Whole-body Imager Research Articles

Proton Spectroscopy of Human Brain with Very Short Echo Time Using High Gradient Amplitudes

In localized proton magnetic resonance spectroscopy very short echo times (TE) are achieved to diminish signal loss due to T 2 relaxation and to avoid phase distortions due to J-coupling. A sequence for single volume spectroscopy in human brain is described with a TE as low as 5 ms. Examinations were performed on a 1.5 T whole-body imager with actively shielded gradients. A self-designed stimulated echo acquisition mode (STEAM) sequence with very high amplitude spoiling gradients of 24 mT/m was used to take advantage of the whole potential of the gradient system. Optimization of TE was carried out by controlling spectral quality and localization in both phantom and volunteer measurements. Proton spectra of human brain were acquired in 21 healthy volunteers. Spectra of occipital white matter, parieto-occipital grey/white matter, and cerebellum revealed none or only small eddy current distortions at a TE of 5 ms. The volume of interest was 8–12 ml, repetition time was 1.5 s, and mixing time was 5 ms. Peak ratios of major metabolites referring to creatine were estimated and the relative standard deviations were calculated to determine interindividual reproducibility. The relative standard deviation of myo-inositol ranged from 6% to 11% within these brain regions whereas for glutamine and glutamate 7% to 16% were found.

T2 accuracy on a whole-body imager.

MR oximetry requires a T2 measurement that is accurate within 5% in vivo. Simple methods are susceptible to signal loss and tend to underestimate T2. Current methods utilize RF pulses or RF cycling patterns that prevent signal loss at each data acquisition. However, using these methods with imperfect pulses, T2 tends to be overestimated due to temporary storage of the magnetization along the longitudinal axis where it decays more slowly with a time constant T1 > T2. To reduce the T1 dependence while preventing signal loss, we utilize simple 90x180y90x composite pulses and good RF cycling patterns. These trains are critical for T2 accuracy over typical ranges of RF and static field inhomogeneities and refocusing intervals. T1 signal decay during each 90x180y90x pulse must be accounted for to yield accuracy within 5% when the pulse-width is 10% or more of the refocusing interval. A simple correction scheme compensates for this T1-related error effectively.

Focal liver masses: characterization with diffusion-weighted echo-planar MR imaging.

To evaluate diffusion-weighted echo-planar magnetic resonance (MR) imaging for improving the specificity of characterization of liver tumors. Diffusion-weighted echo-planar imaging was performed with a 1.5-T whole-body imager with use of a body phased-array coil in 51 patients with 59 hepatic masses (41 malignant tumors, nine hemangiomas, and nine cysts). Apparent diffusion coefficient (ADC) values were obtained with two motion-probing gradients (b = 30 and 1,200 sec/mm2) during each of the breath-hold periods, and an ADC map was constructed. The T2 was derived from spin-echo echo-planar images with echo times of 47 and 99 msec. The ADC value of malignant masses (1.04 x 10(-3) mm2/sec) was significantly lower (P < .01) than that of benign masses (hemangiomas [1.95 x 10(-3) mm2/sec] and cysts [3.05 x 10(-3) mm2/sec]), although the T2s showed considerable overlap. A small amount of overlap in ADC values occurred among malignant tumors, hemangiomas, and cysts. ADC values of two cystic masses from ovarian carcinomas were within the range of those of hemangiomas. These preliminary results indicate that diffusion-weighted MR imaging can be useful in characterizing focal liver masses. With the exception of cystic metastatic tumors, the technique may be especially useful in tumors that appear markedly hyperintense on T2-weighted images due to a long T2.

Phosphorus J-coupling constants of ATP in human brain.

Proton decoupled 31P NMR spectroscopy of the occipital brain of healthy volunteers was performed with a 1.5 T whole-body imager. By use of two-dimensional chemical-shift imaging in combination with slice-selective excitation well resolved localized spectra (38 ml) were obtained within 34 min from which the homonuclear 31P-31P J-coupling constants of ATP could be determined: J(gammabeta) = 16.1 Hz +/- 0.2 Hz and J(alphabeta) = 16.3 Hz +/- 0.1 Hz (mean +/- SEM, n = 14). Both, the J-coupling constants and the chemical-shift difference between alpha- and beta-ATP (delta(alphabeta) = 8.61 ppm +/- 0.01 ppm) were used to calculate the concentration of intracellular free magnesium. The concentrations are 0.39 mM +/- 0.09 mM by using the average of both coupling constants of each spectrum, which is in fair agreement with 0.32 mM +/- 0.01 mM obtained from the chemical shift of alpha and beta phosphate resonances, which is the more accurate result.

