Quantitative bone matrix density measurement by water‐ and fat‐suppressed proton projection MRI (WASPI) with polymer calibration phantoms

The density of the organic matrix of bone substance is a critical parameter necessary to clinically evaluate and distinguish structural and metabolic pathological conditions such as osteomalacia in adults and rickets in growing children. Water‐ and fat‐suppressed proton projection MRI (WASPI) was developed as a noninvasive means to obtain this information. In this study, a density calibration phantom was developed to convert WASPI intensity to true bone matrix density. The phantom contained a specifically designed poly(ethylene oxide)/poly(methyl methacrylate) (PEO/PMMA) blend, whose MRI properties (T₁, T₂, and resonance linewidth) were similar to those of solid bone matrix (collagen, tightly bound water, and other immobile molecules), minimizing the need to correct for differences in T₁ and/or T₂ relaxation between the phantom and the subject. Cortical and trabecular porcine bone specimens were imaged using WASPI with the calibration phantom in the field of view (FOV) as a stable intensity reference. Gravimetric and amino acid analyses were carried out on the same specimens after WASPI, and the chemical results were found to be highly correlated (r² = 0.98 and 0.95, respectively) to the WASPI intensity. By this procedure the WASPI intensity can be used to obtain the true bone matrix mass density in g cm^–3. Magn Reson Med 60:1433–1443, 2008. © 2008 Wiley‐Liss, Inc.     

Bone matrix imaged in vivo by water‐ and fat‐suppressed proton projection MRI (WASPI) of animal and human subjects

Purpose:

To demonstrate water‐ and fat‐suppressed proton projection MRI (WASPI) in a clinical scanner to visualize the solid bone matrix in animal and human subjects.

Materials and Methods:

Pig bone specimens and polymer pellets were used to optimize the WASPI method in terms of soft‐tissue suppression, image resolution, signal‐to‐noise ratio, and scan time on a 3T MRI scanner. The ankles of healthy 2–3‐month‐old live Yorkshire pigs were scanned with the optimized method. The method was also applied to the wrists of six healthy adult human volunteers to demonstrate the feasibility of the WASPI method in human subjects. A transmit/receive coil built with proton‐free materials was utilized to produce a strong B₁ field. A fast transmit/receive switch was developed to reduce the long receiver dead time that would otherwise obscure the signals.

Results:

Clear 3D WASPI images of pig ankles and human wrists, showing only the solid bone matrix and other tissues with high solid content (eg, tendons), with a spatial resolution of 2.0 mm in all three dimensions were obtained in as briefly as 12 minutes.

Conclusion:

WASPI of the solid matrix of bone in humans and animals in vivo is feasible. J. Magn. Reson. Imaging 2010;31:954–963. ©2010 Wiley‐Liss, Inc.     

Density of organic matrix of native mineralized bone measured by water- and fat-suppressed proton projection MRI.

Water- and fat-suppressed projection MR imaging (WASPI) utilizes the large difference between the proton T(2) (*)s of the solid organic matrix and the fluid constituents of bone to suppress the fluid signals while preserving solid matrix signals. The solid constituents include collagen and some molecularly immobile water and exhibit very short T(2) (*). The fluid constituents include mobile water and fat, with long T(2) (*). In WASPI, chemical shift selective low-power pi/2 pulses excite mobile water and fat magnetization which is subsequently dephased by gradient pulses, while the magnetization of collagen and immobile water remains mostly in the z-direction. Additional selective pi pulses in alternate scans further cancel the residual water and fat magnetization. Following water and fat suppression, the matrix signal is excited by a short hard pulse and the free induction decay acquired in the presence of a gradient in a 3D projection method. WASPI was implemented on a 4.7 T MR imaging system and tested on phantoms and bone specimens, enabling excellent visualization of bone matrix. The bone matrix signal per unit volume of bovine trabecular specimens was measured by this MR technique and compared with that determined by chemical analysis. This method could be used in combination with bone mineral density measurement by solid state (31)P projection MRI to determine the degree of bone mineralization.     

Bone mineral imaged in vivo by 31P solid state MRI of human wrists

Purpose:

To implement solid state ³¹P MRI (³¹P SMRI) in a clinical scanner to visualize bone mineral.

Materials and Methods:

Wrists of seven healthy volunteers were scanned. A quadrature wrist ³¹P transmit/receive coil provided strong B₁ and good signal‐to‐noise ratio (SNR). A ¹H‐³¹P frequency converter was constructed to enable detection of the ³¹P signal by means of the ¹H channel. Data points lost in the receiver dead time were recovered by a second acquisition with longer dwell time and lower gradient strength.

Results:

Three‐dimensional ³¹P images, showing only bone mineral of the wrist, were obtained with a clinical 3 Tesla (T) scanner. In the best overall case an image with isotropic resolution of ～5.1 mm and SNR of 30 was obtained in 37 min. ³¹P NMR properties (resonance line width 2 kHz and T₁ 17–19 s) of in vivo human bone mineral were measured.

