thickness, 3.5 mm) breath-hold images. (c, d) Corresponding transverse short-TE BBEPI (TR/TE, 6300/25; section thickness, 2 mm; b value, 10 sec/mm2) breath-hold images show locations of geometric distortion. Air from small bowel near liver (solid arrows) induces geometric distortion and signal loss in short-TE BBEPI image. In this volunteer, geometric distortions close to liver-lung interface (dotted arrows) were largest of all volunteers evaluated (worst image).
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Figure 4d: FSE and EPI MR image comparison shows geometric distortion. (a, b) Transverse fat-suppressed T2-weighted FSE (TR/TE, 5000/80; section thickness, 3.5 mm) breath-hold images. (c, d) Corresponding transverse short-TE BBEPI (TR/TE, 6300/25; section thickness, 2 mm; b value, 10 sec/mm2) breath-hold images show locations of geometric distortion. Air from small bowel near liver (solid arrows) induces geometric distortion and signal loss in short-TE BBEPI image. In this volunteer, geometric distortions close to liver-lung interface (dotted arrows) were largest of all volunteers evaluated (worst image).
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Quantitative assessment.—Quantitative assessment showed that SNR was highest in short-TE BBEPI for both liver and spleen (P < .05) (Table 3, Fig 5). CNR was highest in long-TE BBEPI (15, as compared with 7.8 and 4.8 in short-TE BBEPI and fat-suppressed T2-weighted FSE, respectively) (P < .05).
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Figure 5a: MR image quality in healthy patient with small liver hemangioma as incidental finding obtained in optimization study. (a) Transverse 3.0-T fat-suppressed T2-weighted FSE image (TR/TE, 5000/80) shows two hyperintense lesions (arrows). (b) Transverse 3.0-T short-TE BBEPI image (TR/TE, 6300/25; b value, 10 sec/mm2) shows improved suppression of fat and blood signal, with improved lesion conspicuity (arrows). (c) Transverse 3.0-T long-TE BBEPI image (TR/TE, 6300/80; b value, 10 sec/mm2) shows marked liver darkening but with high lesion SNR and CNR (arrows).
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Figure 5b: MR image quality in healthy patient with small liver hemangioma as incidental finding obtained in optimization study. (a) Transverse 3.0-T fat-suppressed T2-weighted FSE image (TR/TE, 5000/80) shows two hyperintense lesions (arrows). (b) Transverse 3.0-T short-TE BBEPI image (TR/TE, 6300/25; b value, 10 sec/mm2) shows improved suppression of fat and blood signal, with improved lesion conspicuity (arrows). (c) Transverse 3.0-T long-TE BBEPI image (TR/TE, 6300/80; b value, 10 sec/mm2) shows marked liver darkening but with high lesion SNR and CNR (arrows).
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Figure 5c: MR image quality in healthy patient with small liver hemangioma as incidental finding obtained in optimization study. (a) Transverse 3.0-T fat-suppressed T2-weighted FSE image (TR/TE, 5000/80) shows two hyperintense lesions (arrows). (b) Transverse 3.0-T short-TE BBEPI image (TR/TE, 6300/25; b value, 10 sec/mm2) shows improved suppression of fat and blood signal, with improved lesion conspicuity (arrows). (c) Transverse 3.0-T long-TE BBEPI image (TR/TE, 6300/80; b value, 10 sec/mm2) shows marked liver darkening but with high lesion SNR and CNR (arrows).
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The near-isotropic data obtained in short-TE BBEPI allowed multiplanar reformations with darkened hepatic vessels (Fig 6).
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Figure 6a: Multiplanar reformatting capability in short-TE BBEPI MR images. (a) Transverse short-TE BBEPI image. (b) Coronal minimum-intensity projection reconstruction (14-mm-thick section). Note homogeneous suppression of blood signal. (c) Minimum intensity reconstruction in sagittal plane (14-mm-thick section).
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Figure 6b: Multiplanar reformatting capability in short-TE BBEPI MR images. (a) Transverse short-TE BBEPI image. (b) Coronal minimum-intensity projection reconstruction (14-mm-thick section). Note homogeneous suppression of blood signal. (c) Minimum intensity reconstruction in sagittal plane (14-mm-thick section).
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Figure 6c: Multiplanar reformatting capability in short-TE BBEPI MR images. (a) Transverse short-TE BBEPI image. (b) Coronal minimum-intensity projection reconstruction (14-mm-thick section). Note homogeneous suppression of blood signal. (c) Minimum intensity reconstruction in sagittal plane (14-mm-thick section).
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DISCUSSION
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The results of our study that evaluated the application of BBEPI at 3.0 T suggest that thin-section volumetric imaging of the entire liver can be performed in a single breath hold at 3.0 T, providing good overall image quality and liver delineation, acceptable geometric distortions, good suppression of the fat and blood signal, and high SNR and CNR.
We believe the results of our preliminary study are clinically relevant for a number of reasons. First, the selection of short-TE and long-TE BBEPI sequences may potentially provide an alternative for more SAR-intensive fat-suppressed T2-weighted FSE sequences. Routinely, T2-weighted sequences for liver imaging are performed using FSE readout modules, which are RF-intensive owing to multiple refocusing RF pulses. When 3.0 T is compared with 1.5 T, RF pulses with four times more energy (higher B1 levels) are needed to excite the protons, resulting in more RF heating and a fourfold higher SAR at 3.0 T (8,13). This diminishes total anatomic coverage in a single breath hold because the increased SAR limits the amount of RF pulses per unit of time, which results in several breath-hold sessions to cover the entire liver by using thin sections. Since T2-weighted fat-suppression sequences are essential for diagnostic liver imaging, development of low-SAR T2-weighted protocols is important for improvement of liver imaging at a higher magnetic field strength.
