Quantitative lung perfusion mapping at 0.2 T using FAIR True-FISP MRI

MR Perfusion Imaging of the Lung

Lung perfusion assessment is critical for diagnosing and monitoring a variety of respiratory conditions. MRI perfusion provides a radiation-free technique, making it an ideal choice for longitudinal imaging in younger populations. This review focuses on the techniques and applications of MRI perfusion, including contrast-enhanced (CE) MRI and non-CE methods such as arterial spin labeling (ASL), fourier decomposition (FD), and hyperpolarized 129-Xenon (129-Xe) MRI. ASL leverages endogenous water protons as tracers for a non-invasive measure of lung perfusion, while FD offers simultaneous measurements of lung perfusion and ventilation, enabling the generation of ventilation/perfusion mapsHyperpolarized 129-Xe MRI emerges as a novel tool for assessing regional gas exchange in the lungs. Despite the promise of MRI perfusion techniques, challenges persist, including competition with other imaging techniques and the need for additional validation and standardization. In conditions such as cystic fibrosis and lung cancer, MRI has displayed encouraging results, whereas in diseases like chronic obstructive pulmonary disease, further validation remains necessary. In conclusion, while MRI perfusion techniques hold immense potential for a comprehensive, non-invasive assessment of lung function and perfusion, their broader clinical adoption hinges on technological advancements, collaborative research, and rigorous validation.

Update on state-of-the-art for arterial spin labeling (ASL) human perfusion imaging outside of the brain

This review article provides an overview of developments for arterial spin labeling (ASL) perfusion imaging in the body (i.e., outside of the brain). It is part of a series of review/recommendation papers from the International Society for Magnetic Resonance in Medicine (ISMRM) Perfusion Study Group. In this review, we focus on specific challenges and developments tailored for ASL in a variety of body locations. After presenting common challenges, organ-specific reviews of challenges and developments are presented, including kidneys, lungs, heart (myocardium), placenta, eye (retina), liver, pancreas, and muscle, which are regions that have seen the most developments outside of the brain. Summaries and recommendations of acquisition parameters (when appropriate) are provided for each organ. We then explore the possibilities for wider adoption of body ASL based on large standardization efforts, as well as the potential opportunities based on recent advances in high/low-field systems and machine-learning. This review seeks to provide an overview of the current state-of-the-art of ASL for applications in the body, highlighting ongoing challenges and solutions that aim to enable more widespread use of the technique in clinical practice.

Imaging Pulmonary Blood Flow Using Pseudocontinuous Arterial Spin Labeling (PCASL) With Balanced Steady-State Free-Precession (bSSFP) Readout at 1.5T

BACKGROUND

Quantitative assessment of pulmonary blood flow and visualization of its temporal and spatial distribution without contrast media is of clinical significance.

PURPOSE

To assess the potential of electrocardiogram (ECG)-triggered pseudocontinuous arterial spin labeling (PCASL) imaging with balanced steady-state free-precession (bSSFP) readout to measure lung perfusion under free-breathing (FB) conditions and to study temporal and spatial characteristics of pulmonary blood flow.

STUDY TYPE

Prospective, observational.

SUBJECTS

Fourteen volunteers; three patients with pulmonary embolism.

FIELD STRENGTH/SEQUENCES

1.5T, PCASL-bSSFP.

ASSESSMENT

The pulmonary trunk was labeled during systole. The following examinations were performed: 1) FB and timed breath-hold (TBH) examinations with a postlabeling delay (PLD) of 1000 msec, and 2) TBH examinations with multiple PLDs (100-1500 msec). Scan-rescan measurements were performed in four volunteers and one patient. Images were registered and the perfusion was evaluated in large vessels, small vessels, and parenchyma. Mean structural similarity indices (MSSIM) was computed and time-to-peak (TTP) of parenchymal perfusion in multiple PLDs was evaluated. Image quality reading was performed with three independent blinded readers.

STATISTICAL TESTS

Wilcoxon test to compare MSSIM, perfusion, and Likert scores. Spearman's correlation to correlate TTP and cardiac cycle duration. The repeatability coefficient (RC) and within-subject coefficient of variation (wCV) for scan-rescan measurements. Intraclass correlation coefficient (ICC) for interreader agreement.

