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1.5T / 3.0T Fast spectroscopic imaging

ExamCard
de Kok, Wendy Philips Healthcare Philips Global

ExamCards for fast spectroscopic imaging for both 1.5T and 3.0T are included:

ExamCard Fast spectroscopic imaging for 1.5T (Release 2):

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ExamCard Fast spectroscopic imaging for 3.0T (Release 2):

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ExamCard Overview
Scan 1TSI_TE 288ms
Scan 2SENSE_2DSI_TE 288ms
Scan 3SENSE_2DSI_TE 144ms
Scan 4SENSE_2DSI_TE shortest
Scan 5SENSE_TSI_TE 288ms

Introduction

Scan time in spectroscopic imaging is generally long, as spatial encoding is only performed by means of phase encoding. Frequency encoding, as applied in MR imaging, is not used.

The scan time in spectroscopic imaging is TR multiplied by number of phase encoding steps (in both directions) and NSA.

Note that use of K-space shutter and the acquisition time of B0-measurements are not included in this calculation.

 

As an example, the scan time in a sequence with matrix 22 x 18, TR 2000 ms and 1 NSA equals 792 s = 13:12 min.

 

This document describes ways to reduce scan time in spectroscopic imaging. It contains an ExamCard with several fast spectroscopic imaging procedures, for both 1.5T and 3.0T.

2DSI, 1.5T matrix 22 x 18, TR / TE 2000 / 288 ms, scan time 13:12 min. One voxel selected for display
2DSI, 1.5T
matrix 22 x 18, TR / TE 2000 / 288 ms, scan time 13:12 min. One voxel selected for display

Spectral resolution

A very important parameter in MR spectroscopy is the spectral resolution, which is the smallest frequency difference that can be detected in a spectrum. It is expressed in Hz / point and is determined by the total length of the time domain data acquisition period, or the readout time. A longer readout time leads to increased spectral resolution.

 

As an example, a very common readout time is 512 ms, leading to a spectral resolution of ~ 2Hz.

 

Spectral resolution should be sufficient to separate the signals of the various metabolites in the tissue under examination. For brain spectroscopy, the main metabolites of interest are NAA, lactate, creatine and choline. The frequency difference of creatine and choline is very small: a spectral resolution of ~ 4Hz at 1.5T (or ~ 8 Hz at 3.0T) is required to separate the signals.

Turbo Spectroscopic Imaging

In Turbo Spectroscopic Imaging, additional refocussing pulses are added in the sequence to generate a train of echo signals. Each echo is sampled while applying a different phase encoding step (like Turbo Spin Echo).

 

The echo spacing is the time in which the respective echoes can be sampled. It should be long enough to achieve good spectral resolution. Therefore, the turbo factor should be low, to make sure that signal decay is not complete before the last echo of the TSI-train is sampled.

 

As an example, commonly used echo spacing is 288 ms at 1.5T and 3.0T (or 144 ms at 3.0T) in combination with a turbo factor of 3, resulting in a scan time reduction by a factor of 3.

Spectral resolution is reduced to ~ 4 Hz (or ~ 8 Hz if echo spacing is reduced to 144 ms).

 

As long echo spacing is required, turbo spectroscopic imaging can only be combined with long TE. Furthermore, to maximally use the available sample time, a full echo must be sampled, which always leads to modulus spectra. Important phase information (sign of lactate signal at TE = 144 ms) is not available.

T2-blurring and effect on metabolite ratios

If the echo train length is long, blurring may occur if T2-decay decreases the amplitude of the respective echoes to a large extent.

In a TSI-scan with turbo factor 3, the first echo is measured at 288 ms, the second echo at 576 ms, and the third echo even at 864 ms. If the signal intensity is low in the last echo, the inspected spatial resolution in the metabolite map might be lower than expected.

