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

Abstract

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.

In vivo methods and applications of xenon-129 magnetic resonance

“…Advances in scanner hardware and pulse sequence design have enabled the assessment of lung morphology (structure) with 1 H MRI [32] , for example using ultra-short echo-time (UTE) sequences [34] , [35] , which can provide an alternative to CT in some situations [35] , [36] . Functional lung images can be obtained with 1 H MRI [37] either by using external contrast agents which alter T 1 relaxation times, as with dynamic contrast enhanced perfusion imaging [38] and oxygen-enhanced imaging [39] , or by using the modulation of the proton signal caused by respiratory and cardiac motion to infer lung ventilation and perfusion indirectly [40] . While these techniques offer the advantage of functional imaging without the need for additional hardware, hyperpolarised gas MRI allows direct imaging of the inhaled contrast agent and can be tuned to different aspects of lung function, and also probe more distal organs in the case of hyperpolarised 129 Xe MRI.…”

3D phase‐resolved functional lung ventilation MR imaging in healthy volunteers and patients with chronic pulmonary disease

“…Despite the inherent limitations due to low proton density of the pulmonary parenchyma and susceptibility artifacts, 1 proton MRI of the lung is gaining interest. 2 Aside from its acquisition without radiation exposure, which allows for repeated measurements, MRI has functional imaging capacities, for example, assessment of ventilation, perfusion, or diffusion.…”

“…The measurement of regional pulmonary ventilation abnormalities is of great importance in patients with chronic lung diseases such as asthma, cystic fibrosis (CF), or chronic obstructive pulmonary disease (COPD). Despite the inherent limitations due to low proton density of the pulmonary parenchyma and susceptibility artifacts, 1 proton MRI of the lung is gaining interest 2 . Aside from its acquisition without radiation exposure, which allows for repeated measurements, MRI has functional imaging capacities, for example, assessment of ventilation, perfusion, or diffusion.…”

Imaging of congenital lung diseases presenting in the adulthood: a pictorial review

“…Dissolved molecular oxygen and deoxyhemoglobin reduce the relaxation time of T1 and increase signal intensity on T1 images. So, the regional oxygen transfer which is affected by ventilation, perfusion, and diffusion capacity of pulmonary parenchyma can be shown [ 79 , 83 , 84 ]. ASL-MRI is a non-contrast method for the evaluation of pulmonary ventilation and perfusion.…”

“…Functional analysis of 3D UTE MRI can provide the evaluation of ventilation inhomogeneity and hyperinflation in patients with congenital lung diseases such as cystic fibrosis [ 76 – 78 ]. Lung function can also be assessed with dynamic contrast-enhanced (DCE) MRI, oxygen-enhanced (OE) MRI, arterial spin labeling (ASL) and, phase-resolved functional lung (PREFUL) [ 79 , 80 ]. DCE-MRI is the most well-known and well-established method of lung perfusion.…”