Basic Principles & Sequences for Whole Organ MRSI - Brain & Body

Dennis W. J. Klomp^1,2

¹Radiology, Center of Image Science, University Medical Center Utrecht, Utrecht, the Netherlands

²MR Coils BV, Utrecht, Drunen and Zaltbommel, the Netherlands

(2016 ISMRM Annual Meeting in Singapore. Advanced MR Spectroscopy. Sunday, May 8, 2016)

Metabolic imaging is a powerful means to investigate function of organs, diagnose diseases, or assess treatment efficacy. In comparison to morphologic imaging, detection of metabolism may more directly assess the viability of tissue, and potentially can reveal alterations in disease or treatment progression at earlier stage. While Positron Emission Tomography (PET) can reveal influx of specifically labeled metabolites non-invasively in the human body, Magnetic Resonance Spectroscopy (MRS) can show many naturally abundant metabolites at the same scan time. Moreover, MRS can be combined with spatial encoding facilitating MRS Imaging (MRSI) that can be used for non-invasive metabolic imaging in the human body.

In comparison to PET, MRSI has a much lower intrinsic sensitivity, caused by limited signal to noise ratio (SNR). Moreover, in comparison to MRI, MRSI is more sensitive to MR system imperfections causing potential signal overlap from the many metabolites.

In this course, the basics of maximizing SNR while minimizing sensitivity to system imperfections in MRSI of the human body are discussed using example applications in brain, prostate, breast, and body tuned for the nucleus of ¹H, ³¹P and ¹⁹F.

¹H brain MRSI

The challenges in optimizing 1H MRSI of the human brain are providing a homogenous magnetic field throughout the brain, while excluding signal bleed from the orders of magnitude higher signals originating outside the brain (i.e. skull). Traditionally, preselected boxes of spins are excited and refocused to exclude any signal outside the brain. However, these require high bandwidth RF pulses that either cannot be provided by the MRI system, or cause substantial RF power deposition that hinder the efficiency of MRSI. Alternative means to suppress these outer volume signals will be demonstrated in order to obtain spatiotemporal resolutions of 3x3x5mm in 5 minutes.

¹H prostate MRSI

The use of preselection boxes in 1H MRSI of the prostate is crucial as the surrounded volume of tissue covered by the receiver coils is orders of magnitude larger than the prostate itself. Moreover, the magnetic field surrounding the prostate is far from uniform, which could cause ineffective water and lipid suppression. The RF pulses that provide the preselection coincide with increased echo times, giving rise to complex spectral appearance of signals from strongly coupled spin systems like in the highly abundant metabolite of citrate. Tuning optimal echo times while providing highly robust lipid suppression techniques will be demonstrated to facilitate high quality 3D MRSI of the human prostate.

Breast MRSI

Even more challenging than brain and prostate MRSI is 1H MRSI of the human breast. This is because of the high prevalence of lipid tissue which gives rise to orders of magnitude higher signal levels than the metabolites of interest, which are often the pool of choline metabolites. An alternative means would be to use 31P MRSI for imaging the individual choline levels throughout the breast, which is not hindered at all by signals from the lipids. The consequence of using 31P rather than 1H MRSI will be discussed for metabolic imaging in the breast.

³¹P Body MRSI

Not being hindered by the several orders of magnitude higher signals of water and lipids, 31P MRSI can be easily translated for metabolic imaging in the human body. Multi-echo approaches are discussed that regain the signal loss caused by magnetic field distortions. Moreover, full body coil solutions are demonstrated that can provide the uniform RF fields in order to obtain full body 31P MRSI at high temporal resolutions.

¹⁹F MRSI

Finally, an example is given that demonstrates full body fluorine MRSI at ultra-high fields. It will be demonstrated how the inherent non-uniform RF fields at 7T can be steered into uniform spin excitations using multi transmit solutions. Moreover, it will be shown how the very high bandwidth in chemical shifts of fluorine metabolites can be excited using multiband RF pulses.

In summary, using examples from a several challenging MRSI applications, the basic principles and sequences for MRSI in the human brain and body will be explained.