Accepted Manuscript

Faster Imaging with a Portable Unilateral NMR Device

Asaf Liberman, Elad Bergman, Yifat Sarda, Uri Nevo

PII: S1090-7807(13)00084-0

DOI: http://dx.doi.org/10.1016/j.jmr.2013.03.009

Reference: YJMRE 5190

To appear in: Journal of Magnetic Resonance

Received Date: 24 December 2012

Revised Date: 18 March 2013

Please cite this article as: A. Liberman, E. Bergman, Y. Sarda, U. Nevo, Faster Imaging with a Portable Unilateral

NMR Device, Journal of Magnetic Resonance (2013), doi: http://dx.doi.org/10.1016/j.jmr.2013.03.009

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Faster Imaging with a Portable Unilateral NMR Device

Asaf Liberman, Elad Bergman, Yifat Sarda, and Uri Nevo∆

The Iby and Aladar Fleischman Faculty of Engineering, Tel Aviv University, Tel Aviv 69978, Israel ∆Correspondence to: nevouri@tau.ac.il

March 2013

Keywords: Fast imaging; Unilateral NMR; Compressed sensing; Desktop NMR;

Portable NMR; Fast spin echo.

Abstract

Unilateral NMR devices are important tools in various applications such as non-

destructive testing and well logging, but are not applied routinely for imaging,

primarily because B0 inhomogeneity in these scanners leads to a relatively low signal

and requires use of the slow single point imaging scan scheme. Enabling high quality,

fast imaging could make this affordable and portable technology practical for various

imaging applications as well as for new applications that are not yet feasible with

MRI technology.

The goal of this work was to improve imaging times in a portable unilateral NMR

scanner. Both Compressed Sensing and Fast Spin Echo were modified and applied to

fit the unique characteristics of a unilateral device. Two printed phantoms, allowing

high resolution images, were scanned with both methods and compared to a

standard scan and to a low pass scan to evaluate performance. Both methods were

found to be feasible with a unilateral device, proving ways to accelerate single point

imaging in such scanners. This outcome encourages us to explore how to further

accelerate imaging times in unilateral NMR devices so that this technology might

become clinically applicable in the future.

Introduction

Imaging with Open NMR Scanners

Unilateral portable NMR [1-4] devices are used mainly for non-destructive testing

applications where imaging capabilities are not critical. Other applications of low-

NMR and open architecture NMR scanners include oil well logging, food analysis and

quality control [5-8], elastomer quality control [5, 9, 10], cultural heritage [11], and

skin and tendon profiling [12, 13].

Slice selective lateral imaging with open NMR scanners is challenging due to the

limitations of their geometry [14]. The open geometry generates an inhomogeneous

magnetic field with a strong, constant gradient defining non-flat and extremely

thin slices, restricting the acquired signal. A sensitive volume that is sufficiently flat

for imaging can be generated only at a defined distance from the magnet, severely

restricting the device's penetration depth. The constant gradient also leads to a

significant attenuation of the diffusion-weighted signal. Frequency encoding cannot

be utilized in the device under static field conditions, which introduce an

overwhelming read encoding gradient in the direction and cause rapid dephasing

of the signal [15].

2D lateral imaging can thus be done by applying a pure spin echo phase encoding

in both planar directions (also known as Single Point Imaging, SPI), as implemented

by Casanova et al. on the NMR-MOUSE [16]. This procedure limits imaging since scan

times with SPI scale as (for an sampling matrix).

To improve the Signal to Noise Ratio (SNR), Perlo et al. developed a CPMG-like

pulse sequence where a train of nominal pulses generates a train of echoes,

which are accumulated [17]. Introducing such a sequence in the presence of a strong

static magnetic gradient and grossly inhomogeneous and fields causes a

severe distortion in the phase encoding, so that one of the components of the echo

signal goes to zero after a transient period. The proposed solution is to obtain the

full complex signal in two experiments, by phase-cycling the RF refocusing pulse in

two orthogonal directions to acquire a different complex component in each step

[17].

