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Coil Sensitivity Encoding in MR Imaging

Advantages and Disadvantages in Clinical Practice

¹ Department of Radiology, St. Marianna University School of Medicine, 2-16-1 Sugao, Miyamae-Ku, Kawasaki City, Kanagawa, 216-8511, Japan.
² Philips Medical Systems Corporation, Philips Bldg., 13-37 Kohnan 2-chome Minato-Ku, Tokyo, 108, Japan.

Received May 29, 2001; accepted after revision November 15, 2001.

 
Address correspondence to Y. Kurihara.

Introduction

Top Introduction SENSE Imaging Procedure Fast Imaging Improved SNR with Longer... Correction of the Heterogeneous... Effects Due To the... Decreased Specific Absorption... SNR Artifacts Erroneous Field-of-View Setting References

 
Coil sensitivity encoding (SENSE) is a new technique that considerably enhances MR imaging. This technique allows a reduction in scan time in any imaging mode through the use of multiple receiver coils [1]. We have used the technique in clinical practice, especially in body imaging, on 1000 patients for more than a year. On the basis of our initial experience, we summarize the image quality and applications of the SENSE technique for body imaging, focusing on its advantages and disadvantages.

SENSE Imaging Procedure

Top Introduction SENSE Imaging Procedure Fast Imaging Improved SNR with Longer... Correction of the Heterogeneous... Effects Due To the... Decreased Specific Absorption... SNR Artifacts Erroneous Field-of-View Setting References

 
All studies were performed on a Gyroscan ACS-NT15 scanner (Philips, Best, The Netherlands) equipped with a Syncra4 SENSE research software patch (Philips). All images were taken using a Synergy body coil (Philips), which has four individual coil elements and preamplifiers.

MR imaging with the SENSE technique requires the following steps. First, for a reliable sensitivity mapping of each coil element, a reference measurement scan is obtained using the coils and related images acquired with the homogeneous quadrature body coil. In our preliminary clinical research setup, the reference scan was a fast field-echo sequence (TR/TE, 8.9/05.; flip angle, 8°; matrix, 32 x 64 x 64; excitations, 9; scan time, 1 min 4 sec). Second, SENSE data are acquired with a reduced number of phase-encoding steps resulting in an aliased or back-folded image with a reduced field of view (Fig. 1A). Scan time is reduced by decreasing the number of phase-encoding steps. This reduction is defined by the SENSE reduction factor R (the SENSE scan time equals the full scan time divided by R). Third, in the SENSE technique, unlike phased array coils, the coil elements are not used to cover separate anatomic regions to increase the signal-to-noise ratio (SNR) but are used to simultaneously measure the same region for scan-speed increase. The SENSE reconstruction algorithm separates the superimposed signals using information on the individual coil sensitivities and restores the full field-of-view image (Fig. 1B).

View larger version (149K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —Coil sensitivity encoding (SENSE) procedure in MR imaging. Conventional Fourier transformation produces fewer phase encoding steps (R factor, 2) and results in reduction of field of view, causing typical foldover or aliasing artifact.

View larger version (160K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —Coil sensitivity encoding (SENSE) procedure in MR imaging. SENSE reconstruction algorithm separates superimposed signals using information on individual coil sensitivities and restores full field-of-view image. R factor = SENSE reduction factor.

Fast Imaging

Top Introduction SENSE Imaging Procedure Fast Imaging Improved SNR with Longer... Correction of the Heterogeneous... Effects Due To the... Decreased Specific Absorption... SNR Artifacts Erroneous Field-of-View Setting References

 
Fast imaging is an essential function of the SENSE technique. A reduction in scan time can minimize image quality deterioration due to respiratory motion or bowel peristalsis and therefore can improve the image quality (Fig. 2A,2B). In a dynamic study, temporal resolution is dramatically improved with the SENSE technique.

View larger version (122K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —49-year-old woman with ovarian cysts. T2-weighted sagittal spin-echo MR image (TR/TE, 3500/1100) shows blurred bowel tract and muscle (arrows) due to respiratory motion and bowel peristalsis. Scan time for 15 slices was 1 min 35 sec.