RESULTS

Image registration resulted in a significant (P < 0.05) increase of MSSIM. FB perfusion values were 6% higher than TBH (3.28 ± 1.09 vs. 3.10 ± 0.99 mL/min/mL). TTP was highly correlated with individuals' cardiac cycle duration (Spearman = 0.89, P < 0.001). RC and wCV were better for TBH than FB (0.13-0.19 vs. 0.47-1.54 mL/min/mL; 6-7 vs. 19-60%). Image quality was rated very good, with ICCs 0.71-0.89.

DATA CONCLUSION

ECG-triggered PCASL-bSSFP imaging of the lung at 1.5T can provide very good image quality and quantitative perfusion maps even under FB. The course of labeled blood through the lung shows a strong dependence on the individuals' cardiac cycle duration.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 2 J. MAGN. RESON. IMAGING 2020;52:1767-1782.

Proton MRI of the Lung: How to Tame Scarce Protons and Fast Signal Decay

Pulmonary proton MRI techniques offer the unique possibility of assessing lung function and structure without the requirement for hyperpolarization or dedicated hardware, which is mandatory for multinuclear acquisition. Five popular approaches are presented and discussed in this review: 1) oxygen enhanced (OE)-MRI; 2) arterial spin labeling (ASL); 3) Fourier decomposition (FD) MRI and other related methods including self-gated noncontrast-enhanced functional lung (SENCEFUL) MR and phase-resolved functional lung (PREFUL) imaging; 4) dynamic contrast-enhanced (DCE) MRI; and 5) ultrashort TE (UTE) MRI. While DCE MRI is the most established and well-studied perfusion measurement, FD MRI offers a free-breathing test without any contrast agent and is predestined for application in patients with renal failure or with low compliance. Additionally, FD MRI and related methods like PREFUL and SENCEFUL can act as an ionizing radiation-free V/Q scan, since ventilation and perfusion information is acquired simultaneously during one scan. For OE-MRI, different concentrations of oxygen are applied via a facemask to assess the regional change in T₁ , which is caused by the paramagnetic property of oxygen. Since this change is governed by a combination of ventilation, diffusion, and perfusion, a compound functional measurement can be achieved with OE-MRI. The known problem of fast T₂ * decay of the lung parenchyma leading to a low signal-to-noise ratio is bypassed by the UTE acquisition strategy. Computed tomography (CT)-like images allow the assessment of lung structure with high spatial resolution without ionizing radiation. Despite these different branches of proton MRI, common trends are evident among pulmonary proton MRI: 1) free-breathing acquisition with self-gating; 2) application of UTE to preserve a stronger parenchymal signal; and 3) transition from 2D to 3D acquisition. On that note, there is a visible convergence of the different methods and it is not difficult to imagine that future methods will combine different aspects of the presented methods.

Nonuniform Fourier-decomposition MRI for ventilation- and perfusion-weighted imaging of the lung

PURPOSE

To improve the robustness of pulmonary ventilation- and perfusion-weighted imaging with Fourier decomposition (FD) MRI in the presence of respiratory and cardiac frequency variations by replacing the standard fast Fourier transform with the more general nonuniform Fourier transform.

THEORY AND METHODS

Dynamic coronal single-slice MRI of the thorax was performed in 11 patients and 5 healthy volunteers on a 1.5T whole-body scanner using a 2D ultra-fast balanced steady-state free-precession sequence with temporal resolutions of 4-9 images/s. For the proposed nonuniform Fourier-decomposition (NUFD) approach, the original signal with variable physiological frequencies that was acquired with constant sampling rate was retrospectively transformed into a signal with (ventilation or perfusion) frequency-adapted sampling rate. For that purpose, frequency tracking was performed with the synchro-squeezed wavelet transform. Ventilation- and perfusion-weighted NUFD amplitude and signal delay maps were generated and quantitatively compared with regularly sampled FD maps based on their signal-to-noise ratio (SNR).

RESULTS

Volunteers and patients showed statistically significant increases of SNR in frequency-adapted NUFD results compared to regularly sampled FD results. For ventilation data, the mean SNR increased by 43.4 % ± 25.3 % and 24.4 % ± 31.9 % in volunteers and patients, respectively; for perfusion data, SNR increased by 93.0 % ± 36.1 % and 75.6 % ± 62.8 % . Two patients showed perfusion signal in pulmonary areas with NUFD that could not be imaged with FD.