This work demonstrates the application of two methods, Compressed Sensing

(CS) and Fast Spin Echo (FSE) for the purpose of accelerating imaging on a unilateral

low field NMR scanner. Both methods can overcome the basic restriction posed by

SPI: CS allows a subset of the k-space to be read while ensuring a high-quality

reconstruction; FSE enables more than one coefficient to be read following a single

excitation. By overcoming the limitations imposed by SPI, the minimum scan times

previously thought to be required with such a portable unilateral NMR device can be

reduced.

Theory

Compressed Sensing (CS)

Compressed Sensing is a novel acquisition and reconstruction scheme that uses

the sparsity of natural signals together with a non-linear algorithm to provide a high

quality reconstruction of a signal with significantly low sampling rates/percentages.

CS dictates an incoherent sub-sampling of the signal while demanding a

reconstruction based on norm error minimization between the initially sub-

sampled, zero filled image and the reconstructed image, both represented in a

sparse domain. The sub-sampling of the signal described allows for scan time

reduction with little compromise to the image quality.

As CS complements the scanning regime of MRI, combining the two has been

heavily researched in recent years. Lustig et al. have implemented CS in a clinical MRI

device to undersample the k-space, thus reducing scan times (or, interchangeably, to

improve scan resolution) [18]. Parasoglou et al. have combined CS with a single point

imaging scanning scheme to more rapidly image dynamic processes with short

[19].

Fast Spin Echo (FSE)

Fast Spin Echo is an imaging method engineered to acquire more than one k-line

following a single excitation [20]. FSE accelerates the basic spin echo imaging by

applying a CPMG sequence with an additional phase encoding gradient prior to each

echo. Each echo thus adds an additional k-line, leading to the acquisition of

significant segments of the k-space following a single excitation. In cases where is

long enough, full coverage of the k-space can be achieved in one shot (i.e., following

a single pulse) [21].

Materials and Methods

Hardware

NMR-scanner

The NMR-MOUSE scanner (ACT GmbH , Aachen, Germany) was used in this work.

This scanner has a permanent magnet combined with a mounted RF coil system and

gradient coils. The magnet is composed of two permanent rectangular blocks set on

an iron yoke. This setup provides a static magnetic field of about at a distance

of from the magnet (Larmor frequency of ), with a strong

gradient of along the z-direction, away from the face of the magnet. Combining

the strong gradient with hard RF pulses produces selective excitation of thin flat

slices.

An LVC-7700 amplifier (AE Techron Inc., Elkhart, IN, USA) was used to power the

gradient coils and generate pulsed gradients along the and directions. A surface

RF coil is positioned on top of the magnet and gradient coils, and was used to excite

and detect the NMR signal. A spectrometer (Magritek LTD, Wellington, New

Zealand) controls the operation of the gradient coils and the RF coil (input/output).

Phantoms

Two circular-shaped phantoms were used in this work: a phantom containing two

zero shapes and two curved lines and a phantom containing several rectangles and

several circles (hereinafter "phantom A" and "phantom B", respectively) (Fig. 1). The

phantoms contain fine features with width as low as , and a size less than

across, to be scanned with an -diameter coil. The phantoms were filled

with glycerol, a suitable material for testing in a unilateral NMR scanner due to its

short and times and high viscosity, which leads to low self-diffusion.

Fig. 1: Phantom diagrams. (a) Phantom A. Features are relatively scattered and

have different shapes and orientations to test the quality of reconstruction. (b)

Phantom B. Features are concentrated near the center of the FOV. Smallest feature

width is about .

Implementation of CS in a unilateral scanner

The Daubechies-6 wavelet is used as the sparsifying transform for the

reconstructions. The algorithms for the CS acquisition and reconstruction were

based on the work of Lustig et al. as described in [18], and using the supplementary

freeware codes attached (http://www.eecs.berkeley.edu/~mlustig/Software.html).

