25Fat Suppression and Parallel Imaging

S. B. Reeder, MD, PhD

Department of Radiology, E3/311 CSC, 600 Highland Avenue, Madison, WI 53792-3252, USA

C. A. McKenzie, PhD

Department of Radiology, Beth Israel-Deaconess Medical Center, Boston, MA

C O N T E N T S

25.1 Introduction 269 25.2 Chemical-Shift-Based Water-Fat Separation 270 25.2.1 Noise Performance of

Water-Fat Separation Methods 273 25.3 Parallel Imaging and Water-Fat Separation:

Complementary Methods 274 25.3.1 Combining Parallel Imaging and

Water-Fat Separation Methods 274 25.3.2 Applications of Parallel Imaging and Water-Fat Separation 278 25.4 Conclusion 280

References 282

Advanced Methods of 25

Fat Suppression and Parallel Imaging

Scott B. Reeder and Charles A. McKenzie

25.1

Introduction

Uniform and reliable suppression of fat is essential for accurate diagnoses in many areas of MR imaging.

This is particularly true for sequences such as T1- weighted spoiled gradient echo sequences (SPGR), steady-state free precession (SSFP), and fast spin- echo (FSE) imaging where fat is bright and may obscure underlying pathology. Conventional fat- saturation is often adequate for areas of the body with relatively homogeneous main (B

) magnetic fi eld; however, there are many applications where fat-saturation commonly fails. Off-isocenter imaging,

extremity imaging, large fi eld of view (FOV) imaging, and areas such as the skull base and brachial plexus, as well as many others, are all examples of imaging applications where B

is rarely homogenous. Conven- tional fat-saturation pulses are also sensitive to RF (B

) inhomogeneities, which may be problematic for transmit surface coil applications.

Short-TI inversion recovery (STIR) imaging pro- vides very uniform fat-suppression, but has mixed contrast that is highly dependent on T1, and has reduced the signal-to-noise ratio (SNR) (Bydder et al. 1985). In addition, the possibility of suppress- ing contrast-enhanced tissue limits STIR to proton density or T2-weighted applications, and most clini- cal T1-weighted applications rely solely on conven- tional fat-saturation methods. STIR also has reduced sequence effi ciency because of the need to play an inversion pulse followed by a relatively long inver- sion time (TI) (approximately 200 ms at 1.5 T). Other fat-suppression techniques include water-selective

“spectral-spatial” pulses, and although these water- selective methods are insensitive to B

inhomoge- neities, they remain sensitive to B

inhomogeneities (Meyer et al. 1990; Block et al. 1997).

First described in 1984 by Dixon (1984), “in and out

of phase” imaging exploits the difference in chemical

shifts between water and fat in order to separate water

and fat into separate images. This approach was fur-

ther refi ned by Glover using a “three-point” method

that acquired three separate images at different echo

times to compensate for B

fi eld inhomogeneities

(Glover 1991; Glover and Schneider 1991). Three-

point water-fat separation methods have been success-

fully applied to other sequences including fast spin-

echo (FSE) (Hardy et al. 1995), gradient echo (GRE)

imaging (Wang et al. 1998), steady-state free preces-

sion (SSFP) (Reeder et al. 2003) and spiral methods

(Moriguchi et al. 2003). Chemical-shift-based water-

fat separation methods that measure fi eld inhomo-

geneities, and subsequently compensate for them,

provide robust water-fat separation in areas that are

challenging for conventional fat-saturation methods.

In addition, water-fat separation methods are insen- sitive to B

inhomogeneities and are well suited for transmit surface coil applications. A fi nal advantage of water-fat separation methods compared with fat sup- pression methods is the availability of the fat image, which improves tissue characterization through direct visualization of fat in addition to water.

A recently described method, IDEAL (Iterative Decomposition of water and fat with Echo Asymme- try and Least-squares estimation), has been applied to FSE (Reeder, Pindeda, Wen et al. 2005) and gradi- ent echo imaging (Reeder, Pindeda, Yu et al. 2005).

This method uses an iterative method to determine the fi eld inhomogeneity map, allowing the algorithm to remove phase shifts in the source images caused by fi eld inhomogeneities, and subsequently calculates separate water and fat images in the least squares sense. This method allows for arbitrary and unequally spaced echoes, allowing the optimal combination of echo shifts to be used to maximize the SNR perform- ance of the water-fat decomposition (see below). In this chapter, examples of water-fat decompositions will be shown using the IDEAL method, although other chemical shift-based water-fat methods can also be combined with parallel imaging.