CONCLUSION

This study demonstrates that using nonuniform Fourier transform in combination with frequency tracking can significantly increase SNR and reduce frequency overlaps by collecting the signal intensity onto single frequency bins.

Non-contrast quantitative pulmonary perfusion using flow alternating inversion recovery at 3T: A preliminary study

PURPOSE

To demonstrate the initial feasibility of non-contrast quantitative pulmonary perfusion imaging at 3T using flow alternating inversion recovery (FAIR), and to evaluate the intra-session and inter-session reliability of FAIR measurements at 3T.

MATERIALS AND METHODS

Nine healthy volunteers were imaged using our own implementation of FAIR pulse sequence at 3T. Quantitative FAIR perfusion, both with and without larger pulmonary vessels, was correlated with global phase contrast (PC) measured blood flow in the right pulmonary artery (RPA). The same volunteers were also imaged with SPECT perfusion using technetium-99m-macroaggregated albumin and relative dispersion (RD) was assessed between FAIR and SPECT perfusion. Four additional healthy volunteers were evaluated for FAIR repeatability, using intra-class correlation coefficient (ICC) and Bland-Altman analysis. p<0.05 was considered statistically significant.

RESULTS

FAIR perfusion across all subjects was 858±605mL/100g/min (with vessels) and 629±294mL/100g/min (without vessels) and correlated significantly with the PC measured blood flow in the RPA (r=0.62, p<0.01 with vessels; r=0.73, p<0.001 without vessels). The median RD of FAIR perfusion across all subjects was 0.73 (with vessels) and 0.49 (without vessels), compared against 0.23 with SPECT perfusion. The intra/inter-session ICC of FAIR perfusion with vessels was 0.95/0.59 and improved to 0.96/0.72, when vessels were removed.

CONCLUSIONS

Non-contrast quantitative pulmonary perfusion imaging using FAIR is feasible at 3T. This may serve as a reliable method to assess regional lung perfusion at 3T to characterize and monitor treatment response in chronic lung disease without the concerns of repeated exposure to ionizing radiation or the accumulation of exogenous contrast agent.

Volumetric Arterial Spin-labeled Perfusion Imaging of the Kidneys with a Three-dimensional Fast Spin Echo Acquisition

RATIONALE AND OBJECTIVES

Renal perfusion measurements using noninvasive arterial spin-labeled (ASL) magnetic resonance imaging techniques are gaining interest. Currently, focus has been on perfusion in the context of renal transplant. Our objectives were to explore the use of ASL in patients with renal cancer, and to evaluate three-dimensional (3D) fast spin echo (FSE) acquisition, a robust volumetric imaging method for abdominal applications. We evaluate 3D ASL perfusion magnetic resonance imaging in the kidneys compared to two-dimensional (2D) ASL in patients and healthy subjects.

MATERIALS AND METHODS

Isotropic resolution (2.6 × 2.6 × 2.8 mm(3)) 3D ASL using segmented FSE was compared to 2D single-shot FSE. ASL used pseudo-continuous labeling, suppression of background signal, and synchronized breathing. Quantitative perfusion values and signal-to-noise ratio (SNR) were compared between 3D and 2D ASL in four healthy volunteers and semiquantitative assessments were made by four radiologists in four patients with known renal masses (primary renal cell carcinoma).

RESULTS

Renal cortex perfusion in healthy subjects was 284 ± 21 mL/100 g/min, with test-retest repeatability of 8.8%. No significant differences were found between the quantitative perfusion value and SNR in volunteers between 3D ASL and 2D ASL, or in 3D ASL with synchronized or free breathing. In patients, semiquantitative assessment by radiologists showed no significant difference in image quality between 2D ASL and 3D ASL. In one case, 2D ASL missed a high perfusion focus in a mass that was seen by 3D ASL.

CONCLUSIONS

3D ASL renal perfusion imaging provides isotropic-resolution images, with comparable quantitative perfusion values and image SNR in similar imaging time to single-slice 2D ASL.

