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Lung imaging has traditionally relied on x-ray methods, since proton MRI is limited to some extent by low proton density in the lung parenchyma and static field inhomogeneities in the chest. The relatively recent introduction of MRI of hyperpolarized noble gases has led to a rapidly evolving field of pulmonary MRI, revealing functional information of the lungs, which were hitherto unattainable. This review article briefly describes the physical background of the technology, and subsequently focuses on its clinical applications. Four different techniques that have been used in various human investigations are discussed: ventilation distribution, ventilation dynamics, and small airway evaluation using diffusion imaging and oxygen uptake assessment.

We have developed a new method for estimation of regional CBF (rCBF) and cerebrovascular reserve capacity on a pixel-by-pixel basis by means of dynamic magnetic resonance imaging (MRI). Thirteen healthy volunteers, 8 patients with occlusion and/or high grade stenosis of the internal carotid artery (ICA), and 2 patients with acute stroke underwent dynamic susceptibility-weighted contrast enhanced MRI. Using principles of indicator dilution theory and deconvolution analysis, maps of rCBF, regional cerebral blood volume, and of the mean transit time (MTT) were calculated. In patients with ICA occlusion/stenosis, cerebrovascular reserve capacity was assessed by the rCBF increase after acetazolamide stimulation. Mean gray and white matter rCBF values in normals were 67.1 and 23.7 mL x 100 g(-1) x min(-1), respectively. Before acetazolamide stimulation, six of eight patients with ICA occlusions showed decreased rCBF values; and in seven patients increased MTT values were observed in tissue ipsilateral to the occlusion. After acetazolamide stimulation, decreased cerebrovascular reserve capacity was observed in five of eight patients with ICA occlusion. In acute stroke, rCBF in the central core of ischemia was less than 8 mL x 100 g(-1) x min(-1). In peri-infarct tissue, rCBF and MTT were higher than in unaffected tissue but rCBF was normal. Dynamic MRI provides important clinical information on the hemodynamic state of brain tissue in patients with occlusive cerebrovascular disease or acute stroke.

Purpose:To develop and test a saturation-recovery True-FISP (SR-TrueFISP) pulse sequence for first-pass myocardial perfusion imaging. Materials and Methods:First-pass magnetic resonance imaging (MRI) of Gd-DTPA (2 mL) kinetics in the heart was performed using an SR-TrueFISP pulse sequence (TR/TE/ ␣ ϭ 2.6 msec/1.4 msec/55°) with saturation preparation TD ϭ 30 msec before the TrueFISP readout. Measurements were also performed with a conventional saturation-recovery TurboFLASH (SRTF) pulse sequence for comparison.Results: SR-TrueFISP images were of excellent quality and demonstrated contrast agent wash-in more clearly than SRTF images. The signal increase in myocardium was higher in SR-TrueFISP than in SRTF data. Precontrast SNR and peak CNR were not significantly different between both sequences despite 57% improved spatial resolution for SRTrueFISP. A SUFFICIENT BLOOD SUPPLY IS essential for tissue function. Contrast enhanced myocardial perfusion imaging (MPI) with magnetic resonance imaging (MRI) has been shown to have the potential to provide insights into myocardial microcirculation qualitatively (1), semiquantitatively (2-6), and quantitatively (6 -12). ConclusionFast magnetization-prepared (e.g., saturation recovery) spoiled gradient echo pulse sequences have been used previously for MPI (2,8 -11). However, for semiquantitative (i.e., slope) and quantitative (i.e., determination of myocardial blood flow (MBF)) evaluation of first-pass MPI, a linear relation between the signal intensity and the contrast medium (CM) concentration is of advantage, because the conversion of signal intensities to CM concentrations can then be accomplished easily (6,9,10). The linearity can be achieved by using a very short delay time (TD) between the saturation pulse and the gradient echo imaging part of the sequence, and a low CM dosage. Moreover, to keep the imaging time short, very short sequence repetition times (TR) have to be used. Consequently, the MR images and the signal-time curves (STCs) derived thereof demonstrate poor signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR).With regard to SNR and CNR, conventional spoiled gradient echo pulse sequences such as the snapshotfast low-angle single-shot (TurboFLASH) (13) sequence are inefficient in that all transverse magnetization is destroyed before the next phase encoding step. In steady-state free precession pulse sequences (SSFP) (14) such as TrueFISP (15), however, the transverse magnetization is refocused after data acquisition, resulting in an addition of the refocused transverse magnetization with the newly excited transverse magnetization of the next phase encoding step. Moreover, spin echos from previous excitations are refocused. At very short TRs, TrueFISP offers higher SNRs than spoiled gradient echo pulse sequences, in particular for tissues with large T2/T1 values (T1, longitudinal relaxation time; T2, transverse relaxation time) (16). Moreover, Zur et al (16) showed that the SSFP sequence is more efficient in terms of SNR per unit time than the conve...

MRI of the lungs using hyperpolarized helium-3 ( 3 He) allows the determination of intrapulmonary oxygen partial pressures (p O2 ). The need to separate competing processes of signal loss has hitherto required two different imaging series during two different breathing maneuvers. In this work, a new imaging strategy to measure p O2 by a single series of consecutive scans is presented. Within the last 5 years, helium-3 magnetic resonance imaging ( 3 He-MRI) of the lungs has been employed for both morphological and functional imaging. The first encompasses studies of overt or subclinical lung disease, e.g., in chronic obstructive pulmonary disease (COPD) and emphysema patients (1,2), asthma patients (3), and smokers (4). The latter includes the use of fast imaging sequences (5-7), diffusion studies (8 -10), and the determination of intrapulmonary oxygen partial pressures (11-13). All of these methods provide exciting new approaches to lung function analysis. Complementary to the diagnostic information obtained from morphological imaging, rapid lung imaging allows time-resolved ventilation studies. Diffusion-weighted imaging and oxygen-sensitive MRI yield physiological information that was previously unavailable or was obtainable only by invasive means. (Comprehensive reviews are given in Refs. 14 and 15.)Oxygen-sensitive 3 He-MRI makes use of the oxygen-induced nuclear relaxation of 3 He (16) to compute the intrapulmonary oxygen partial pressure p O2 during short breath-holds. However, true relaxation due to molecular oxygen must be distinguished from RF-induced signal loss, i.e., the two depolarization effects must be decoupled. As neither is known a priori, this requires two different imaging sequences: with different RF excitation amplitudes and, hence, flip angles (11), or with different interimage time intervals (12). A subtraction of the logarithmic intensities of both image series serves to mathematically eliminate one unknown and allows precise determination of the other. To date, the two image series have been acquired during two separate breathing maneuvers.The analyses described in Refs. 11 and 12 rely on identical physiological conditions during paired image acquisition. It has to be assumed that the p O2 at the start of the imaging sequence and its temporal development are equal in both breath-holds. The imaged subject thus has to perform two identical breathing maneuvers. However, preliminary studies in healthy human volunteers (13) and patients (17) have shown that this condition is not always met.A second disadvantage of the double-acquisition approach lies in its 3 He consumption. In previous experiments (12,13), a 3 He amount of ϳ2 ϫ 200 cm 3 was required for each examination. In view of the limited resources of 3 He, a more economical measurement technique is desirable.In this work, we present a new imaging algorithm which permits the determination of intrapulmonary p O2 using a single-breath, single-bolus imaging sequence. Its strategy is to employ a simultaneous change in both excitation ampli...