25.2

Chemical-Shift-Based Water-Fat Separation For most clinical MR applications an image will con- tain both water and fat. The signal from a voxel will

have a complex dependence on the amount of water and fat in that voxel, as well as the echo time and the local fi eld inhomogeneity. Mathematically, the signal from such a voxel can be written,

s t ( )

  W  Fe

e

⁽¹⁾ where t

is the echo shift from the spin-echo or TE=0 for gradient echo imaging, W and F are signal intensities from water and fat, ∆f

is the chemical shift between water and fat, approximately –210 Hz at 1.5 T or –420 Hz at 3.0 T, and ψ is the off-resonance frequency shift (Hz) resulting from local B

inhomo- geneities. Figure 25.1 shows a vector diagram of the signal contributions from water (red) and fat (blue) to the observed signal (purple) when water and fat are in-phase (t=0) and at some time t chosen to achieve a phase θ between water and fat. The angle ϕ = 2πψt is the phase resulting from local fi eld inhomogeneity, ψ, acting on both the water and fat vectors.

By sampling s(t

) at three or more different echo times t

, there is suffi cient information to decom- pose water from fat. The specifi c choice of echo times greatly infl uences the best possible SNR perform- ance of the water-fat decomposition. For example, conventional three-point methods, fi rst described by Glover (Glover 1991; Glover and Schneider 1991), acquired echoes with a relative water-fat phase shift ( θ = 2π∆f

t

) of - π, 0, π, i.e.: out of phase, in-phase, and out of phase. With this echo combination, the effective number of signal averages (NSA) can be shown to be approximately 2.7, about 10% less SNR than the maxi- mum possible NSA of 3.0, which would be achieved if all the information from the three source images was

Fig. 25.1. Water (red) and fat (blue) signal add to form the observed signal (purple), shown at two different echo times, t=0 (in-phase) and t = 1/2 πθ∆f

t = 0

t = 1/( πθ∆f

) ϕ θ

Water Fat Signal

used effi ciently to separate water from fat. Recently, it has been shown that the SNR of the water and fat image is maximized when the relative phase of water and fat is perpendicular (i.e.: θ = 2π∆f

t

= π/2, 3π/2, etc.), for one of the echoes, and the other two echoes are acquired with a relative phase shift 2 π/3 before and after the per- pendicular echo (Pineda et al. 2005). This would cor- respond to echo shifts of -0.4ms, 1.2 ms and 2.8 ms for FSE imaging at 1.5 T. For this combination of echoes, the SNR performance reaches the maximum possible with NSA=3.0. This work has also shown that in gen- eral, there is a strong dependence of the SNR behavior on the proportion of water and fat in a voxel, except for the above combination of echo shifts when the NSA is 3.0 for all ratios of water and fat. Figure 25.2 sum- marizes one possible water-fat separation approach using IDEAL. It is important to note that although it is possible to acquire all three echoes within a single TR using an echo-planar readout, multi-echo approaches are restrictive, and it is diffi cult to achieve the optimal echo shifts while maintaining fl exibility in the acquisi- tion paramters such as bandwidth and matrix size.

Figure 25.3 shows an example of a calculated water T2-weighted FSE image of the brachial plexus of a normal volunteer acquired with a neurovascular coil, compared with a conventional T2-weighted fat-satu- rated FSE image. Large areas of failed fat-suppres- sion seen in the fat-saturated image, caused by severe

susceptibility commonly seen in the head and neck region, show uniform separation of water from fat in the IDEAL image.

Extremities such as the ankle and foot are also chal- lenging areas where conventional fat-saturation com- monly fails as a result of unfavorable geometry that exacerbates susceptibility differences. Figure 25.4 is an example of IDEAL-FSE imaging in an ankle with a metallic fi xation plate, demonstrating improved sup- pression of fat compared to conventional fat-satura- tion, despite the presence of metallic hardware.

Characterization of tissues through direct visuali- zation of fatty tissue is a distinct advantage of water- fat separation methods. For example, Fig. 25.5 shows separated T2-weighted-FSE water and fat images, as well as recombined images from the pelvis of a woman with an adnexal mass. A large component of this bright mass separates into the fat-only image, confi rming the diagnosis of ovarian dermoid with high confi dence.