A dynamic perfusion bioreactor approach for engineering respiratory tissues in-vitro

In vitro culture of respiratory tissues poses many challenges due to the intrinsic complexity of the respiratory system. Multiple cellular phenotypes comprise the respiratory epithelium and operate under dynamic, gas-interchanging conditions that should be replicated for near-physiologic cultivation of functional tissues in vitro. A novel biomimetic perfusion bioreactor system has been proposed to reconstitute key functional conditions of the human lung. This portable system consists of several biologically-inspired components: (i) a 3-dimensional (3-D) elastomeric soft tissue scaffold construct, (ii) a mechanical actuator, (iii) a perfusion system and (iv) gaseous exchange capabilities. These integrated components operate synergistically to create a unique, dynamic air-liquid interface (ALI) environment that allows controlled application of physiological and pathological strain while complementing standard cell culture techniques. This system holds potential for engineering 3-D tissues to meet growing demand for a range of applications, from more ethical and efficient pharmaceutical screening to clinical graft transplants.

Arterial spin labeling-fast imaging with steady-state free precession (ASL-FISP): a rapid and quantitative perfusion technique for high-field MRI

Arterial spin labeling (ASL) is a valuable non-contrast perfusion MRI technique with numerous clinical applications. Many previous ASL MRI studies have utilized either echo-planar imaging (EPI) or true fast imaging with steady-state free precession (true FISP) readouts, which are prone to off-resonance artifacts on high-field MRI scanners. We have developed a rapid ASL-FISP MRI acquisition for high-field preclinical MRI scanners providing perfusion-weighted images with little or no artifacts in less than 2 s. In this initial implementation, a flow-sensitive alternating inversion recovery (FAIR) ASL preparation was combined with a rapid, centrically encoded FISP readout. Validation studies on healthy C57/BL6 mice provided consistent estimation of in vivo mouse brain perfusion at 7 and 9.4 T (249 ± 38 and 241 ± 17 mL/min/100 g, respectively). The utility of this method was further demonstrated in the detection of significant perfusion deficits in a C57/BL6 mouse model of ischemic stroke. Reasonable kidney perfusion estimates were also obtained for a healthy C57/BL6 mouse exhibiting differential perfusion in the renal cortex and medulla. Overall, the ASL-FISP technique provides a rapid and quantitative in vivo assessment of tissue perfusion for high-field MRI scanners with minimal image artifacts.

Lung Parenchymal Signal Intensity in MRI: A Technical Review with Educational Aspirations Regarding Reversible Versus Irreversible Transverse Relaxation Effects in Common Pulse Sequences

Lung parenchyma is challenging to image with proton MRI. The large air space results in ~l/5th as many signal-generating protons compared to other organs. Air/tissue magnetic susceptibility differences lead to strong magnetic field gradients throughout the lungs and to broad frequency distributions, much broader than within other organs. Such distributions have been the subject of experimental and theoretical analyses which may reveal aspects of lung microarchitecture useful for diagnosis. Their most immediate relevance to current imaging practice is to cause rapid signal decays, commonly discussed in terms of short T2* values of 1 ms or lower at typical imaging field strengths. Herein we provide a brief review of previous studies describing and interpreting proton lung spectra. We then link these broad frequency distributions to rapid signal decays, though not necessarily the exponential decays generally used to define T2* values. We examine how these decays influence observed signal intensities and spatial mapping features associated with the most prominent torso imaging sequences, including spoiled gradient and spin echo sequences. Effects of imperfect refocusing pulses on the multiple echo signal decays in single shot fast spin echo (SSFSE) sequences and effects of broad frequency distributions on balanced steady state free precession (bSSFP) sequence signal intensities are also provided. The theoretical analyses are based on the concept of explicitly separating the effects of reversible and irreversible transverse relaxation processes, thus providing a somewhat novel and more general framework from which to estimate lung signal intensity behavior in modern imaging practice.

Comparison between high-resolution CT and MRI using a very short echo time in patients with cystic fibrosis with extra focus on mosaic attenuation

BACKGROUND

It would be beneficial to establish pulmonary MRI as a complementary approach to CT for direct visualization of mosaic perfusion, bullae, and emphysema in patients with cystic fibrosis.

OBJECTIVES

The purpose of this study was to compare both modalities, CT and MRI, using the Helbich-Bhalla score with a special focus on reliable detection of a mosaic pattern.