A 4.2 K receiver coil and SQUID amplifier used to improve the SNR of low-field magnetic resonance images of the human arm

We describe the design and use of a 48 mm diameter, liquid-helium-cooled MRI receiver coil and DC SQUID pre-amplifier. Comparison of images of a non-conducting room temperature test object collected with the SQUID-based system and those collected with an equivalent-area room-temperature surface coil show that the SQUID system SNR is approximately a factor of four greater, despite a 15 mm vacuum gap between sample and coil in the SQUID case. SQUID images of the lower arm also display improved SNRs over those of the room-temperature coil, this time by a factor of between two and three, and as a result reveal greater anatomical detail. We show that the performance is currently limited by inductively coupled losses from metal components in the imager, but that, by using the same system in a whole-body imager, the SNR of SQUID images of the arm will exceed the room-temperature coil's performance by a factor of between 2.8 and 4.5. We believe that these are the first magnetic resonance images of a living sample to have been produced with a SQUID-based receiver.

Determination of the Chemical-Shift Difference between the Lactate Multiplets and Its pH Dependence

A volume-selective NMR method is presented for very exact determination of the difference of the Larmor frequencies between the coupled resonances of homonuclear AX3spin systems, as, for example, lactate. The frequency difference can be determined with an accuracy of 0.003 ppm even if only the doublet of the spin system can be detected. The method is based on the effects of homonuclear polarization transfer which occurs in localized double-spin-echo spectroscopy. It is used for determination of the chemical-shift difference of the lactate multiplets, which depends on the degree of dissociation and consequently on pH. Measurements were performed with a 1.5 T Siemens Magnetom SP 63 whole-body imager on solutions of lactate and acetic acid in physiological sodium chloride solution with pH from 1 to 11. As a consequence of these measurements, conclusions are possible for the optimum echo time for PRESS measurements which avoid signal losses from polarization transfer. Furthermore, the possibility of localized pH determination by this effect is discussed.

Modification of a whole-body NMR imager into a radio frequency EPR spectrometer suitable for in vivo measurements

We report the modification of a low-field whole-body NMR imager to allow radio frequency EPR spectroscopy. The instrument is designed to give optimum sensitivity for in vivo detection of free radicals. The RF circuit is able to operate over a wide frequency range (240 - 320 MHz) and is designed to handle input power levels of up to 12.5 W. The EPR resonator is of the loop - gap type suitable for samples up to 400 ml. A remote resonator coupling method has been developed enabling convenient matching adjustment. An automatic frequency control circuit is able to adjust for frequency deviations caused by animal motion. Where possible, existing imager hardware and commercially available instruments have been used. The instrument is controlled from a central computer via an IEEE 488 instrumentation bus. Here we present sensitivity measurements obtained from a variety of large aqueous samples containing nitroxide free radicals. We show that the instrument is suitable for the detection of exogenous free radicals in 200 g rats.

Functional MRI of the pharynx in obstructive sleep apnea using rapid 2D FLASH sequences

Functional imaging of the pharynx used to be the domain of cineradiography, CT and ultrafast CT. The development of modern MRI techniques led to new access to functional disorders of the pharynx. The aim of this study was to implement a new MRI technique to examine oropharyngeal obstructive mechanisms in patients with obstructive sleep apnea (OSA). Sixteen patients suffering from OSA and 6 healthy volunteers were examined on a 1.5 T whole-body imager ("Vision", Siemens, Erlangen Medical Engineering, Germany) using a circular polarized head coil. Imaging was performed with 2D flash sequences in midsagittal and axial planes. Patients and volunteers were asked to breathe normally through the nose and to simulate snoring and the Mueller maneuver during magnetic resonance imaging (MRI). Prior to MRI, all patients underwent an ear, nose and throat (ENT) examination, functional fiberoptic nasopharyngoscopy and polysomnography. A temporal resolution of 6 images/s and an in-plane resolution of 2.67 x 1.8 mm were achieved. The mobility of the tongue, soft palate and pharyngeal surface could be clearly delineated. The MRI findings correlated well with the clinical examinations. We propose ultrafast MRI as a reliable and non-invasive method of evaluating pharyngeal obstruction and their levels.