As a companion case to the images shown in Fig. 25.5, Fig. 25.6 shows T1-weighted IDEAL-FSE images through the pelvis of a woman with endome- triosis. A small hyperintense mass along the right pelvic side-wall separates into the water-only image, indicating that this is not a fatty mass as may have been initially suspected on a non-fat suppressed image.

Fig. 25.2. Schematic of the IDEAL water-fat separation method. Three complex images acquired at optimized echo shifts ( θ

, θ

, and θ

) are used by an iterative fi eld map calculation to determine the fi eld map ( ψ). The effects of the fi eld map are removed from the source images and water and fat images are subsequently calculated using a least-squares solution

Iterative fi eld map calculation θ

= – π/6

θ

= 7 π/6 θ

= π/2

Field map ( ψ ⁾

Least-squares solution for water & fat Water

Fat Fat

Water

Complex

Source Images

An interesting variation of chemical shift-based imaging is the separation of water and silicone.

Silicone and water have a relative chemical shift of approximately –315 Hz at 1.5 T (Schneider and Chan 1993). This opens the potential for water-sili- cone separation, an application particularly impor- tant for evaluation of silicone breast implant rupture.

An effective approach using a STIR inversion pulse

Fig. 25.3a,b. a Coronal water T2-weighted IDEAL-FSE image of the brachial plexus and cervical spine shows tre- mendously improved fat suppression in comparison to b conventional fat-saturated FSE imaging, which shows large areas of failed fat saturation and inadvertent water suppression leading to a non-diagnostic image

Fig. 25.4a,b. Coronal T2-weighted im- ages of an ankle with a metallic fi bular screw acquired with a IDEAL-FSE and b conventional fat-saturated FSE. Areas of failed fat-saturation near the screw as well as elsewhere are seen in the fat- saturated image (arrows)

a b

a b

to suppress signal from fat within the breast has been described by Ma et al. (2004). In this approach, the separation of water and silicone is performed by adjusting the echo shifts slightly to achieve the same phase shifts normally used between water and fat.

Because water and silicone have a relative chemical

shift larger than that between water and fat, the echo

shifts are smaller than those used for water-fat sepa-

Fig. 25.5a–c. T1-weighted IDEAL-FSE a recombined, b water and c fat images in a woman with a right adnexal mass. A large component of this mass separates into the fat image (thin ar- row) confi rming the diagnosis of ovarian dermoid with high confi dence. Note the small amount of free pelvic fl uid (thick arrow) in the water image

Fig. 25.6a–c. T1-weighted IDEAL-FSE a recombined, b water and c fat images in a woman with endometriosis showing a small hyperintense mass along the right pelvic sidewall, representing a small endometrioma. It is not a fatty mass as was initially suspected on non-fat suppressed imaging

b

c

b c a

a

ration. Figure 25.7 shows an example of T2-weighted IDEAL-STIR imaging of a breast containing a com- posite saline-silicone implant demonstrating uni- form separation of silicone and water with uniform suppression of fat. The silicone component has rup- tured and is clearly visualized in the silicone-only image.

25.2.1

Noise Performance of

Water-Fat Separation Methods

As mentioned above, the noise performance of an estimation technique such as water-fat decomposi- tion used in chemical shift-based separation methods can be described with the effective number of signal averages (NSA), which is equivalent to signal aver- aging commonly used to describe MR acquisitions.

With conventional three-point methods, NSA is 2.7, and with IDEAL, NSA is 3.0, the maximum possible for any three-point method, utilizing all available signal in an effi cient manner. An effective average of three implies that the SNR of the decomposed water and fat images will increase by 3 (≈1.73) compared to the SNR of a single source image.

25.3

Parallel Imaging and Water-Fat Separation:

Complementary Methods

Accelerating a water-fat separation acquisition with

parallel imaging will help alleviate the three-fold

increase in scan time compared with conventional

fat-saturated methods. However, from Chaps. 2 and

3, we know that there is an inherent SNR penalty that occurs when we use parallel imaging to accelerate an acquisition. As described in those chapters, the local SNR at a pixel within an accelerated image is given by,

SNR SNR

R

 g R

(2)

where SNR

is the SNR of the pixel in an unac- celerated image, R is the “reduction” or acceleration factor, and g is the “geometry” or g-factor that refl ects additional noise amplifi cation due to ill-condition- ing of the unwrapping process. Although the g-factor always has a value of 1.0 or higher, it is close to 1.0 for well-designed coil arrays and low acceleration factors.