Since the sampled domain in MRI is the spatial frequency domain, most of the

information in scans of natural images will be concentrated in the center of the k-

space. Thus, by using a probability density function (PDF) that decreases gradually

according to a power of distance from the center with a predetermined full sampling

radius, a higher concentration of center sampling is prioritized, while simultaneously

assuring the incoherence required for the CS acquisition [18].

Implementation of an FSE-like sequence in a unilateral scanner

As opposed to the implementation of FSE in common MRI, the prototype FSE-like

sequence implemented in a unilateral NMR scanner in this work scans two k-space

coefficients following a single excitation (instead of several k-space lines as in

common MRI systems). (We limited the number of coefficients per excitation to

accumulate a sufficient signal per k-space coefficient, given the low signal

amplitude). Phase encoding in FSE is generated by addition of a gradient blip (See

Fig. 2 for a scheme of the pulse sequence). Time to echo (TE) is usually kept at a

minimum to maximize the generated number of echoes [17], nevertheless, to avoid

gradient overshoots and eddy currents, TE was extended in these experiments. It

should be noted here that attempting to extend the TE only for the application of the

blipped gradient pulses results in distortion of the echo train after the pulses, due to

the increased influence of stimulated echoes.

Within the echo train ( echoes), the first coefficient was encoded based on the

integration of the first echoes, and the second coefficient on the remaining

echoes (omitting from integrations the two echo periods used for

encoding of the second coefficient). The exact value of was calculated by

demanding equal energy for both parts of the FID, given the apparent (found in a

preliminary measurement).

Fig. 2: The Fast Spin Echo pulse sequence in a unilateral NMR. The sequence is

divided into four periods: the first encoding period, the first CPMG period, the

second encoding period (gradient blips), and the second CPMG period. The first

period is used to encode the phase of the initial coefficient using gradients employed

in the x and y directions (The encoding time to echo, TEe, is indicated). The second

period is used to accumulate data to be read as the first k-space coefficient with a

series of pulses (The detection time to echo, TEd is indicated). The third period is

used to shift phase to encode the next coefficient using gradient blips during two TE

periods. The fourth period is used to accumulate data to be read as the second k-

space coefficient with a series of pulses.

As noted, the CPMG-like sequence in the unilateral scanner nulls one of the

coefficients of the complex signal after a transient period, prompting the use of a 2-

step phase cycling scheme (repetition of the sequence in two orthogonal directions)

to read both components of the magnetization (real and imaginary) [17]. Since in FSE

two k-space coefficients are read, an extended 4-step phase cycling scheme is

employed to gather all four components of magnetization (see Table 1 and Fig. 3 for

the phase cycling scheme and extraction of the coefficients, respectively). By

alternating the post-blip orthogonal direction of the refocusing RF pulses, all four

combinations of the products of the pre-blip and post-blip phases (complex

components) are encoded. Using trigonometric relations between the encoded

products, the accumulated phases are calculated and the second k-space coefficient

is extracted.

Fig. 3: Recovery of complex coefficient values using the phase cycling scheme. The

real component of the first coefficient is obtained by applying pulses and the

imaginary component is obtained by applying pulses. In addition to this an

additional cycling is applied for the pulses after the gradient 'blip' ( and ).

Altogether the four combinations of acquired components result in reconstruction of

two complex k-space coefficients.