Conclusion:

In vivo ³¹P SMRI visualization of human wrist bone mineral with a clinical MR scanner is feasible with suitable modifications to circumvent the scanners' limitations in reception of short‐T₂ signals. Frequency conversion methodology is useful for implementing ³¹P SMRI measurements on scanners which do not have multinuclear capability or for which the multinuclear receiver dead time is excessive. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Ultrashort echo time spectroscopic imaging (UTESI) of cortical bone.

Cortical bone in the mature skeleton has a short T(2)* and produces no detectable signal with conventional MR sequences. A two-dimensional ultrashort echo time (UTE) sequence employing half radio frequency (RF) pulse excitations and radial ramp sampling reduces the effective TE to 8 micros and is capable of detecting signals from cortical bone. We propose a time-efficient UTE spectroscopic imaging (UTESI) technique based on an interleaved variable TE acquisition, preceded by long T(2)* signal suppression using either a 90 degrees pulse and gradient dephasing or an inversion pulse and nulling. The projections were divided into multiple groups with the data for each group being collected with progressively increasing TE and interleaved projection angles. The undersampled projections within each group sparsely covered k-space. A view sharing and sliding window reconstruction algorithm was implemented to reconstruct images at each TE, followed by Fourier transformation in the time domain to generate spectroscopic images. T(2)* was quantified through either exponential fitting of the time domain images or line fitting of the magnitude spectrum. Relative water content and the resonance frequency shift due to bulk susceptibility were also evaluated. The feasibility of this technique was demonstrated with phantom and volunteer studies on a clinical 3T scanner.     

Comparison of optimized soft-tissue suppression schemes for ultrashort echo time MRI

Ultrashort echo time (UTE) imaging with soft-tissue suppression reveals short-T(2) components (typically hundreds of microseconds to milliseconds) ordinarily not captured or obscured by long-T(2) tissue signals on the order of tens of milliseconds or longer. Therefore, the technique enables visualization and quantification of short-T(2) proton signals such as those in highly collagenated connective tissues. This work compares the performance of the three most commonly used long-T(2) suppression UTE sequences, i.e., echo subtraction (dual-echo UTE), saturation via dual-band saturation pulses (dual-band UTE), and inversion by adiabatic inversion pulses (IR-UTE) at 3 T, via Bloch simulations and experimentally in vivo in the lower extremities of test subjects. For unbiased performance comparison, the acquisition parameters are optimized individually for each sequence to maximize short-T(2) signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) between short- and long-T(2) components. Results show excellent short-T(2) contrast which is achieved with these optimized sequences. A combination of dual-band UTE with dual-echo UTE provides good short-T(2) SNR and CNR with less sensitivity to B(1) homogeneity. IR-UTE has the lowest short-T(2) SNR efficiency but provides highly uniform short-T(2) contrast and is well suited for imaging short-T(2) species with relatively short T(1) such as bone water.     

CT and MRI of the semicircular canals in the normal and diseased temporal bone

Imaging of the semicircular canals specifically is part of the imaging process of the temporal bone in general. The semicircular canals are easily seen on CT images and 3DFT-CISS-weighted MR images, both performed with 1.0-mm-thick slices, or even thinner slices. In selected cases, the T1-weighted images give unique information on the semicircular canals. This article briefly reviews the variety of semicircular canal anomalies that are most frequently present and can be routinely seen on CT and MR examinations of the temporal bone. It also provides a list that can be used by the radiologist in clinical practice to decide which technique, CT or MR, should be used to detect specific anomalies at the level of the semicircular canals.     

Magnetization transfer contrast imaging in bovine and human cortical bone applying an ultrashort echo time sequence at 3 Tesla

Magnetization transfer (MT) contrast imaging reveals interactions between free water molecules and macromolecules in a variety of tissues. The introduction of ultrashort echo time (UTE) sequences to clinical whole‐body MR scanners expands the possibility of MT imaging to tissues with extremely fast signal decay such as cortical bone. The aim of this study was to investigate the MT effect of bovine cortical bone in vitro on a 3 Tesla whole‐body MR unit. A 3D‐UTE sequence with a rectangular‐shaped on‐resonant excitation pulse and a Gaussian‐shaped off‐resonant saturation pulse for MT preparation was applied. The flip angle and off‐resonance frequency of the MT pulse was systematically varied. Measurements on various samples of bovine cortical bone, agar gel, aqueous manganese chloride solutions, and solid polymeric materials (polyurethane) were performed, followed by preliminary applications on human tibial bone in vivo. Direct on‐resonant saturation effects of the MT prepulses were calculated numerically by means of Bloch's equations. Corrected for direct saturation effects dry and fresh bovine cortical bone showed “true” MTR values of 0.26 and 0.21, respectively. In vivo data were obtained from three healthy subjects and showed MTR values of 0.30 ± 0.08. In vivo studies into MT of cortical bone might have the potential to give new insights in musculoskeletal pathologies. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Investigation of MR decay rates in microphantom models of trabecular bone