The g-factor depends on a wide variety of parameters including orientation of the phase encoding direc- tion, the acceleration factor used, fi eld of view, the object being imaged, and the design of the coil ele- ments in the array used for data reception.

For a water-fat separation acquisition, the SNR is given by

SNR SNR NSA

R

 g R

(3)

From this equation, we see that the decrease in SNR from accelerating an acquisition ( 1 R ) is offset by the effective averaging that occurs from decomposing water and fat from multiple images A( NSA ). The maximum possible NSA is 3.0, just as if the three images had been averaged together.

If our water-fat decomposition has been optimized, as is the case with IDEAL (i.e., NSA=3.0), and if we assume that the g-factor is low (i.e.: g,1.0), we see that SNR

=SNR

when R=3, i.e., there is no SNR penalty.

Therefore, when R=3, we effectively achieve water-fat separation for “free.”

As we can see, parallel imaging and water-fat sepa- ration methods are complementary methods: parallel imaging alleviates the scan time penalty inherent to multi-point water-fat separation methods, while the high SNR behavior of water-fat separation compen- sates for the SNR penalty of parallel imaging.

25.3.1

Combining Parallel Imaging and Water-Fat Separation Methods

Figure 25.8a shows a schematic of the three k-space data matrices acquired for a fully sampled data set required for a three-point water-fat separation. If the three data sets are under-sampled in the phase encoding direction (Fig. 25.8b, R=2 in this example), we can unwrap the subsequent aliasing with a paral- lel imaging reconstruction algorithm of our choice and accelerate the acquisition. Separate calibration images containing coil sensitivities would be required to reconstruct the data set acquired in Fig. 25.8b.

Alternatively, only the edges of k-space can be undersampled, leaving full sampling at the center of k- space (Fig. 25.8c). The higher sampling density at the

Fig. 25.7a–c. Sagittal T2-weighted IDEAL-STIR a recombined, b water, and c silicone images of a breast containing a composite water- silicone implant with rupture of the silicone component (long arrow) and a small amount of pericapsular fl uid (short arrow). The STIR pulse is used to suppress fat, while IDEAL exploits the chemical shift between water and silicone to separate these components

b c

center of k-space provides the necessary coil sensitivity information needed to unwrap the undersampled por- tions of k-space. This additional information provides a “self ” or “auto”-calibration, eliminating the need for an external calibration (cf. chapters 2 and 8). Because the three images acquired at the different echo times must be at the same location, they will have the same coil sensitivities, and therefore can use the same coil sensitivity information, either from a separate calibra- tion scan, or from central k-space lines. This scheme is shown in Fig. 25.8c where fully sampled central k- space lines are acquired for echo 1. It is important to realize that these additional self-calibration lines can be acquired for any of the three echoes and can be applied for all three images, making this an effi cient use of the self-calibration. This approach is described in more detail elsewhere (McKenzie et al. 2004). In this work, we apply both image domain unwrapping

(SENSE) (Pruessmann et al. 1999) as well as a gener- alized k-space based algorithm (generalized encoding matrix (GEM), Sodickson and McKenzie 2001) for reconstruction of undersampled data.

The major disadvantage of self-calibration meth- ods in parallel imaging is the penalty required to acquire the additional central k-space lines. How- ever, since this information can be used for all three images in the multi-point acquisition, the overall time penalty is relatively small. For this reason, we prefer the modifi ed self-calibration approach, which uses calibration data from one of the three images, because we achieve the same benefi ts of self-cali- bration, most notably the decreased motion sensi- tivity from misregistration of calibration data and under-sampled data, while paying a relatively minor penalty for the acquisition of a few additional self- calibrating lines.

Fig. 25.8. a Fully sampled k-space for the three ech- oes acquired at different echo times. b Under- sampling of echoes will accelerate the acquisi- tion, although a separate calibration image would be required. c Full sam- pling of central k-space is performed for one of the three images (echo 1 in this example), providing a “self-calibration” used to unwrap aliasing for all three images

a

b

c

Further reductions in sampling can be made through the use of reduced sampling strategies. In general, fi eld inhomogeneities vary slowly over the image, and can be well characterized with low-reso- lution images, which require little time to acquire.