METHODS

Out of 51 patients examined by MRI on a 1.5-Tesla system during a period of 2 years, 19 patients were scheduled for additional low-dose CT in a clinical context. The MRI protocol comprised a gradient echo (GRE) sequence with a very short echo time (TE = 0.8 ms) in inspiration and expiration, a 3-D GRE sequence in breath hold, and a fast spin echo sequence with respiration and ECG triggering. MDCT was carried out in inspiration and adapted to body weight using 100 or 120 kV, 30-60 mA, 1- and 3-mm slice thicknesses, as well as low and high kernels. Additionally incremental slices in 3 positions were recorded in expiration for distinct detection of air trapping. CT and MRI analyses were performed by two radiologic readers in consensus unaware of the clinical parameters. The Helbich-Bhalla score of both examinations was correlated. Mean difference and accordance were assessed in each category.

RESULTS

There was a strong correlation between CT and MRI (R = 0.87, p < 0.01). The mean Helbich-Bhalla score for CT was 12.2 (range 1-18) and for MRI it was 11.7 (range 2-19). The mean difference was 0.5 points. Besides this strong correlation for findings (bronchiectasis, mucus plugging, peribronchial thickening, and consolidation) with a prolonged T2 TE in MRI, we could also state a qualitative agreement of 95-100% in the categories with short T2 and low signal intensity in MRI as emphysema, bullae, and mosaic perfusion.

CONCLUSIONS

These results suggest that in our patient group none of the relevant findings were missed by MR imaging and reading.

A gated-7T MRI technique for tracking lung tumor development and progression in mice after exposure to low doses of ionizing radiation

A gated-7T magnetic resonance imaging (MRI) application is described that can accurately and efficiently measure the size of in vivo mouse lung tumors from ∼0.1 mm(3) to >4 mm(3). This MRI approach fills a void in radiation research because the technique can be used to noninvasively measure the growth rate of lung tumors in large numbers of mice that have been irradiated with low doses (<50 mGy) without the additional radiation exposure associated with planar X ray, CT or PET imaging. High quality, high resolution, reproducible images of the mouse thorax were obtained in ∼20 min using: (1) a Bruker 7T micro-MRI scanner equipped with a 60 mm inner diameter gradient insert capable of generating a maximum gradient of 1000 mT/m; (2) a 35 mm inner diameter quadrature radiofrequency volume coil; and (3) an electrocardiogram and respiratory gated Fast Low Angle Shot (FLASH) pulse sequence. The images had an in-plane image resolution of 98 μm and a 0.5 mm slice thickness. Tumor diameter measured by MRI was highly correlated (R(2) = 0.97) with the tumor diameter measured by electronic calipers. Data generated with an initiation/promotion mouse model of lung carcinogenesis and this MRI technique demonstrated that mice exposed to 4 weekly fractions of 10, 30 or 50 mGy of CT radiation had the same lung tumor growth rate as that measured in sham-irradiated mice. In summary, this high-field, double-gated MRI approach is an efficient way of quantitatively tracking lung tumor development and progression after exposure to low doses of ionizing radiation.

Pulmonary perfusion imaging using MRI: clinical application

BACKGROUND

Lung perfusion is one of the key components of oxygenation. It is hampered in pulmonary arterial diseases and secondary due to parenchymal diseases.

METHODS

Assessment is frequently required during the workup of a patient for either of these disease categories.

RESULTS

This review provides insight into imaging techniques, qualitative and quantitative evaluation, and focuses on clinical application of MR perfusion.

CONCLUSION

The two major techniques, non-contrast-enhanced (arterial spin labeling) and contrast-enhanced perfusion techniques, are discussed.

Improved quantification of brain perfusion using FAIR with active suppression of superior tagging (FAIR ASST)

PURPOSE

To address two problems for perfusion studies in the middle or inferior brain regions: (1) to reduce venous artifacts due to the intrinsic superior labeling of FAIR; (2) to alleviate the discrepancy of the existence of both superior and inferior boluses, but with only the inferior bolus having a temporally defined bolus width with Q2TIPs or QUIPSS.

MATERIALS AND METHODS

Superior tagging suppression methods for FAIR with different combinations of pre- and postinversion superior saturation pulses were evaluated and compared with FAIR with Q2TIPS for producing perfusion maps of superior, middle, and inferior brain regions.