MR measurements of transverse relaxation time, T2*, in trabecular bone may provide both structural and density-related information for assessment of bone mineral status in osteoporosis. Using submillimeter scale glass phantoms as simplified models of trabecular bone, we have made a quantitative investigation of the dependence of T2* decay on modeled trabecular microstructure and MR image resolution. The experimental MR data are in excellent agreement with predictions from a computer simulation. Decreasing the modeled trabecular bone volume fraction, ζ, decreases the decay rate, as expected. However, if trabecular width and spacing are both increased without changing ζ, the decay rate is unchanged. The measured decay curves closely follow the predicted dependence on trabecular orientation. The decay rates are independent of image resolution, provided that the pixel dimensions are larger than the intertrabecular spacing. For smaller pixel sizes, the decay rate decreases with decreasing pixel size.     

Demonstration of achilles tendon rupture by three phase bone scintigraphy and MRI

A man with complaint of soreness in the right medial ankle underwent three-phase bone scintigraphy; the results of the study suggested chronic active osteomyelitis or cellulitis, he was on antibiotics and was not experiencing any improvement. MR imaging confirmed Achilles tendon rupture. This case illustrates that a positive three-phase study is non-specific disease entity.     

Dual inversion recovery,ultrashort echo time (DIR UTE) imaging: Creating high contrast for short‐T2 species

Imaging of short‐T₂ species requires not only a short echo time but also efficient suppression of long‐T₂ species in order to maximize the short‐T₂ contrast and dynamic range. This paper introduces a method of long‐T₂ suppression using two long adiabatic inversion pulses. The first adiabatic inversion pulse inverts the magnetization of long‐T₂ water and the second one inverts that of fat. Short‐T₂ species experience a significant transverse relaxation during the long adiabatic inversion process and are minimally affected by the inversion pulses. Data acquisition with a short echo time of 8 μs starts following a time delay of inversion time (TI1) for the inverted water magnetization to reach a null point and a time delay of TI2 for the inverted fat magnetization to reach a null point. The suppression of long‐T₂ species depends on proper combination of TI1, TI2, and pulse repetition time. It is insensitive to radiofrequency inhomogeneities because of the adiabatic inversion pulses. The feasibility of this dual inversion recovery ultrashort echo time technique was demonstrated on phantoms, cadaveric specimens, and healthy volunteers, using a clinical 3‐T scanner. High image contrast was achieved for the deep radial and calcified layers of articular cartilage, cortical bone, and the Achilles tendon. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Clinically compatible MRI strategies for discriminating bound and pore water in cortical bone

Advances in modern magnetic resonance imaging (MRI) pulse sequences have enabled clinically practical cortical bone imaging. Human cortical bone is known to contain a distribution of T₁ and T₂ components attributed to bound and pore water, although clinical imaging approaches have yet to discriminate bound from pore water based on their relaxation properties. Herein, two clinically compatible MRI strategies are proposed for selectively imaging either bound or pore water by utilizing differences in their T₁s and T₂s. The strategies are validated in a population of ex vivo human cortical bones, and estimates obtained for bound and pore water are compared to bone mechanical properties. Results show that the two MRI strategies provide good estimates of bound and pore water that correlate to bone mechanical properties. As such, the strategies for bound and pore water discrimination shown herein should provide diagnostically useful tools for assessing bone fracture risk, once applied to clinical MRI. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Accelerated proton echo planar spectroscopic imaging (PEPSI) using GRAPPA with a 32-channel phased-array coil.

Parallel imaging has been demonstrated to reduce the encoding time of MR spectroscopic imaging (MRSI). Here we investigate up to 5-fold acceleration of 2D proton echo planar spectroscopic imaging (PEPSI) at 3T using generalized autocalibrating partial parallel acquisition (GRAPPA) with a 32-channel coil array, 1.5 cm(3) voxel size, TR/TE of 15/2000 ms, and 2.1 Hz spectral resolution. Compared to an 8-channel array, the smaller RF coil elements in this 32-channel array provided a 3.1-fold and 2.8-fold increase in signal-to-noise ratio (SNR) in the peripheral region and the central region, respectively, and more spatial modulated information. Comparison of sensitivity-encoding (SENSE) and GRAPPA reconstruction using an 8-channel array showed that both methods yielded similar quantitative metabolite measures (P > 0.1). Concentration values of N-acetyl-aspartate (NAA), total creatine (tCr), choline (Cho), myo-inositol (mI), and the sum of glutamate and glutamine (Glx) for both methods were consistent with previous studies. Using the 32-channel array coil the mean Cramer-Rao lower bounds (CRLB) were less than 8% for NAA, tCr, and Cho and less than 15% for mI and Glx at 2-fold acceleration. At 4-fold acceleration the mean CRLB for NAA, tCr, and Cho was less than 11%. In conclusion, the use of a 32-channel coil array and GRAPPA reconstruction can significantly reduce the measurement time for mapping brain metabolites.