Once the fi eld map is known, only two full-resolution images acquired at different echoes times are required to decompose water from fat (Reeder, Wen et al. 2004;

Brau et al. 2005). For single coil applications, this offers an approximately 30% decrease in minimum scan time. When used with multi-coil and parallel imaging applications, substantial decreases in scan time can be achieved even with small parallel reduction factors (R=2–3) (Brau et al. 2005). This under-sampled two- point approach is illustrated in Fig. 25.9. The image acquired at echo 1 is a low-resolution full-fi eld-of-view (FOV) image used as a calibration scan to unwrap the under-sampled full-resolution reduced-FOV images

acquired at echo times 2 and 3. The unwrapped images at echo times 2 and 3 (Fig. 25.9b) are then low-pass fi l- tered (Fig. 25.9c) to obtain three low-resolution images that are then used to measure the fi eld map, ψ. Finally, the fi eld map is demodulated from images acquired at echo times 2 and 3, which are then used to decompose water and fat. Note that the data from the low-resolu- tion image is used for both fi eld map calculation and for parallel imaging calibration, further enhancing the effi ciency of this approach. Details of this approach are described elsewhere (Brau et al. 2005). Using a paral- lel reduction factor of 2, scan time reductions of 63%

compared with a non-accelerated three-point scan can be achieved, while a reduction factor of 3 provides 75%

scan time reduction, providing scan times faster than conventional fat-saturated imaging.

A fi nal approach that has also shown early initial success is one-point water-fat separation (Yu et al. 2004;

Fig. 25.9a–c. Under-sam- pled two-point sampling strategy. a Image acquired at echo time 1 is a low- resolution, full-FOV image used as a calibration scan to unwrap the under- sampled full-resolution, reduced-FOV images acquired at echo times 2 and 3. The central lines of k-space from unwrapped images b at echoes 2 and 3 are used to obtain three low-resolution data sets c that are then used to determine the fi eld map, ψ. Finally, the fi eld map is demodulated from images acquired at echo times 2 and 3, which are then used to decompose water and fat

c a

b

Hoory et al. 2005; Son et al. 2005; Yu et al. 2005). This approach is particularly useful for dynamic contrast- enhanced imaging and CINE cardiac imaging where repeated high spatial and temporal resolution imaging at the same location is desired (Yu et al. 2004; Hoory et al. 2005; Son et al. 2005; Yu et al. 2005). Repetitive same location imaging permits the acquisition of a pre-contrast calibration scan of the site to be imaged to measure the fi eld map and constant phase shifts from coils, dielectric effects, etc. This three-image calibration is used to remove the local phase shift, φ, that results from a combination of constant phase shifts and fi eld inhomogeneity. This approach differs from two- and three-point water-fat separation meth- ods where only the effect of the fi eld inhomogeneity needs to be removed and the resulting water and fat images are complex images that contain the additional

constant phase shifts, which are removed through the magnitude operation. With the one-point technique, images are then acquired during dynamic imaging with the TE adjusted so that water and fat are perpen- dicular ( θ=π/2) (Fig. 25.10). The calibration phase and fi eld map images are used to remove the local phase shift, φ, leaving the water and fat signal in the real and imaginary components of the signal, respectively.

Changes in the fi eld map from the presence of gadolin- ium in dynamic contrast-enhanced imaging have been shown to be negligible for concentrations of gadolin- ium expected in vivo (Hoory et al. 2005).

The acquisition time of the calibration image can be reduced using parallel imaging and/or the low-reso- lution fi eld map approaches described above. More importantly, the images acquired during dynamic contrast bolus injection can also be accelerated, using

Fig. 25.10. Dynamic scanning with single-image water-fat separation. A three-image pre-contrast calibration scan is used to the local fi eld map and additional constant phase shifts that will result in a combined phase shift, φ. The effects of φ are removed from single images acquired with θ=π/2, during the injection of contrast, leaving the resulting water and fat signal in the real and imaginary components of the signal, respectively

θ

Imag Pre-contrast 3-pt calibration scan

Single image dynamic scan

Field map Phase map

Real F

W φ ^(TE) F W

Repeat . . . θ

θ

the initial calibration images as coil sensitivity maps to unwrap the single image dynamic acquisitions. This approach permits the acquisition of separate water and fat images with scan times that are half (R=2), one third (R=3) or less (R>3) than conventional non-accel- erated fat-saturated imaging (Yu et al. 2005).

