Phosphorus-31 MRI of hard and soft solids using quadratic echo line-narrowing

Abstract

Magnetic resonance imaging (MRI) of solids is rarely attempted. One of the main reasons is that the broader MR linewidths, compared to the narrow resonance of the hydrogen ( 1 H) in free water, limit both the attainable spatial resolution and the signal-to-noise ratio. Basic physics research, stimulated by the quest to build a quantum computer, gave rise to a unique MR pulse sequence that offers a solution to this long-standing problem. The "quadratic echo" significantly narrows the broad MR spectrum of solids. Applying field gradients in sync with this line-narrowing sequence offers a fresh approach to carry out MRI of hard and soft solids with high spatial resolution and with a wide range of potential uses. Here we demonstrate that this method can be used to carry out three-dimensional MRI of the phosphorus ( 31 P) in ex vivo bone and soft tissue samples.bone mineral | cell membranes M agnetic resonance imaging (MRI) has become an invaluable tool for clinical medicine, fundamental biomedical research, the physical sciences, and engineering (1). Typically, MRI detects only the signal from free water, using just a single nuclear isotope ( 1 H). Extending the reach of MRI to the study of other elements, and to hard or soft solids, opens new frontiers of discovery. One example is phosphorus ( 31 P), which is abundant in both bone mineral and cell membranes, so 31 P MRI of tissues is, in principle, possible. In practice, however, the slower motion of 31 P in those environments (compared to 1 H in water) results in much broader MR spectra (Fig. S1), limiting both the attainable spatial resolution and the signal-to-noise ratio (2). In this paper, we describe the use of a pulse sequence to narrow the 31 P MR spectrum of solids to that of a liquid (3), making high-resolution imaging possible. This line-narrowing MR sequence was discovered in the course of basic research (3-6) initiated by Kane's proposal (7) to build a quantum computer using phosphorus spins in silicon. Applying field gradients in synch with this sequence, we have obtained high-resolution 3D MR images of the 31 P in a variety of ex vivo animal bone samples. This is a unique probe of a key element in bone mineral, which complements existing assessments of bone quality. Using the same approach, we have obtained 3D MR images of 31 P in several ex vivo soft tissues.Bone is a composite material (8), containing approximately 45% bone mineral by volume (9). Bone mineral is similar to calcium hydroxyapatite (i.e., Ca 10 ðOHÞ 2 ðPO 4 Þ 6 Þ, but it is less crystalline, and it has a unique stoichiometry (10). The spatial distribution, composition, and quantity of bone mineral are primarily responsible for the compressive strength and stiffness of bone (8-10). While a few 31 P MRI studies have successfully targeted in vivo (10-12) and ex vivo (10, 13, 14, 15) bone, the broad MR spectra have limited the achievable spatial resolution to no better than 0.5 mm (15) and more typically in the range of 2 mm. There is currently a great need to probe the internal compositi...

Determination of mean ionization potential using magnetic resonance imaging for the reduction of proton beam range uncertainties: theory and application

“…The average ratio of minerals to P from Table 2 was 5.57 and in line with the theoretical value from HA. As MRI for 31 P in solid materials is still an active area of research (Frey et al, 2012;Seifert et al, 2014;Seifert and Wehrli, 2016a, b) with potential for 31 P density measurements, we note the theoretical potential for signal acquisition but will not show imaging data support in this study. (White et al, 1991) and are regularly cited as "standard" tissue compositions for the purposes of DECT and stoichiometric calibrations and proton stopping power ratio calculations.…”

Accelerating multidimensional NMR and MRI experiments using iterated maps

“…In Figure 5, our reconstruction algorithm is applied to sparsely sampled 3D 31 P MRI of solids data (see [13] and Supporting Information for more information on how this data was acquired). In this experiment, the image is restricted to a portion of the full field of view (FOV) in order to avoid known artifacts (see Figure S2), which enables the construction of strong constraints in the P̂ 1 mask (where k⃗ and r⃗ in MRI map onto t⃗ and f⃗ in NMR).…”

“…We became interested in this problem while developing a 3D MRI of solids technique, since the long T 1 of phosphorus-31 in bone meant that one dense data set required T acq > 45 hours [13]. Taking the first step towards sparse MRI, or compressed-sensing (CS) MRI [14, 15], k⃗ space was sparsely-sampled in a pseudo-random way for T acq < 1.5 hours.…”

“…Taking the first step towards sparse MRI, or compressed-sensing (CS) MRI [14, 15], k⃗ space was sparsely-sampled in a pseudo-random way for T acq < 1.5 hours. We then Fourier transformed this data, yielding an image of modest quality (Figure S3 in [13]) but in much less time . Typically, the next step to improve upon this image is to adopt non-Fourier reconstruction algorithms like l 1 minimization [16, 17, 18].…”

Long-lived selective spin echoes in dipolar solids under periodic and aperiodicπ-pulse trains

“…Preserving quantum coherence is of importance to a variety of fields, from quantum computing and information processing 1,2 , where unitary control of qubits is a critical requirement of many proposed algorithms, to biomedical applications, where longlived signals can help improve resolution in imaging applications 3,4 . As such, a variety of techniques have been developed to increase the lifetime of the coherence being studied by controlling the system-environment interactions.…”