Experiments

Feasibility and performance of CS in a unilateral NMR scanner

To validate the real-time feasibility of CS scans with a low-SNR unilateral NMR and

to compare its performance to that of a basic undersampling method, both

phantoms were scanned with a 32 X 32 matrix (FOV of , spatial

resolution of ). Five CS scans were performed in real time, with sampling

percentages of 30%, 40%, 50%, 60%, and 70% and full sampling radii of 0.41, 0.48,

0.55, 0.6, and 0.67 respectively. CS scanning schemes for phantom A are shown in

Fig. 4l-p. Scans for phantom A were performed with parameters ,

with 350 echoes acquired, and

, which resulted in a total experimental time of 48

minutes per image. Scans for phantom B were performed with similar parameters,

except for and 500 echoes acquired, which resulted in a total

experimental time of 34 minutes per image. Times for CS scans can be calculated

accordingly, with the shortest CS scan for phantom B being the one with a sampling

percentage of 30%, which lasted 10 minutes. Full sampling radii were chosen based

on a preliminary experiment, which demonstrated an advantage for scans acquired

with a high (yet not maximal) full sampling radius.

Low Pass (LP), a basic undersampling scheme, was used for comparison. Each CS

scan was compared to an LP scan with an equal sampling percentage (and with equal

scan duration), similar to approaches in previous works [18]. All LP scans were

performed by extracting a square-shaped subset of the coefficients from the center

of a single fully scanned k-space. All scans were performed with two averaging

repetitions (each includes phase cycling, resulting in four excitations per experiment)

and compared via RMS error (RMSE) to a fully scanned k-space with 12 averaging

repetitions. RMSE is given by the following equation,

(1.1)

where is the reconstructed image, and is the gold standard reference

image.

Feasibility and performance of an FSE-like sequence in a unilateral NMR scanner

To validate the feasibility of the FSE-like sequence in a unilateral device, both

phantoms were scanned with a pixel matrix. The phantoms were scanned

twice: a standard full sampling scan with 24 averaging repetitions and an FSE scan

with 48 averaging repetitions (lasting the same amount of time and acquiring the

same number of total echoes as the standard scan). Scans for phantom A were

performed with parameters , to allow for blip gradients

with duration of to be inserted, with 200 echoes acquired,

and . Scans for

phantom B were performed with similar parameters except for and

450 echoes acquired. Images are presented in the results section.

The initial accuracy of the FSE method was verified by joining two k-spaces

scanned in opposite spatial directions resulting in the entire k-space being composed

of post-blip coefficients. The k-space was transformed into an image that was

validated relative to a standard image.

Results

Feasibility and performance of CS in a unilateral NMR scanner

Figure 4 presents the imaging results of phantom A. Images clearly show an

improvement in reconstruction with a higher sampling percentage. It is also evident

CS is superior to LP sampling, which blurs the image by cutting high spatial frequency

information (Fig. 4b-d), effectively causing a loss of sharpness. CS undersampling, on

the other hand, better preserves even small details and contrast, and reduces the

amount of visible noise in the imaged object (Fig. 4g-i).

Sampling 50% (or more) of the coefficients with CS (Fig. 4i-k) is sufficient to

recover most of the fine features of the 100% sampled image. On the other hand, LP

sampling of 50%-60% of the coefficients yields mostly recognizable features, but

with blurry edges and lower contrast. Sampling of 70% of the coefficients with both

methods produces good images. RMS error results are lower for CS sampling

throughout the experiment relative to the gold standard scan (Fig. 4a).

Fig. 4: Results of the experiment comparing CS and LP reconstructions of phantom

A with similar scan percentages. (a) 100% sampled image with 12 averaging

repetitions. (b-f) Low pass reconstructed images: sampling of 30%, 40%, 50%, 60%,

and 70% of the coefficients respectively. (g-k) Compressed sensing reconstructed

images: sampling of 30%, 40%, 50%, 60%, and 70% of the coefficients respectively.

RMS error relative to the high-averaged, 100% sampled image is indicated below

each image. (l-p) Scanning masks for CS scans: white pixels were scanned while black

pixels were skipped.

Figure 5 presents the phantom B imaging results. Both CS and LP lead to

reconstructed images that improve as the sampling percentage increases. CS

reconstructions are consistently less noisy, are sharper, and are more detailed (Fig.

5g-k).