RESULTS

One preinversion plus two postinversion superior saturation radio frequency pulses effectively suppressed the superior tagging of FAIR and sufficiently eliminated venous artifacts without negative effects, avoiding the overestimations of cerebral blood flow that can occur in FAIR.

CONCLUSION

FAIR ASST improves FAIR with Q2TIPS and provides more reliable and accurate blood flow estimations for perfusion studies of middle and lower brain regions. FAIR ASST confers the advantages of asymmetric PASL techniques, such as PICORE, in which only the inferiorly labeled blood is used for perfusion quantification, to the symmetric PASL technique FAIR, while preserving the robustness of FAIR against MT effects.

Perspectives on optimizing trial design and endpoints in peripheral arterial disease: a case for imaging-based surrogates as endpoints of functional efficacy

Surrogate endpoints are important for validation of mechanism, early proof of concept, and the rational design of clinical trials for regulatory approval of drugs. The recent failure of several drugs in peripheral arterial disease (PAD) and in atherosclerosis highlights the importance of understanding drug effect and is a clarion call for better endpoints. This review focuses on aspects relating to the current state of surrogate endpoints in PAD and reviews emerging endpoints using imaging approaches that may have the potential of improving study design in PAD.

Magnetic resonance perfusion imaging without contrast media

PURPOSE

Principles of magnetic resonance imaging techniques providing perfusion-related contrast weighting without administration of contrast media are reported and analysed systematically. Especially common approaches to arterial spin labelling (ASL) perfusion imaging allowing quantitative assessment of specific perfusion rates are described in detail. The potential of ASL for perfusion imaging was tested in several types of tissue.

METHODS

After a systematic comparison of technical aspects of continuous and pulsed ASL techniques the standard kinetic model and tissue properties of influence to quantitative measurements of perfusion are reported. For the applications demonstrated in this paper a flow-sensitive alternating inversion recovery (FAIR) ASL perfusion preparation approach followed by true fast imaging with steady precession (true FISP) data recording was developed and implemented on whole-body scanners operating at 0.2, 1.5 and 3 T for quantitative perfusion measurement in various types of tissue.

RESULTS

ASL imaging provides a non-invasive tool for assessment of tissue perfusion rates in vivo. Images recorded from kidney, lung, brain, salivary gland and thyroid gland provide a spatial resolution of a few millimetres and sufficient signal to noise ratio in perfusion maps after 2-5 min of examination time.

CONCLUSIONS

Newly developed ASL techniques provide especially high image quality and quantitative perfusion maps in tissues with relatively high perfusion rates (as also present in many tumours). Averaging of acquisitions and image subtraction procedures are mandatory, leading to the necessity of synchronization of data recording to breathing in abdominal and thoracic organs.

Non-contrast-enhanced perfusion and ventilation assessment of the human lung by means of fourier decomposition in proton MRI

Assessment of regional lung perfusion and ventilation has significant clinical value for the diagnosis and follow-up of pulmonary diseases. In this work a new method of non-contrast-enhanced functional lung MRI (not dependent on intravenous or inhalative contrast agents) is proposed. A two-dimensional (2D) true fast imaging with steady precession (TrueFISP) pulse sequence (TR/TE = 1.9 ms/0.8 ms, acquisition time [TA] = 112 ms/image) was implemented on a 1.5T whole-body MR scanner. The imaging protocol comprised sets of 198 lung images acquired with an imaging rate of 3.33 images/s in coronal and sagittal view. No electrocardiogram (ECG) or respiratory triggering was used. A nonrigid image registration algorithm was applied to compensate for respiratory motion. Rapid data acquisition allowed observing intensity changes in corresponding lung areas with respect to the cardiac and respiratory frequencies. After a Fourier analysis along the time domain, two spectral lines corresponding to both frequencies were used to calculate the perfusion- and ventilation-weighted images. The described method was applied in preliminary studies on volunteers and patients showing clinical relevance to obtain non-contrast-enhanced perfusion and ventilation data.