View larger version (125K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —49-year-old woman with ovarian cysts. T2-weighted sagittal spin-echo MR image (3500/1100) using coil sensitivity encoding (R factor, 2) shows anatomic structures (arrows) more clearly than A because of suppression of motion artifact. Scan time for 15 slices was 53 sec.

If there is no need to reduce the scan time, the SENSE technique can provide improved spatial resolution and an increased number of slices.

In three-dimensional Fourier transformation reconstruction, phase-encoding steps are performed along the normal phase-encoding direction and also along the slice-selection direction. Therefore, the SENSE technique can be applied in both directions and can result in a further speeding up of data acquisition. The SENSE reduction factor in both directions (phase and slice) can be set to 2, resulting in an overall SENSE reduction factor of 4 (2 x 2). This technique is useful for gadolinium-enhanced MR angiography (Fig. 3A,3B,3C,3D).

View larger version (54K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —61-year-old woman with Takayasu's arteritis. High-resolution contrast-enhanced MR angiography carotid images (TR/TE, 4.0/1.0) using two-dimensional coil sensitivity encoding technique (R factor, 4) provide high temporal-resolution dynamic study. Maximum-intensity-projection angiograms from three-dimensional volumes are shown acquired at 8 (A), 12 (B), 16 (C), and 20 sec (D) after injection of gadolinium-enhanced contrast agent. Field of view is 529 x 529 mm. Each maximum-intensity-projection image is reconstructed with 24 slices of source images.

View larger version (65K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —61-year-old woman with Takayasu's arteritis. High-resolution contrast-enhanced MR angiography carotid images (TR/TE, 4.0/1.0) using two-dimensional coil sensitivity encoding technique (R factor, 4) provide high temporal-resolution dynamic study. Maximum-intensity-projection angiograms from three-dimensional volumes are shown acquired at 8 (A), 12 (B), 16 (C), and 20 sec (D) after injection of gadolinium-enhanced contrast agent. Field of view is 529 x 529 mm. Each maximum-intensity-projection image is reconstructed with 24 slices of source images.

View larger version (70K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3C. —61-year-old woman with Takayasu's arteritis. High-resolution contrast-enhanced MR angiography carotid images (TR/TE, 4.0/1.0) using two-dimensional coil sensitivity encoding technique (R factor, 4) provide high temporal-resolution dynamic study. Maximum-intensity-projection angiograms from three-dimensional volumes are shown acquired at 8 (A), 12 (B), 16 (C), and 20 sec (D) after injection of gadolinium-enhanced contrast agent. Field of view is 529 x 529 mm. Each maximum-intensity-projection image is reconstructed with 24 slices of source images.

View larger version (73K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3D. —61-year-old woman with Takayasu's arteritis. High-resolution contrast-enhanced MR angiography carotid images (TR/TE, 4.0/1.0) using two-dimensional coil sensitivity encoding technique (R factor, 4) provide high temporal-resolution dynamic study. Maximum-intensity-projection angiograms from three-dimensional volumes are shown acquired at 8 (A), 12 (B), 16 (C), and 20 sec (D) after injection of gadolinium-enhanced contrast agent. Field of view is 529 x 529 mm. Each maximum-intensity-projection image is reconstructed with 24 slices of source images.

Improved SNR with Longer TR

Top Introduction SENSE Imaging Procedure Fast Imaging Improved SNR with Longer... Correction of the Heterogeneous... Effects Due To the... Decreased Specific Absorption... SNR Artifacts Erroneous Field-of-View Setting References

 
Ultrashort (2-4 msec) TR is frequently used for advanced gadolinium-enhanced MR angiography, providing high temporal and spatial resolution but with a decreased SNR. In this situation, a bolus injection of contrast agent is needed to achieve a high-signal level. The SENSE technique is another choice for improved SNR (Fig. 4). If there is no need to further reduce the scan time, a longer TR combined with SENSE can be chosen (e.g., using a TR twice as long and a SENSE reduction factor of 2). This procedure results in an increase of SNR (typically more than 40%) for the same scan time. There are two reasons for this increase in SNR [2, 3]. First, the acquisition bandwidth is lower, resulting in a higher SNR. Second, the longer TR causes further T1 relaxation to a higher steady-state level, again resulting in an increase in the SNR.