Fine features in phantom B are already clear and sharp with only 30% of the

coefficients sampled with a CS acquisition (Fig. 5g), while LP sampling yields

identifiable features that are very blurry (Fig. 5b-d). It is important to note that

above 50% sampling (Fig. 5i-k) there is almost no improvement in feature sharpness

(although block artifacts are visible in lower sampling), indicating that there is no

need to sample more than 50% of the coefficients in such cases. A difference is

noticeable between the 50%-and-above scans with LP sampling (Fig. 5d-f), where the

boundaries are sharper, but the images are noisier.

RMS error results are lower for CS sampling throughout the experiment relative

to the gold standard scan (Fig. 5a).

Fig. 5: Results of the experiment comparing CS and LP reconstructions of phantom

B with a similar scan percentage. (a) 100% sampled image with 12 averaging

repetitions. (b-f) Low pass reconstructed images: sampling of 30%, 40%, 50%, 60%,

and 70% of the coefficients respectively. (g-k) Compressed sensing reconstructed

images: sampling of 30%, 40%, 50%, 60%, and 70% of the coefficients respectively.

RMS error relative to the high-averaged, 100% sampled image is indicated below

each image.

The fidelity provided by CS is further demonstrated by the one-dimensional cross

section profiles of two representative lines (lines 9 and 18) from the 30% and 50%

sampling reconstructions displayed in Figure 6. CS reconstructions (darker grey,

triangle markers, dash-dotted line) are indeed closer to the gold standard

reconstruction (black) relative to the LP reconstructions (lighter grey, circular

markers, dashed line). Edges in CS reconstructions are steeper and better preserved.

As expected, cross sections of images scanned with 50% of the coefficients are closer

to the gold standard (fig. 6b, d) image, relative to images scanned with 30% of the

coefficients (fig. 6a, c). (Note that most, but not all, one-dimensional profiles show

the superior quality of the CS over the LP images).

Fig. 6: Cross section results for the experiment comparing CS and LP phantom B

reconstructions with a similar scan percentage for two representative lines (the 9th

and 18th

). (a) 100% sampled image with 12 averaging repetitions. (b-c) Cross section

results for the 9th

line for GS-, CS-, and LP-reconstructed images with 30% and 50% of

the coefficients, respectively. The solid black line with rectangular markers

represents the GS; the dashed light grey line with circular markers represents the LP;

and the dark grey dash-dotted line with triangular markers represents the CS. (d-e)

Cross section results for the 18th

line for GS-, CS-, and LP-reconstructed images with

30% and 50% of the coefficients, respectively.

Feasibility and performance of FSE in a unilateral NMR scanner

Figure 7 presents the imaging results for phantoms A and B. An FSE scan with 48

averaging repetitions (Fig. 7b for phantom A, Fig. 7d for phantom B) is compared to a

standard scan with 24 averaging repetitions (Fig. 7a for phantom A, Fig. 7c for

phantom B) resulting in an equal scan time. The total number of echoes acquired is

equal for these scans. Both images present high quality and are very similar,

although the FSE image has a higher relative noise level. FSE is thus feasible in a

unilateral NMR.

Fig. 7: FSE experiment results with phantoms A and B. (a) Standard sampling

image with 24 averaging repetitions for phantom A. (b) FSE sampling image with 48

averaging repetitions for phantom A. (c) Standard sampling image with 24 averaging

repetitions for phantom B. (d) FSE sampling image with 48 averaging repetitions for

phantom B.

Discussion

This work demonstrates the feasibility of using techniques to accelerate imaging

performed using a unilateral NMR device. The low SNR caused by the

inhomogeneous magnetic field is a limiting factor in a unilateral device. The high

number of averaging repetitions needed for an acceptable SNR and the inability to

apply lateral frequency encoding gradients force long scan times. Our goal here was

not to generate scan times sufficiently short for clinical or biological use, since these

depend on hardware and will improve over time, but to demonstrate that significant

acceleration of pure phase-encoding imaging times is feasible and to develop specific

ways to do so.