25.3.2

Applications of Parallel Imaging and Water-Fat Separation

Although the three-fold increase in scan time of water- fat separation methods can be used effi ciently from an SNR perspective, long minimum scan times are unac- ceptable for many applications. These include cardiac and abdominal imaging that requires rapid imaging to acquire high-resolution images within a breath- hold. For example, a recently described three-point water-fat separation approach in the heart acquires three complete sets of SSFP CINE images at different

echo shifts in order to decompose separate water and fat CINE images of the heart (Reeder, Markl et al.

2005; Reeder, McKenzie et al. 2004). The three-fold increase in scan time limits acquisitions to single slices with reduced spatial and temporal resolution.

Figure 25.11 shows an example of water-only CINE SSFP images from a normal volunteer acquired during a 26-s breath-hold compared with images acquired with a parallel acceleration of 2, using the three-point self-calibration method described above (Fig. 25.8c). Comparable image quality has been achieved in nearly half the scan time.

Figure 25.12 shows T2-weighted IDEAL-FSE and T1-weighted IDEAL-SPGR images of the ankle acquired using the two-point method without paral- lel acceleration, compared with conventional fat-sat- urated methods. Without parallel imaging, the two- point approach offers a modest, but important 30%

scan time reduction.

Contrast-enhanced breast imaging with MRI has become a gold standard for the diagnosis of inva-

Fig. 25.11a–f. CINE SSFP water-only images shown at end-diastole a,d, mid-systole b,e and end-diastole c,f acquired without a–c and with d–f parallel acceleration (R=2) using the self-calibrated three-point method. Although image quality is comparable, the scan time is nearly half using the accelerated method

b c a

d e f

sive breast cancers, with sensitivity and specifi city exceeding 90% (Agostan et al. 2001). Robust fat sup- pression is of paramount importance when imaging the breast, which is largely comprised of fatty tissue that can obscure enhancing lesions. Imaging of both breasts simultaneously is highly desirable to avoid two separate exams with repeated contrast injections.

Although conventional fat saturation works relatively well for imaging one breast, which is relatively easy to shim for magnetic fi eld inhomogeneities, uniform suppression of fat for bilateral breast imaging is very challenging because it is very diffi cult to achieve

uniform fi eld homogeneity over both breasts. For this reason, water-fat separation methods that com- pensate for fi eld inhomogeneities are well suited for bilateral breast imaging application.

The increased scan time of three-point water-fat separation methods may be prohibitive, however, and reduction of scan times with parallel imaging would reduce or eliminate the scan time penalty and provide uniform fat-suppression. Figure 25.13 shows an exam- ple from a pre-contrast three-point 3D-IDEAL-SPGR acquisition in the breast of a normal volunteer, using an acceleration of three (R=3). Excellent separation of

Fig. 25.12a–d. Sagittal imaging of the foot with a two-point T2-weighted IDEAL-FSE and c two- point IDEAL-SPGR.

Conventional fat-satu- rated T2-weighted FSE b and SPGR d imaging is shown for comparison and reveals large areas of failed fat suppression (arrows). Images courtesy of Anja CS. Brau, PhD, GE Healthcare

Fig. 25.13a–c. Axial three-point 3D-IDEAL-SPGR recombined a, water b and fat c breast images acquired in a normal volunteer using a reduction factor of three (R=3). SNR performance and scan time of this acquisition are comparable to conventional fat-saturated imaging, but with uniform separation of water and fat across both breasts. Scan time was 2:10 min for 48 slices covering both breasts with 512×256 matrix size and 0.6×1.2×4 mm

resolution. TR/TE=10.6/4.0 ms

c b

a

c

b a

d

water and fat is seen across the entire image in images that were acquired in approximately the same scan time as a conventional fat-saturated image.

Although high-resolution morphological imag- ing of breast lesions is very important in the charac- terization of breast malignancies, dynamic contrast- enhanced (DCE) imaging of the breast has been shown to improve the specifi city greatly in the detection of

Fig. 25.14a–c. Dynamic contrast-enhanced 3D-IDEAL-SPGR water-only single-point images obtained with an acceleration of three (R=3), before a and after b,c contrast injection. Scan time of each phase was 43 s for 48 slices covering both breasts with 512×256 matrix size and 0.6×1.2×4 mm

resolution. TR/

TE=10.6/4.0 ms

invasive breast cancers (Agostan et al. 2001). However, DCE imaging is a highly challenging application: the optimal approach provides complete coverage of both breasts with high spatial and high temporal resolution with uniform fat-suppression. The use of single-point water-fat separation methods in combination with par- allel imaging may be able to satisfy these requirements.