31P NMR studies of human soleus and gastrocnemius show differences in theJ γβ coupling constant of ATP and in intracellular free magnesium

Localized proton decoupled 31P in vivo NMR spectroscopy of the human calf muscle was performed using a 1.5-T whole-body imager and the slice selective two-dimensional chemical-shift-imaging (2D-CSI) technique. The 31P-31P coupling constants and the chemical shifts of ATP were compared in gastrocnemius and soleus. Significant differences were found in the coupling constant J gamma beta: (18.1 +/- 0.7) Hz versus (17.1 +/- 0.6) Hz (means +/- SD, P < 10-5). Differences were also observed in the chemical shift separation delta alpha beta between the alpha- and beta-ATP signal: (8.498 +/- 0.023) ppm versus (8.522 +/- 0.222) ppm (p < 0.001) in gastrocnemius and soleus, respectively. A higher [MgATP]/[ATPfree] ratio and a significantly higher level of intracellular free magnesium of (0.52 +/- 0.06) mM in gastrocnemius versus (0.46 +/- 0.05) mM in soleus (p < 0.001) can be derived based on delta alpha beta and KDMgATP. Heterogeneity needs to be taken into account in clinical studies on magnesium by NMR methods in calf muscle. The coupling constant J gamma beta provides additional information, possibly on enzymatic processes, and correlates with [Mg2+free]. The detailed analysis of muscles with different fiber type characteristics lends support to the significance of this parameter in evaluating metabolism. The data reported can be used as prior knowledge for fits in which the coupling constants are set to a fixed value.

Erste Ergebnisse der MRT-gesteuerten laserinduzierten interstitiellen Thermotherapie von Kopf- und Halstumoren

Laser-induced interstitial thermo-therapy (LITT) was introduced as a minimally invasive form of therapy for tumors in different anatomic regions. However, in the orofacial region, it has not been used so far for inoperable T4 carcinomas. Since vascular and neural structures are often close to the tumor or are even involved, online monitoring of LITT is necessary. The aim of our study was to establish a method of monitoring LITT with MRI (magnetic resonance imaging) in the orofacial region. Five patients with T4 carcinomas of the orofacial region underwent LITT under anesthesia. A 1.5 T whole-body imager with a circular polarized head coil was used. Before and after the intervention, the region of interest was studied using T1- and T2-weighted sequences in axial and coronal planes, with and without contrast enhancement (intravenous Gd-DTPA). Temperature distribution was monitored with a T1-weighted 2D-FLASH (fast low angle shot) sequence. The positioning of the optical fibers was monitored with MRI. Nd:YAG laser equipment was used for laser application. The necrosis was best seen on contrast-enhanced MRI. Immediately after LITT, the outcome could be determined by MRI. We proposed that MRI-guided LITT be used for neoplasms in the orofacial region at advanced stages.

Pulse Angle Dependence of Double-Spin-Echo Proton NMR Spectra of Citrate—Theory and Experiments

The dependence of the signal of the strongly coupled AB spin system on the pulse angles in double spin-echo sequences was examined. Experimental1H NMR spectra of citrate were recorded on a 1.5 T whole-body imager. Theoretically calculated spectra were generated by an adapted computer-algebra program and found to correlate well with the experimental spectra. The dependence of the signal intensity on the sequence timing and the pulse angles was calculated analytically. For longer echo times, the ratio of the signal intensity from citrate to the signal intensity from uncoupled nuclei depends strongly on the applied flip angles. These findings are important for the quantitation of citrate from localized spectra, because frequency-selective pulses necessary for the localization inevitably produce deviations from the nominal pulse angles in certain areas of the selected volume.

Change in chemical shift and splitting of 31P gamma-ATP signal in human skeletal muscle during exercise and recovery.

Proton decoupled 31P in vivo NMR spectroscopy of the human finger flexor muscles was performed during exercise and recovery using a 1.5 T whole-body imager. Predominantly the gamma-ATP signal shows a splitting caused by different signal contributions with chemical shifts that vary independently. Studies on the human gastrocnemius and biceps femoris muscle were undertaken to investigate the appearance of the splitting in these muscles as well. In all cases more than one signal contribution was found which might represent the different muscle fibre types and their recruitment pattern following exercise. An analysis of the chemical shifts (delta) of ATP results in changes of up to 0.4 ppm and 0.1 ppm for delta gamma- and delta beta-ATP, respectively. Based solely on the chemical shifts of the ATP 31P signals the tissue pH value following exercise was determined. The result was in good agreement with the value derived from delta Pi.