View larger version (169K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4. —35-year-old man with intralobar sequestration. High-resolution pulmonary MR angiogram using two times longer TR (8 msec) and coil sensitivity encoding (R factor, 2) shows perfusion defect in left lower lobe due to intra-lobar sequestration. Note increased signals from lung parenchyma because of longer T1 recovery.

Correction of the Heterogeneous Sensitivity of the Coils

Top Introduction SENSE Imaging Procedure Fast Imaging Improved SNR with Longer... Correction of the Heterogeneous... Effects Due To the... Decreased Specific Absorption... SNR Artifacts Erroneous Field-of-View Setting References

 
A synergy coil or phased array coil is composed of multiple surface coils. This results in a difference in intensity on an image because of the heterogeneous sensitivity of each coil. The SENSE technique uses the coil sensitivity, measured with a reference scan, to make the final image intensity completely homogeneous. This technique not only provides us with a correction of the heterogeneous sensitivity of each surface coil, but also with a wider field of view without decreasing the signal intensity at the periphery of the coils (Fig. 5A,5B).

View larger version (151K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5A. —49-year-old woman with cervical carcinoma of uterus. T2-weighted sagittal spin-echo MR image (TR/TE, 3500/1100) shows decreased signal intensity in marginal areas of field of view (arrows).

View larger version (152K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5B. —49-year-old woman with cervical carcinoma of uterus. T2-weighted sagittal spin-echo MR image (3500/1100) using coil sensitivity encoding (R factor, 2) shows uniform intensity of image (arrows).

Effects Due To the Decrease in Echo-Train Length

Top Introduction SENSE Imaging Procedure Fast Imaging Improved SNR with Longer... Correction of the Heterogeneous... Effects Due To the... Decreased Specific Absorption... SNR Artifacts Erroneous Field-of-View Setting References

 
SENSE may be used to reduce the length of the echo train by decreasing the number of phase-encoding steps, improving the image quality.

In turbo spin-echo sequences, a decreased echo-train length or reduced turbo factor results in suppression of the strong fat intensity with J-coupling effects (Fig. 6A,6B), improvement in SNR by avoiding the sampling of weak signals in the latter part of the echo train, improvement in contrast material within soft tissue, and reduction of T2 blurring due to the diminished T2 filtering effect.

View larger version (181K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6A. —27-year-old healthy male volunteer. T2-weighted axial spin-echo MR image (TR/TE, 3000/120; turbo factor, 13) results in strong fat intensity with J-coupling effects.

View larger version (179K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6B. —27-year-old healthy male volunteer. Coil sensitivity encoding image (R factor, 2) with same TR/TE as A, turbo factor of 7, and longer echo spacing shows suppression of bright fat signal.

In echoplanar imaging, decreased echo-train length or reduced echoplanar imaging factor causes two effects. The first effect involves a decrease in the chemical shift artifact along the phase direction that arises because of the accumulation of phase shift during sequential phase-encoding steps, of which the number is reduced in SENSE. The second effect involves a decrease in the susceptibility artifact due to reduced phase accumulation from the dephasing in field inhomogeneities [4, 5].

Decreased Specific Absorption Rate

Top Introduction SENSE Imaging Procedure Fast Imaging Improved SNR with Longer... Correction of the Heterogeneous... Effects Due To the... Decreased Specific Absorption... SNR Artifacts Erroneous Field-of-View Setting References

 
SENSE may be used to reduce the number of radiofrequency pulses or to slow the switching of gradients, resulting in a smaller specific absorption rate and reduced neurostimulation.

SNR

Top Introduction SENSE Imaging Procedure Fast Imaging Improved SNR with Longer... Correction of the Heterogeneous... Effects Due To the... Decreased Specific Absorption... SNR Artifacts Erroneous Field-of-View Setting References

 
As in all images, the SNR of SENSE images is proportional to the square root of the acquisition time. Therefore, when a SENSE reduction factor of R is applied, the overall SNR of the SENSE image reduces by one divided by the square root (R). Furthermore, in SENSE, additional noise appears when the geometric relations of the coil sensitivities are not ideal [1]. This specific effect is quantitatively described by the local geometric factor g. The local SNR of a SENSE image is then given by

The geometric factor describes the ability to separate pixels superimposed by aliasing with the coil configuration used. Note that g varies over the image. The g is always greater than 1 and is higher when multiple aliasing occurs. If the SENSE reduction factor is less than 2.5, the geometry factor is usually less than 1.1 and is, in practice, negligible.