Compressed Sensing

While the pure phase-encoding scan scheme is the main drawback in a scan

performed with a unilateral NMR device, this scheme actually is beneficial for CS as it

enables freedom in the creation of the scanning trajectory. On the other hand, the

limitation it poses may be overcome by using CS to scan fewer of the k-space

coefficients, thus significantly reducing scan times.

It is important to note that the limitations of a unilateral NMR device still apply,

regardless of the offered improvements. The above characteristics (small FOV, small

sampling matrix, low SNR) together with the currently limited field of view leave

little headroom for reducing the sampling percentage as each reduction might

potentially cause distortion. For example, attempting to transform these images to

the wavelet domain and then disposing of the weakest coefficients (thus executing a

basic method of compression) fails when a low percentage of the coefficients is kept.

Fast Spin Echo

An FSE-like pulse sequence was already suggested and applied by Casanova et al.

[22] in 2003. By applying two gradient pulses with opposite polarization before and

after the echo formation, an independent phase encoding was performed on each

echo, leaving a zero phase shift, prior to the next echo. The method presented here

offers several improvements over this pulse sequence: (a) The power dissipated by

the gradients is minimized since the blip gradients are only used to shift the phase

(from one k-space coefficient to an adjacent one), as opposed to dephasing and

rephasing it entirely. (b) The sensitivity is greatly increased due to the nominal

pulse train which maximizes the number of echoes read during the experiment. This

is possible due to the significantly shorter TE time, which is possible since it must

contain only a gradient blip, rather than two full gradients and an acquisition period.

(c) The number of echoes dedicated to the acquisition of each coefficient is

calculated based on the of the materials, thus avoiding a weighted

acquisition.

The implementation of FSE in a unilateral NMR scanner required the use of a 4-

step phase cycle to extract the components of both complex coefficients. With the

current application, extension of the FSE scan to coefficients encoded in each

CPMG train will demand longer phase cycling schemes ( cycles for coefficients).

This extended scheme can, in principle, be reduced back to a 2-step scheme by, for

example, estimating the complex magnetization components of a primary set of

echoes, and then using this data as a priori information for estimating the secondary

set of echoes (see fig. 3). Currently, small errors that arise in the factors of such

estimation due to low sensitivity lead to severe image distortion. However, a

reconstruction scheme that improves sensitivity might enable the use of this

shortened phase cycling scheme. FSE should be further optimized with a higher

amplitude gradient blip that will minimize the length of TE.

Conclusions

Imaging in a unilateral NMR device with implementation of CS was demonstrated.

Even sampling of as low as 50% of the coefficients produced clean, sharp images. A

Fast Spin Echo-like sequence was also implemented in a unilateral NMR,

demonstrating the feasibility of further accelerating imaging time. Since FSE and CS

reduce imaging times in entirely different ways, a combination of the two techniques

is also possible.

Unilateral NMR devices are not yet feasible for biomedical imaging applications.

Sensitivity requires improvement, penetration depth should be increased, and

imaging times should be further reduced. With the above reported methods, small

objects with details of the order of can be scanned in ~10 minutes for a

pixel image. A combination of hardware and software improvements may

lead to a further reduction in imaging times.

Acknowledgments

This study was supported by an IRG grant (MMDTIAN) of the Marie Curie Foundation

(EU). UN acknowledges support by the Colton family scholarship. We wish to thank

Mr. Ezra Shaked, as well as members of the ACT and Magritek companies, for

continuous technical support.

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Tables:

Table 1: Phase cycling scheme for FSE pulse sequence. Using a 4-step phase cycle

four complex components of two coefficients are read.

Phase \ Cycle No. 1st 2nd 3th 4th

phase x x x x

1st CPMG phase x x y y

2nd

CPMG phase x y x y

Figure Captions:

Fig. 1: Phantom diagrams. (a) Phantom A. Features are relatively scattered and

have different shapes and orientations to test the quality of reconstruction. (b)

Phantom B. Features are concentrated near the center of the FOV. Smallest feature

width is about 0.3mm.