PhosphorusJCoupling Constants of ATP in Human Myocardium and Calf Muscle

Proton-decoupled31P NMR spectroscopy of the heart and calf muscle of healthy volunteers was performed with a 1.5 T whole-body imager. By use of two-dimensional chemical-shift imaging in combination with slice-selective excitation, well-resolved localized spectra (elements of 38 ml) were obtained within 20 to 35 min from which the homonuclear J coupling constants of ATP could be determined. In myocardium,Jγβ= 16.03 ± 0.17 Hz andJαβ= 15.82 ± 0.23 Hz were obtained, while the values in calf muscle wereJγβ= 17.16 ± 0.12 Hz andJαβ= 16.04 ± 0.09 Hz. The difference inJγβwas significant. According to the literature, a possible reason for greater ATP J coupling constants is a smaller fraction of ATP complexed to magnesium. However, the chemical-shift difference between α- and β-ATP, which is also a measure for the fraction of ATP complexed to magnesium, showed only a small difference in ATP complexation: 88% in myocardium and 90% in calf muscle. This small difference cannot account for the observed difference in Jγβ.

Localized double spin echo proton spectroscopy part II: Weakly coupled homonuclear spin systems

The localized 90°-180°-180° double spin echo method uses three slice-selective radio frequency pulses to successively excite three orthogonal slices. Pulse angle distributions within the excited slices create different unwanted coherences in weakly coupled homonuclear spin systems that give rise to signal loss. The amount of signal loss depends on the time intervals between the pulses, so only a proper design of the sequence results in a high signal strength of coupled spins and in corresponding multiplets that are free of phase anomalies. The dependence on the timing of the double spin echo sequence and the pulse angle dependence are examined theoretically using the product operator formalism and experimentally using a Siemens Magnetom 1.5 T whole-body imager. © 1996 John Wiley & Sons, Inc.

Relaxation of AB Spin Systems in Stimulated-Echo Spectroscopy

The behavior of the strongly coupled AB spin system during the STEAM (stimulated echo acquisition mode) sequence was calculated analytically. Relaxation during the TM interval, in which longitudinal magnetization and zero-quantum coherences (ZQCs) occur, was accounted for by following the course of the different density-matrix terms. The result allows one to determine sequence timings to provide high signal intensities or signals resulting from certain coherences. Theoretically calculated spectra can be generated, using an analytical function. Series of proton spectra were recorded from a 0.1maqueous solution of citrate on a 1.5 T whole-body imager at 22°C. Spectra series with constant echo time TE were used to evaluate the longitudinal relaxation timeT1as well as the zero-quantum relaxation timeTZQby fitting the theoretically predicted curve to the experimental data. The evaluated proton relaxation timesT1andTZQin citrate differ strongly:T1= 770 ms,TZQ= 1300 ms.

High Resolution NMR Spectroscopy on Whole-Body Imagers:Optimized Single-and Multi-Pulse Experiments in Vitro

Abstract From 100 ml spherical glass bottles filled with aqueous solutions and suspended in a homogeneous magnetic field, NMR spectra with linewidths of about 0.7 Hz were achieved in single-pulse and multi-pulse spectra. A relatively wide receiver coil as the body coil or the standard head coil of the manufacturer were employed to acquire spectra after different non-localized pulse sequences. Examples of single-pulse spectra and double spin-echo spectra of aqueous solutions with lactate, citrate, or glucose are demonstrated and discussed. The fact that all experiments can be performed using well-defined pulse angles acting on the entire sample at the field strenght of the whole-body unit allows to determine the characteristics (e.g. chemical shift differences, coupling constants) of spin systems of biologically important molecules precisely, without need for additional spectrometers. Constant flip angles are advantageous for adequate theoretical analysis of spectra from coupled spin systems. The effects of a defined "misadjustment" of the transmitter on the spectra can be measured directly, whereas localized methods always yield a superposition of signals due to the distribution of flip angles inside the selected volume. In some cases, optimized sequence parameters for localized examinations in vivo can be derived numerically from the analyzed coupling data.

Magnetization transfer by simple sequences of rectangular pulses.

