Markus Kotas

Inversion recovery TrueFISP: Quantification of T1, T2, and spin density

A novel procedure is proposed to extract T1, T2, and relative spin density from the signal time course sampled with a series of TrueFISP images after spin inversion. Generally, the recovery of the magnetization during continuous TrueFISP imaging can be described in good approximation by a three parameter monoexponential function S(t) = Sstst(1‐INV exp(‐t/T  *1 ). This apparent relaxation time T  *1 ≤ T1 depends on the flip angle as well as on both T1 and T2. Here, it is shown that the ratio T1/T2 can be directly extracted from the inversion factor INV, which describes the relation of the signal value extrapolated to t = 0 and the steady‐state signal. Analytical expressions are given for the derivation of T1, T2, and relative spin density directly from the fit parameters. Phantom results show excellent agreement with single point reference measurements. In human volunteers T1, T2, and spin density maps in agreement with literature values were obtained. Magn Reson Med 51:661–667, 2004.

Quantitative tissue perfusion measurements in head and neck carcinoma patients before and during radiation therapy with a non-invasive MR imaging spin-labeling technique

PURPOSE Tumor blood flow, tumor tissue perfusion and oxygen supply have substantial influence on the responsiveness of tumors to radiotherapy. This study was aimed at implementing and evaluating a non-invasive functional magnetic resonance (MR) imaging spin-labeling technique at a main magnetic field strength of 2T for measuring tissue perfusion changes in head and neck carcinoma patients before and during radiotherapy. METHODS Tissue perfusion was determined quantitatively in ten patients with head and neck cancer. Five patients were investigated twice during radiation therapy. For perfusion measurements, a non-invasive MR spin-labeling technique was employed: The longitudinal relaxation time T(1) was measured with segmented Snapshot-FLASH imaging after either slice-selective or non-selective spin inversion. Perfusion values were calculated pixelwise employing a two-compartment tissue model. With this technique no contrast agents are required so that repetitive measurements are possible. Perfusion images with a slice thickness of 10mm and an in-plane resolution of 1.9x2.8mm(2) were acquired at a total scan time of 8:30min per scan. RESULTS With the non-invasive MR imaging technique it was possible to visualize tumor and normal tissue perfusion as well as perfusion changes in the course of radiotherapy with a spatial resolution of less than 3mm. Among the investigated subjects measured tumor perfusion and changes in perfusion were heterogenous. In 4/5 patients studied at the start and end of radiotherapy, perfusion decreased, while in one patient there was an increase. CONCLUSIONS A method is presented that allows non-invasive and repetitive characterization of tissue perfusion. This parameter may be used for treatment stratification, especially in treatments that use vasomodulation or anti-angiogenic agents.

Quantitative regional oxygen transfer imaging of the human lung.

To demonstrate that the use of nonquantitative methods in oxygen‐enhanced (OE) lung imaging can be problematic and to present a new approach for quantitative OE lung imaging, which fulfills the requirements for easy application in clinical practice.

Monitoring of Tumor Oxygenation Changes in Head-and-Neck Carcinoma Patients Breathing a Hyperoxic Hypercapnic Gas Mixture with a Noninvasive MRI Technique