[Oxygen-enhanced functional MR lung imaging]

Current diagnostic tools for the assessment of lung function are limited by global measurements or the need for radioactive tracers. Ideally, these tools should allow quantitative, regional distinct analyses without exposure to radiation. The current paper presents oxygen-enhanced functional MRI for assessment of lung ventilation. First applied in humans in 1996, a considerable amount of experience is now available on 1.5T scanners. The generation of quantitative T1-maps shows a high clinical potential. Low-field MR scanners, which are mostly open-designed, are especially interesting for functional lung imaging. The open design has advantages in respect to patient comfort by lower noise production and easy access to the patients and the costs are lower (no need for helium cooling). Lower signal-to-noise ratios can be overcome by changing the relaxation times. New navigator techniques allow further compensations. This article focuses on the presentation of low-field scanners and the application of T1 and T2(*) maps is described for healthy volunteers and first patients.

Autoimmune thyroid disease: arterial spin-labeling perfusion MR imaging

PURPOSE

To assess thyroid perfusion in patients with autoimmune thyroid diseases compared with that in healthy control subjects by using an arterial spin-labeling (ASL) magnetic resonance (MR) technique and to assess whether thyroid perfusion is associated with endocrine laboratory abnormalities.

MATERIALS AND METHODS

This study was approved by the local institutional review board. All participants gave written informed consent. Perfusion imaging of the thyroid gland was performed in 10 patients with Graves disease (GD) and 10 patients with Hashimoto thyroiditis (HT). Ten healthy individuals served as control subjects. Perfusion imaging was performed with a 1.5-T MR unit by using a flow-sensitive alternating inversion recovery-true fast imaging with steady-state precession technique. Perfusion maps of the entire thyroid gland were calculated on the basis of extended Bloch equations. Analysis of variance with a post hoc test (Tukey honestly significant difference) was performed to assess differences in perfusion between groups. Associations between perfusion and laboratory parameters were analyzed with univariate regression analysis.

RESULTS

Mean thyroid perfusion was 1596 mL/min/100 g +/- 436 (standard deviation) in patients with GD, 825 mL/min/100 g +/- 264 in patients with HT, and 491 mL/min/100 g +/- 89 in healthy control subjects. Perfusion was significantly higher in patients with GD (P < .0001) and those with HT (P < .05) than in control subjects. A significant difference in thyroid perfusion was detected between the two autoimmune entities (P < .0001). In patients with GD, significant associations were found between perfusion and serum concentrations of free thyroid hormones and anti-thyroid-stimulating hormone receptor antibodies (P < .05 for all).

CONCLUSION

Quantitative ASL perfusion imaging of the thyroid gland revealed significant perfusion differences in the autoimmune thyroid diseases GD and HT. Absolute quantification of thyroid perfusion may be useful in the clinical assessment of autoimmune thyroid disorders and when monitoring therapeutic treatment in GD.

Strategies for reducing respiratory motion artifacts in renal perfusion imaging with arterial spin labeling

Arterial spin labeling (ASL) perfusion measurements may have many applications outside the brain. In the abdomen, severe image artifacts can arise from motions between acquisitions of multiple signal averages in ASL, even with single-shot image acquisition. Background suppression and respiratory motion synchronization techniques can be used to ameliorate these artifacts. Two separate in vivo studies of renal perfusion imaging using pulsed continuous ASL (pCASL) were performed. The first study assessed various combinations of background suppression and breathing strategies. The second investigated the retrospective sorting of images acquired during free breathing based on respiratory position. Quantitative assessments of the test-retest repeatability of perfusion measurements and the image quality scored by two radiologists were made. Image quality was most significantly improved by using background suppression schemes and controlled breathing when compared to other combinations without background suppression or with free breathing, assessed by test-retests (5% level, F-test), and by radiologists' scores (5% level, Mann-Whitney U-test). Under free breathing, retrospectively sorting images based on respiratory position showed significant improvement. Both radiologists found 100% of the images had preferable image sharpness after sorting. High-quality renal perfusion measurements with reduced respiratory motion artifacts have been demonstrated using ASL when appropriate background suppression and breathing strategies are applied.