Second, BBEPI may provide a solution for problematic fat suppression by using fat-suppressed T2-weighted FSE sequences at 3.0 T. Since RF wavelength is reduced at 3.0 T owing to a higher operational frequency of 128 MHz (compared with 64 MHz at 1.5 T), conductivity increases, which results in stronger eddy currents. B1 inhomogeneities, in combination with the nature of the fat suppression and refocusing pulses in echo-train spin-echo sequences, may also be responsible for relatively poorer fat-suppression quality. Fat suppression is important for diagnostic T2-weighted imaging of the liver, as it is utilized to improve the dynamic range of acquisition, which translates to an easier reading when bright lesions are found against a darkened surrounding (2,15). Therefore, improved fat suppression is important, for which the optimized BBEPI may potentially provide an alternative.
Third, in many research centers diffusion-induced EPI with relatively short TE is routinely applied in the liver to obtain apparent diffusion coefficient maps that can potentially help to improve differentiation between benign and malignant liver lesions (16). The results of our preliminary study show that a short-TE EPI sequence can also be used as a BBEPI sequence with the application of a low b value in three orthogonal directions, which may potentially be useful to augment the sensitivity for visualization of smaller liver lesions. Okada et al (19) demonstrated that the inherent high contrast resolution of a low-diffusion gradient EPI sequence provided higher sensitivity for detection of focal liver lesions when compared with a T2-weighted FSE sequence, even at the cost of low in-plane spatial resolution. Therefore, the combination of inherent high contrast resolution, lower magnetic transfer contrast effects (hence with higher liver-to-lesion contrast than with echo-train spin-echo sequences), and dark vessels may potentially allow for better detection of small liver lesions, but this remains to be determined in further studies.
Fourth, the selection of a long-TE BBEPI improves CNR at the cost of thicker sections, while lower TE values provide a lower CNR but higher SNR to provide near-isotropic voxel imaging (with a 17.8-mm3 voxel). The combination of these two acquisitions may potentially prove useful for a simplified, sensitive liver protocol at any magnetic field strength, particularly at 3.0 T, in view of the higher SAR values. In this case, the implementation of a dual-echo BBEPI to acquire both contrasts simultaneously may have the potential to further decrease the total imaging duration, but this remains to be determined.
Our preliminary study is limited by the fact that only volunteers were used as test subjects and that no patients were included. However, our aim was merely to evaluate the potential of BBEPI for morphologic abdominal imaging at 3.0 T and to evaluate image quality including fat suppression and anatomic coverage, as compared with the reference standard fat-suppressed T2-weighted FSE sequences. The FSE sequence that was used for comparison was designed to provide a competitive sequence for imaging at 3.0 T, and may be considered less optimal of the respiratory-gated FSE sequences routinely used at 1.5 T. However, the artifacts related to the introduction of these sequences at 3.0 T including blurring, the consequences of the magnetic transfer contrast effects for the liver-to-lesion contrast, the fat suppression problems, and the SAR issues require a different approach to the design and implementation of these sequences. The current sequence was optimized to obtain rapid liver coverage, with good fat suppression and high in-plane resolution, and was used in the study to demonstrate that an optimized EPI-based sequence may provide an alternative for FSE sequences at 3.0 T. Last, the BBEPI sequence may be hampered by geometric distortions and signal loss around air-tissue interface regions, which were not analyzed in detail in this study. Currently, we are evaluating the described optimized BBEPI sequence in patients with liver disease.
In conclusion, BBEPI is a feasible alternative for fast, low-SAR, thin-section morphologic imaging of the entire liver in a single breath hold at 3.0 T.
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ADVANCES IN KNOWLEDGE
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- Diffusion-induced black-blood echo-planar imaging (BBEPI) at 3.0 T is feasible for fast thin-section morphologic imaging of the entire liver in a single breath hold and has potential for use as an alternative for the more specific absorption rate–intensive echo-train spin-echo sequences for liver imaging at 3.0 T.
- Near-isotropic data obtained with short–echo time BBEPI allow multiplanar reformations, rendering hepatic vessels as dark structures in gray liver parenchyma.
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IMPLICATION FOR PATIENT CARE
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- Echo-planar imaging can provide near-isotropic and single breath-hold fast volumetric imaging of the entire liver at 3.0 T, with potential improvements in lesion detection and patient comfort.
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FOOTNOTES
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Abbreviations: BBEPI = black-blood EPI • CNR = contrast-to-noise ratio • EPI = echo-planar imaging • FSE = fast spin echo • RF = radiofrequency • SAR = specific absorption rate • SNR = signal-to-noise ratio • TE = echo time • TR = repetition time
2 Current address: Department of Radiology, University of Nebraska Medical Center, Omaha, Neb. 
Author contributions: Guarantor of integrity of entire study, I.C.v.d.B.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, I.C.v.d.B., S.M.H., P.A.W.; experimental studies, I.C.v.d.B., P.A.W.; statistical analysis, I.C.v.d.B.; and manuscript editing, all authors
Authors stated no financial relationship to disclose.
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References
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ABSTRACT
INTRODUCTION