Purpose:To implement and evaluate a noninvasive functional MRI technique for measuring tumor tissue oxygenation changes in head-and-neck carcinoma patients.Patients and Methods:Tissue oxygenation changes were determined quantitatively in 13 patients with head-and-neck cancer. The MR examinations were performed on a clinical MR scanner at 1.5 T. Different breathing gases (air, 2% CO2 and 98% O2, 100% oxygen) were administered to induce oxygenation changes. A multigradient echo sequence was used for quantification of the apparent transverse relaxation time T2*.Results:Pixel-by-pixel analysis of the T2* values in tumors showed a shift toward higher values corresponding to oxygenation increase and correlated with a median shift toward positive values in the ΔT2* fraction under carbogen and oxygen breathing in most but not all patients. A slightly pronounced T2* increase breathing oxygen compared with 2% CO2/98% O2 was found. Furthermore, a statistically significant difference in the heterogeneity of oxygenation changes induced by oxygen or 2% CO2/ 98% O2 breathing was seen.Conclusion:Measurement of oxygenation changes in head-and-neck tumor patients is feasible by the presented MRI technique. Tumor oxygenation and oxygenation changes were heterogeneous among the investigated patients. To the authors’ knowledge, they are the first to describe a statistically significant difference in the heterogeneity of oxygenation changes induced by oxygen or 2% CO2/98% O2 breathing using a noninvasive MRI technique.Ziel:Entwicklung und Evaluierung einer nichtinvasiven funktionellen MRT-Technik zur Messung von Tumoroxygenierungsänderungen bei Patienten mit Kopf-Hals-Tumoren.Patienten und Methodik:Änderungen der Gewebeoxygenierung wurden bei 13 Patienten mit Kopf-Hals-Karzinomen quantitativ gemessen. Die MR-Untersuchungen wurden an einem klinischen MR-Gerät bei 1,5 T durchgeführt. Um Oxygenierungsänderungen zu verursachen, wurden verschiedene Atemgase (Raumluft, 2% CO2 und 98% O2, 100% Sauerstoff) verabreicht. Eine Multigradientenechosequenz wurde zur Quantifizierung der Relaxationszeit T2* verwendet.Ergebnisse:Die Pixel-für-Pixel-Analyse der T2*-Werte in den Tumoren zeigte eine Verschiebung zu höheren Werten, die mit einer verbesserten Oxygenierung korrespondierte und mit einer Verschiebung des Medians zu positiven Werten in der ΔT2*-Fraktion bei 2% CO2/98% O2- und Sauerstoffatmung bei den meisten, jedoch nicht allen Patienten korrelierte. Es fand sich ein geringgradig verstärkter T2*-Anstieg bei Sauerstoffatmung im Vergleich zur 2% CO2/98% O2-Atmung. Ferner zeigte sich ein statistisch signifikanter Unterschied in der Heterogenität der durch Sauerstoff- oder 2% CO2/98% O2-Atmung hervorgerufenen Oxygenierungsänderungen.Schlussfolgerung:Die dargestellte MRT-Technik eignet sich zur Messung von Oxygenierungsänderungen bei Patienten mit Kopf-Hals-Tumoren. Bei den untersuchten Patienten waren die Tumoroxygenierung und Oxygenierungsänderungen heterogen. Soweit den Autoren bekannt ist, beschreiben sie als Erste einen statistisch signifikanten Unterschied in der durch Sauerstoff- oder 2% CO2/98% O2-Atmung hervorgerufenen Heterogenität von Oxygenierungsänderungen durch Anwendung einer nichtinvasiven MRT-Methode.

Short‐echo spectroscopic imaging combined with lactate editing in a single scan

A short‐echo spectroscopic imaging sequence extended with a frequency‐selective multiple‐quantum‐ coherence technique (Sel‐MQC) is presented. The method enables acquisition of a complete water‐suppressed proton spectrum with a short echo time and filtering of the J‐coupling metabolite, lactate, from co‐resonant lipids in one scan. The purpose of the study was to validate this combined pulse sequence in vitro and in vivo. Measurements on phantoms confirmed the feasibility of the method, and, for a practical in vivo application, experiments were carried out on eight tumors from two different tumor models [UT‐SCC‐8 (n = 4) and SAS (n = 4)]. T1‐ and T2‐weighted metabolite and lipid ratios were calculated, and the tumors showed different values in the central and outer regions. The ratio of the lipid methylene peak area (1.30 ppm) to choline peak area (3.20 ppm) was significantly (p < 0.01) different in the central tumor area between the two models, and lactate was detected in only three out of four tumors in the SAS tumor line. The present approach of combining short‐echo spectroscopic imaging and lactate editing allows the characterization of tumor‐specific metabolites such as choline, lipid methylene and methyl resonances as well as lactate in a single scan. Copyright