On-resonant radio frequency (RF) sequences composed of a train of short rectangular pulses of the same kind were optimized in order to obtain selective saturation of protons with short transverse relaxation times for magnetization transfer purposes. It is demonstrated that the sequences regarded allow a good adaptation to different requirements for magnetization transfer examinations on whole-body imagers. The sequences presented here provide relatively strong saturation of protons with very short transverse relaxation times T2 approximately less than 50 microseconds, whereas signals from protons with long T2 to be recorded are hardly influenced in a broad frequency range. The sequences are especially advantageous for applications in pulse files with limited numbers of support points.

The Distribution of the Magnetic Field in the Spine Depends on the Composition of Bone Marrow

Although the composition of bone marrow with hemopoietic cells, fat cells, and extracellular fluid can be roughly assessed by standard MR-imaging techniques and especially water and lipid-selective chemical-shift-imaging methods, a new approach to the characterization of the magnetic properties of marrow was performed by special field-mapping techniques. The distribution of the magnetic field inside and outside vertebral bodies containing paramagnetic substances was systematically studied for phantoms and by measurements in vivo. Nineteen healthy volunteers and 26 patients with alterations of the bone marrow due to hematologic diseases were examined. The amount of paramagnetic substances in the marrow was estimated by measuring steps of Larmor frequency of the water resonances at the transition between vertebral bodies and adjacent intervertebral disks. These frequency steps were exhibited by MAGSUS imaging on a 1.5 Tesla whole-body imager. Additional volume-localized H-1 spectroscopy allowed a more quantitative assessment of the spectral components. The measured frequency steps of the water resonances ranged between 0 and 26 Hz for the healthy volunteers examined. In contrast, patients with pathologically altered marrow and high amount of paramagnetic substances revealed frequency steps of up to 85 Hz. The frequency steps in 8 patients after bone marrow transplantation (BMT) with slow reconstitution (mean 48.9 Hz, standard deviation (s.d.) 21.7 Hz) were significantly (p < 0.001) higher than in normal volunteers. Seven BMT patients with good reconstitution (frequency steps: mean 16.7 Hz, s.d. 13.9 Hz) were not clearly different from the healthy subjects. Six patients with acute leukemia showed significantly (p < 0.01) increasing frequency steps during initial cytotoxic treatment: The frequency steps increased from a mean of 4.7 Hz (s.d, 2.7 Hz) before treatment to a mean of 30.2 Hz (s.d. 14.6 Hz) after a few months of therapy.

Adapted techniques for clinical MR imaging of tendons.

To determine whether the echo time of magnetic resonance gradient-echo and spin-echo imaging sequences may be important for the occurrence of high signal strength from tendon with pathological alterations, imaging sequences with sufficient spatial resolution and very short echo times were developed for whole-body imagers with standard gradient system. The sequences were applied on the Achilles tendons of five healthy volunteers and seven patients with achillodynia. Some affected regions inside tendon, probably corresponding with tissue with subtle edema in the collagen bundles were only revealed in images recorded with very short echo times TE < 5 ms, whereas stronger affections and protons in liquids between the fiber bundles were also shown in images with longer echo times TE > 10 ms. Gradient-echo methods allow shorter echo times than spin-echo techniques for a given gradient system of the imager and given spatial resolution. So minimum echo time gradient-echo sequences should be used for sensitive imaging of tendon alterations, because no considerable signal dephasing due to susceptibility effects were found in tendon.

Polarization-Transfer Effects in Localized Double-Spin-Echo Spectroscopy of Weakly Coupled Homonuclear Spin Systems

In point-resolved spectroscopy (PRESS), fast echo-time-dependent signal modulations of the multiplets of weakly coupled homonuclear spin systems occur. They are caused by polarization-transfer effects that are explained by calculations with the product-operator formalism. The impact of slice profiles and transmitter adjustment and the difference of the polarization-transfer effects between AX and AX 3 systems are explained. Experiments with echo-time variation were performed with slice-selective pulses of different quality of profile. Experiments were carried out with a solution of calcium lactate in a 1.5 T Siemens whole-body imager. The experimental results confirm the theoretical calculations. It is stated that in localized double-spin-echo spectroscopy, especially at echo times of TE = 2/ J, the polarization-transfer effects must be considered. This knowledge is important in lactate spectroscopy in vivo, where the assumptions for 2/ J range between 270 and 290 ms, leading to different signal intensities of lactate in PRESS spectra.