Fig. 2: The Fast Spin Echo pulse sequence in a unilateral NMR. The sequence is

divided into four periods: the first encoding period, the first CPMG period, the

second encoding period (gradient blips), and the second CPMG period. The first

period is used to encode the phase of the initial coefficient using gradients employed

in the x and y directions (The encoding time to echo, TEe, is indicated). The second

period is used to accumulate data to be read as the first k-space coefficient with a

series of pulses (The detection time to echo, TEd is indicated). The third period is

used to shift phase to encode the next coefficient using gradient blips during two TE

periods. The fourth period is used to accumulate data to be read as the second k-

space coefficient with a series of pulses.

Fig. 3: Recovery of complex coefficient values using the phase cycling scheme. The

real component of the first coefficient is obtained by applying pulses and the

imaginary component is obtained by applying pulses. In addition to this an

additional cycling is applied for the pulses after the gradient "blip" ( and ).

Altogether the four combinations of acquired components result in reconstruction of

two complex k-space coefficients.

Fig. 4: Results of the experiment comparing CS and LP reconstructions of phantom

A with similar scan percentages. (a) 100% sampled image with 12 averaging

repetitions. (b-f) Low pass reconstructed images: sampling of 30%, 40%, 50%, 60%,

and 70% of the coefficients respectively. (g-k) Compressed sensing reconstructed

images: sampling of 30%, 40%, 50%, 60%, and 70% of the coefficients respectively.

RMS error relative to the high-averaged, 100% sampled image is indicated below

each image. (l-p) Scanning masks for CS scans: white pixels were scanned while black

pixels were skipped.

Fig. 5: Results of the experiment comparing CS and LP reconstructions of phantom

B with a similar scan percentage. (a) 100% sampled image with 12 averaging

repetitions. (b-f) Low pass reconstructed images: sampling of 30%, 40%, 50%, 60%,

and 70% of the coefficients respectively. (g-k) Compressed sensing reconstructed

images: sampling of 30%, 40%, 50%, 60%, and 70% of the coefficients respectively.

RMS error relative to the high-averaged, 100% sampled image is indicated below

each image.

Fig. 6: Cross section results for the experiment comparing CS and LP phantom B

reconstructions with a similar scan percentage for two representative lines (the 9th

and 18th

). (a) 100% sampled image with 12 averaging repetitions. (b-c) Cross section

results for the 9th

line for GS-, CS-, and LP-reconstructed images with 30% and 50% of

the coefficients, respectively. The solid black line with rectangular markers

represents the GS; the dashed light grey line with circular markers represents the LP;

and the dark grey dash-dotted line with triangular markers represents the CS. (d-e)

Cross section results for the 18th

line for GS-, CS-, and LP-reconstructed images with

30% and 50% of the coefficients, respectively.

Fig. 7: FSE experiment results with phantoms A and B. (a) Standard sampling

image with 24 averaging repetitions for phantom A. (b) FSE sampling image with 48

averaging repetitions for phantom A. (c) Standard sampling image with 24 averaging

repetitions for phantom B. (d) FSE sampling image with 48 averaging repetitions for

phantom B.

Tables:

Table 1: Phase cycling scheme for FSE pulse sequence. Using a 4-step phase cycle

four complex components of two coefficients are read.

Phase \ Cycle No. 1st

2nd

3th

4th

phase x x x x

1st

CPMG phase x x y y

2nd CPMG phase x y x y

Article highlights

• Two techniques were used to enable faster imaging in a Unilateral NMR device.

• Compressed Sensing was used to sub-sample the k-space.

• Fast spin echo was used to acquire two coefficients following each excitation.

• Reduction in scan times caused relatively minor deterioration of image quality.

• Further hardware and software improvements will facilitate imaging applications.