Single‐shot quantitative perfusion imaging of the human lung

The major drawback to quantitative perfusion imaging using arterial spin labeling (ASL) techniques is the need to acquire two images (tag and control), which must be subtracted in order to obtain a perfusion‐weighted image. This can potentially result in misregistration artifacts, especially in lung imaging, due to varying lung inflation levels in different breath‐holds. In this work a double inversion recovery (DIR) imaging technique that yields perfusion‐weighted images of the human lung in a single shot is presented. This technique ensures the complete suppression of background tissue while it preserves signal from the blood. Furthermore, the perfusion‐weighted images and an additional (independent) acquired reference scan can be used to obtain quantitative perfusion information from the lungs. Magn Reson Med, 2006.

Assessment of pulmonary perfusion in a single shot using SEEPAGE.

To present a single‐shot perfusion imaging sequence that does not require contrast agents or a subtraction of a tag and a control image to create the perfusion‐weighted contrast. The proposed method is based on SEEPAGE.

Sensitive J-coupled metabolite mapping using Sel-MQC with selective multi-spin-echo readout.

The selective multiple quantum coherence technique is combined with a read gradient to accelerate the measurement of a specific scalar‐coupled metabolite. The sensitivities of the localization using pure phase encoding and localization with the read gradient are compared in experiments at high magnetic field strength (17.6 T). Multiple spin‐echoes of the selective multiple quantum coherence edited metabolite are acquired using frequency‐selective refocusing of the specified molecule group. The frequency‐selective refocusing does not affect the J‐modulation of a coupled spin system, and the echo time is not limited to a multiple of 1/J to acquire pure in‐phase or antiphase signal. The multiple echoes can be used to accelerate the metabolite imaging experiment or to measure the apparent transverse relaxation T2. A simple phase‐shifting scheme is presented, which enables the suppression of editing artifacts resulting from the multiple spin‐echoes of the water resonance. The experiments are carried out on phantoms, in which lactate and polyunsaturated fatty acids are edited, and in vivo on tumors, in which lactate content and T2 are imaged. The method is of particular interest when a fast and sensitive selective multiple quantum coherence editing is necessary, e.g., for spatial three dimensional experiments. Magn Reson Med, 2009.

T-one insensitive steady state imaging: A framework for purely T2-weighted TrueFISP

A new conceptual framework called T‐one insensitive steady state imaging is proposed for fast generation of MR images with pure T2 contrast. This is accomplished by imaging between nonequally spaced inversion pulses, with the magnetization vector alternatively residing in states parallel and antiparallel to B0 for durations TPi and TAi, respectively. With TPi and TAi adequately chosen, identical signal time evolution can be obtained for different T1 values, i.e., T1 contrast can efficiently be removed from resultant images. As a specific realization of this principle, T‐one insensitive steady state imaging sequences are presented which use True free induction steady precession readout blocks between the inversion pulses. While the conventional True free induction steady precession signal time course would be determined by both T2 and T1, a pure T2 dependence is realized with successfully suppressed influence of longitudinal relaxation, and images with essentially T2 contrast alone are obtained. Analytical expressions are provided for the description of the ideal signal behavior, which help in creating pathways for sequence parameter optimization. The performance of the technique is analyzed with Bloch equation simulations. In vivo results obtained in healthy volunteers and brain tumor patients are presented. Magn Reson Med, 2012.

Potential of magnetization transfer MRI for target volume definition in patients with non-small-cell lung cancer.

To develop a magnetization transfer (MT) module in conjunction with a single‐shot MRI readout technique and to investigate the MT phenomenon in non‐small‐cell lung cancer (NSCLC) as an adjunct for radiation therapy planning.