For example, the images shown in Fig. 25.14 (the same volunteer as shown in Fig. 25.13) were acquired during dynamic contrast injection. Both breasts were imaged at 1.5 T with a single-point 3D-IDEAL-SPGR approach combined with a parallel acceleration of three (R=3).

Images with 0.6×1.2×4.0 mm

resolution were acquired through the entire breast every 43 s using a 512×256×48 matrix. The scan time is approximately one third of a conventional non-accelerated fat-saturated scan.

Dynamic contrast-enhanced imaging of the liver is also essential for the characterization of liver lesions such as hepatocellular carcinomas and metastatic dis- ease. Rapid, high-resolution breath-hold imaging of the abdomen with uniform suppression of fat is essen- tial. Figure 25.15 shows breath-held 3D-IDEAL-SPGR water and fat images acquired with an acceleration of two (R=2) in two different patients. One patient is normal and the second patient has diffuse fatty infi l- tration of the liver (steatohepatitis). Separate water and fat images indicate uniform separation of water and fat. These images can be recombined into calcu- lated “in-phase” and “out-of-phase” images, analogous to conventional in-phase and out-of-phase imaging usually acquired as a separate acquisition. Although the fatty infi ltration of the second patient can be seen in the fat image (Fig. 25.15f), it is more apparent in the recombined out-of-phase image (Fig. 25.15h).

Dynamic contrast-enhanced imaging of the liver can also be performed with the accelerated one-point methods described in this chapter. Figure 25.16 shows breath-held 3D-IDEAL-SPGR images of the liver acquired at 3.0 T using the single-image approach.

Images acquired with an accelerated three-point method are shown for comparison. Both approaches used a parallel acceleration of two (R=2).

25.4 Conclusion

Chemical-shift-based water-fat separation methods such as conventional three-point “Dixon” methods and IDEAL are capable of providing high-quality

a

b

c

images with uniform and robust fat suppression despite the presence of fi eld inhomogeneities, even in challenging areas of the body. Although these methods are SNR effi cient, their main drawback is the three-fold increase in the minimum scan time.

Applications that require short minimum scan times, such as cardiac and abdominal breath-held imaging, as well as dynamic contrast-enhanced imaging of the

Fig. 25.15a–h. Breath-held contrast-enhanced 3D-IDEAL-SPGR water a,e, fat b,f, recombined in-phase c,g and recombined out of phase d,h images acquired at 1.5 T with an acceleration of two (R=2). The patient shown in the top row is normal; however, the patient in the bottom row has diffuse fatty infi ltration of the liver seen as low-level signal in the fat image f, but most apparent as signal drop in the calculated out-of-phase image h

Fig. 25.16a–f. Dynamic contrast-enhanced image of the liver acquired at 3.0 T with 3D-IDEAL-SPGR using the one-point method a-c during the arterial a, portal venous b and delayed c phases after the injection of contrast. Three-point images d–f show comparable image quality, but with three times the scan time. Both methods used a parallel acceleration of two (R=2)

c b

a d

g f

e h

c b

a

f e

d

liver or breast, would benefi t from water-fat separa- tion methods if the scan times could be reduced.

Fortunately, parallel imaging can be exploited to

reduce the amount of data required to reconstruct

an image. Importantly, these parallel imaging meth-

ods preserve the phase content of the complex source

images, permitting the combination of parallel imag-

ing with water-fat separation methods. These methods

are highly complementary: parallel imaging allevi- ates the scan time penalty of the separation method, while the SNR penalty of parallel imaging is offset by the high SNR performance of the water-fat decompo- sition. In this work we reviewed the combination of parallel imaging with self-calibrating three-point and two-point methods, as well as with a one-point method well suited for dynamic contrast-enhanced imaging.

Acknowledgments. The authors wish to thank Chris Beaulieu, Anja Brau, Jean Brittain, Garry Gold, Jane Johnson, Norbert Pelc, Angel Pineda, Neil Rofsky, Ann Shimakawa and Huanzhou Yu for their gener- ous assistance.

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