 

Additionally, if T2-relaxation rates are different for the various metabolites, the signal contribution per metabolite changes over the respective echoes. Measured metabolite ratios might therefore be different in a TSI scan as compared to a non-TSI scan, where all echo signals are sampled at the same echo time. This effect is visible in the images below, where three single voxel scans, acquired at TE 288, 576 and 864 ms respectively, are displayed. The ratio cho/cre and NAA/cre increases at longer TE.

SVS TE 288 ms, 3.0T SVS TE 576 ms, 3.0T SVS TE 864 ms, 3.0T
SVS TE 288 ms, 3.0T
SVS TE 576 ms, 3.0T
SVS TE 864 ms, 3.0T

SENSE Spectroscopic Imaging

SENSE can be used to undersample K-space in the phase-encoding direction, in order to reduce scan time by the SENSE factor applied. As phase encoding is applied in two in-plane directions in spectroscopic imaging, SENSE reduction can also be applied in two in-plane directions simultaneously. SENSE Spectroscopic Imaging can therefore easily be performed in a scan time that is equal to the scan time of a Turbo Spectroscopic Imaging scan, while maintaining high spatial resolution and reduction of T2-blurring effects.

 

As an example, if SENSE factor ~3.5 is applied to the scan that is described in the introduction, the scan time is reduced to ~ 4:30 min, which is equal to the scan time of a Turbo Spectroscopic Imaging scan.

 

SENSE CSI can be applied with any echo time, short or long, and half echo acquisition is possible, which means that real spectra can be acquired. Phase information is available. However, SNR will be reduced as compared to a non-SENSE spectroscopic imaging scan.

 

The image examples below show a comparison of TSI and SENSE-CSI for TE = 288 ms and for TE = 144 ms. The latter example clearly shows the reduced spectral resolution in the TSI scan.

TSI, TE 288 ms, 3.0T TSI factor 3, TR 2000 ms, scan time 4:24 minSENSE CSI, TE 288 ms, 3.0T SENSE factor 3.7, TR 2000 ms, scan time 4:28 min
TSI, TE 288 ms, 3.0T
SENSE CSI, TE 288 ms, 3.0T
TSI factor 3, TR 2000 ms, scan time 4:24 min
SENSE factor 3.7, TR 2000 ms, scan time 4:28 min
TSI, TE 144 ms, 1.5T TSI factor 3, TR 2000 ms, scan time 4:24 min. Spectral resolution is poor: choline and creatine signals cannot be separated.SENSE CSI, TE 144 ms, 1.5T SENSE factor 3.7, TR 2000 ms, scan time 4:28 min. Spectral resolution is high: choline and creatine signals can be separated.
TSI, TE 144 ms, 1.5T
SENSE CSI, TE 144 ms, 1.5T
TSI factor 3, TR 2000 ms, scan time 4:24 min. Spectral resolution is poor: choline and creatine signals cannot be separated.
SENSE factor 3.7, TR 2000 ms, scan time 4:28 min. Spectral resolution is high: choline and creatine signals can be separated.

Summary

Both Turbo Spectroscopic Imaging and SENSE Spectroscopic Imaging are techniques to reduce scan time in, lengthy, spectroscopic imaging scans.

 

The following table may help to determine the method of choice.

 

 

TSI

SENSE-CSI

Spectral resolution

Low

high

Echo time

Long

Free

Signal-to-noise ratio

Relatively high

Relatively low

 

Further scan time reduction can obviously be realized by adding SENSE to a Turbo Spectroscopic Imaging scan. If time is the most important factor, sub-minute spectroscopic imaging can be realized, by using relatively low echo spacing and increased turbo factor in combination with a high SENSE factor. 

 

SENSE-TSI, TE 288 ms, 1.5T TSI factor 3, SENSE factor 3, scan time 1:48 min
SENSE-TSI, TE 288 ms, 1.5T
TSI factor 3, SENSE factor 3, scan time 1:48 min


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ExamCard
Achieva 1.5T, Achieva 3.0T
Release 2
Nova, Quasar, Quasar Dual
Brain, Neuro, Spectroscopy
 

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