Artifacts

Top Introduction SENSE Imaging Procedure Fast Imaging Improved SNR with Longer... Correction of the Heterogeneous... Effects Due To the... Decreased Specific Absorption... SNR Artifacts Erroneous Field-of-View Setting References

 
Most of the artifacts associated with SENSE images are caused by a mismatch between the reference scan and the SENSE data acquisition (Fig. 7). These mismatches can be caused by the respiratory pattern, local susceptibility, strong fat signal, or voxel size. Actually, the artifacts are hardly visible in most practical clinical scans because of optimizing of the voxel size and breathing pattern and adjusting the reference scan so that susceptibility has a minimal influence.

View larger version (121K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7. —71-year-old woman with adrenal mass. Abdominal gradient-echo MR image (TR/TE, 215/2.3) using coil sensitivity encoding (SENSE) (R factor, 2) shows residual bright fat signal (large arrow) and "split-line" artifact (small arrows), which causes jump-up intensity at mid portion. Both rare artifacts could be caused by mismatch between reference scan and SENSE data acquisition.

Erroneous Field-of-View Setting

Top Introduction SENSE Imaging Procedure Fast Imaging Improved SNR with Longer... Correction of the Heterogeneous... Effects Due To the... Decreased Specific Absorption... SNR Artifacts Erroneous Field-of-View Setting References

 
If the field-of-view setting is too small for SENSE imaging acquisition, folded areas partially persist on the mid portion of SENSE reconstructed images (Fig. 8A,8B). In SENSE, with a reduction factor of 2, foldover beyond the mid line of the field of view cannot be unfolded. This is not an artifact but a mistake in the field-of-view setting.

View larger version (67K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8A. —45-year-old man with liver cirrhosis. Aliasing image before coil sensitivity encoding (SENSE) reconstruction algorithm shows foldover beyond midline of field of view because of small field-of-view setting.

View larger version (96K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8B. —45-year-old man with liver cirrhosis. SENSE reconstructed images (TR/TE, 520/20; flip angle, 20°; R factor, 2) shows remaining folded areas (arrows).

SENSE is a powerful and promising technique for fast MR imaging for any sequence of body imaging. Advantages of SENSE are based on not only the reduction in total scan time, but also on the increased flexibility of parameters and the improved image quality because of the reduction in echo-train length. Artifacts and decreased image quality usually result from a sub-optimal reference scan and coil setup. However, these disadvantageous effects occur rarely in most daily examinations in clinical practice.

References

Top Introduction SENSE Imaging Procedure Fast Imaging Improved SNR with Longer... Correction of the Heterogeneous... Effects Due To the... Decreased Specific Absorption... SNR Artifacts Erroneous Field-of-View Setting References

 

1. Pruessmann KP, Weiger M, Scheidegger MB, Boesiger P. SENSE: sensitivity encoding for fast MRI. Magn Reson Med 1999;42:952 -962[Medline]
2. Weiger M, Pruessmann KP, Kassner A, et al. Contrast-enhanced 3D MRA using SENSE. J Magn Reson Imaging 2000;12:671 -677[Medline]
3. Parker DL, Goodrich KC, Alexander AL, Buswell HR, Blatter DD, Tsuruda JS. Optimized visualization of vessels in contrast enhanced intracranial MR angiography. Magn Reson Med 1998;40:873 -882[Medline]
4. Golay X, Prussmann KP, Weiger M, et al. PRESTO-SENSE: an ultrafast whole-brain fMRI technique. Magn Reson Med 2000;43:779 -786[Medline]
5. Pruessmann KP, Weiger M, Muiswinkel AMC, Boesiger P. Sensitivity encoding for single-shot diffusion imaging. In: Proceedings of 7th annual scientific meeting of the International Society for Magnetic Resonance in Medicine. Berkley, CA: ISMRM, 1999(P):1815

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