Lung imaging under free-breathing conditions

Respiratory motion and pulsatile blood flow can generate artifacts in morphological and functional lung imaging. Total acquisition time, and thus the achievable signal to noise ratio, is limited when performing breath-hold and/or electrocardiogram-triggered imaging. To overcome these limitations, imaging during free respiration can be performed using respiratory gating/triggering devices or navigator echoes. However, these techniques provide only poor gating resolution and can induce saturation bands and signal fluctuations into the lung volume. In this work, acquisition schemes for nonphase encoded navigator echoes were implemented into different sequences for morphological and functional lung imaging at 1.5 Tesla (T) and 0.2T. The navigator echoes allow monitoring of respiratory motion and provide an ECG-trigger signal for correction of the heart cycle without influencing the imaged slices. Artifact free images acquired during free respiration using a 3D GE, 2D multislice TSE or multi-Gradient Echo sequence for oxygen-enhanced T(2)(*) quantification are presented.

Perfusion imaging of the pancreas using an arterial spin labeling technique

PURPOSE

To investigate the feasibility of perfusion imaging of the pancreas using an arterial spin labeling (ASL) technique.

MATERIALS AND METHODS

An adapted flow-sensitive alternating inversion recovery (FAIR)-TrueFISP ASL technique was implemented on a 1.5 T scanner. Anatomical and perfusion imaging in three different parts of the pancreas were performed in 10 healthy volunteers. Quantitative perfusion values were calculated using the extended Bloch equations.

RESULTS

Perfusion images of all subjects showed diagnostic image quality in the pancreatic tail and the head. Assessment of pancreatic tissue perfusion was possible in all organ parts. Mean perfusion values were 271 +/- 79 mL/100g/min in the head, 351 +/- 112 mL/100g/min in the body, and 243 +/- 55 mL/100g/min in the tail of the pancreas. Total examination time for perfusion imaging of the entire organ was 15.4 minutes.

CONCLUSION

FAIR-TrueFISP permits analysis of pancreatic tissue perfusion with good image quality in a clinically applicable measuring time. Assessment of perfusion disorders may be useful in the diagnosis of inflammatory pancreatic pathologies, endocrine and exocrine pancreatic disorders, and in monitoring of pancreatic transplants.

MR measurement of blood flow in the parotid gland without contrast medium: a functional study before and after gustatory stimulation

PURPOSE

To investigate the feasibility of blood flow imaging in the parotid gland using the arterial spin labeling (ASL) technique for assessment of functional changes in the parotid gland after gustatory stimulation.

MATERIALS AND METHODS

Anatomical and ASL imaging of the parotid gland was performed in eight healthy volunteers before and after gustatory stimulation over a period of 17 min. All measurements were carried out in a 1.5 T whole-body MR unit. ASL and data recording were performed with an adapted FAIR TrueFISP (flow-sensitive alternating inversion-recovery true fast imaging with steady precession) technique. Maps of estimated tissue blood inflow in both parotid glands were derived using a simplified model and the extended Bloch equations.

RESULTS

Delineation of the parotid glands was possible on FAIR TrueFISP images in all cases. In the 160 s period immediately after stimulation, a significant (P < 0.01) mean increase of 62% in the estimated parotid blood flow was observed. Estimated baseline blood flow before gustatory stimulation ranged from 226 to 500 mL/min/100 g (mean +/- SD 335 +/- 86). These rates increased in the 160 s immediately after stimulation to 404-772 mL/min/100 g (mean 542 +/- 108). In all volunteers, blood flow returned to near baseline values within the observation period. No statistically significant difference between the right and left parotid was observed in baseline and peak blood flow.

CONCLUSION

ASL FAIR TrueFISP is feasible for functional characterization of the parotid glands. Assessment of changes in blood flow in the parotid gland could serve as a diagnostic tool in patients suffering from xerostomia.

Health information technology will shift the medical care paradigm

The current paradigm of medical care depends heavily on the autonomous and highly trained doctor to collect and process information necessary to care for each patient. This paradigm is challenged by the increasing requirements for knowledge by both patients and doctors; by the need to evaluate populations of patients inside and outside one's practice; by consistently unmet quality of care expectations; by the costliness of redundant, fragmented, and suboptimal care; and by a seemingly insurmountable demand for chronic disease care. Medical care refinements within the old paradigm may not solve these challenges, suggesting a shift to a new paradigm is needed. A new paradigm could be considerably more reliant on health information technology because that offers the best option for addressing our challenges and creating a foundation for future medical progress, although this process will be disruptive.