Cornelius von Morze

Intracranial time-of-flight MR angiography at 7T with comparison to 3T.

To establish the feasibility of intracranial time‐of‐flight (TOF) MR angiography (MRA) at 7T using phased array coils and to compare its performance to 3T.

Imaging of blood flow using hyperpolarized [13C]Urea in preclinical cancer models

To demonstrate dynamic imaging of a diffusible perfusion tracer, hyperpolarized [13C]urea, for regional measurement of blood flow in preclinical cancer models.

Evaluation of intracranial stenoses and aneurysms with accelerated 4D flow

The aim of this study was to evaluate intracranial arterial stenoses and aneurysms with accelerated time-resolved three-dimensional (3D) phase-contrast MRI or 4D flow. The 4D flow technique was utilized to image four normal volunteers, two patients with intracranial stenoses and two patients with intracranial aneurysms. In order to reduce scan time, parallel imaging was combined with an acquisition strategy that eliminates the corners of k-space. In the two patients with intracranial stenoses, 4D flow velocity measurements showed that one patient had normal velocity profiles in agreement with a previous magnetic resonance angiogram (MRA), while the second showed increased velocities that indicated a less significant narrowing than suspected on a previous MRA, as confirmed by catheter angiography. This result may have prevented an invasive angiogram. In the two patients with 4-mm intracranial aneurysm, one had a stable helical flow pattern with a large jet, while the other had a temporally unstable flow pattern with a more focal jet possibly indicating that the second aneurysm may have a higher likelihood of rupture. Accelerated 4D flow provides time-resolved 3D velocity data in an 8- to 10-min scan. In the stenosis patients, the addition of 4D flow to a traditional MRA adds the velocity data provided from transcranial Doppler ultrasound (TCD) possibly allowing for more accurate grading of stenoses. In the aneurysm patients, visualization of flow patterns may help to provide prognostic information about future risk of rupture.

High Resolution

(13)C steady state free precession (SSFP) magnetic resonance imaging and effective spin-spin relaxation time (T2) mapping were performed using hyperpolarized [(13)C] urea and [(13) C,(15)N2] urea injected intravenously in rats. (15)N labeling gave large T2 increases both in solution and in vivo due to the elimination of a strong scalar relaxation pathway. The T2 increase was pronounced in the kidney, with [(13) C,(15) N2] urea giving T2 values of 6.3±1.3 s in the cortex and medulla, and 11±2 s in the renal pelvis. The measured T2 in the aorta was 1.3±0.3 s. [(13)C] urea showed shortened T2 values in the kidney of 0.23±0.03 s compared to 0.28±0.03 s measured in the aorta. The enhanced T2 of [(13)C,(15)N2] urea was utilized to generate large signal enhancement by SSFP acquisitions with flip angles approaching the fully refocused regime. Projection images at 0.94 mm in-plane resolution were acquired with both urea isotopes, with [(13)C,(15) N2] urea giving a greater than four-fold increase in signal-to-noise ratio over [(13)C] urea.

^{13}

The metabolically inactive hyperpolarized agents HP001 (bis-1,1-(hydroxymethyl)-[1-(13)C]cyclopropane-d(8)) and urea enable a new type of perfusion magnetic resonance imaging based on a direct signal source that is background-free. The addition of perfusion information to metabolic information obtained by spectroscopic imaging of hyperpolarized [1-(13)C]pyruvate would be of great value in exploring the relationship between perfusion and metabolism in cancer. In preclinical normal murine and cancer model studies, we performed both dynamic multislice imaging of the specialized hyperpolarized perfusion compound HP001 (T(1)=95 s ex vivo, 32 s in vivo at 3 T) using a pulse sequence with balanced steady-state free precession and ramped flip angle over time for efficient utilization of the hyperpolarized magnetization and three-dimensional echo-planar spectroscopic imaging of urea copolarized with [1-(13)C]pyruvate, with compressed sensing for resolution enhancement. For the dynamic data, peak signal maps and blood flow maps derived from perfusion modeling were generated. The spatial heterogeneity of perfusion was increased 2.9-fold in tumor tissues (P=.05), and slower washout was observed in the dynamic data. The results of separate dynamic HP001 imaging and copolarized pyruvate/urea imaging were compared. A strong and significant correlation (R=0.73, P=.02) detected between the urea and HP001 data confirmed the value of copolarizing urea with pyruvate for simultaneous assessment of perfusion and metabolism.

C MRI With Hyperpolarized Urea: In Vivo

To demonstrate simultaneous hyperpolarization and imaging of three 13C‐labeled perfusion MRI contrast agents with dissimilar molecular structures ([13C]urea, [13C]hydroxymethyl cyclopropane, and [13C]t‐butanol) and correspondingly variable chemical shifts and physiological characteristics, and to exploit their varying diffusibility for simultaneous measurement of vascular permeability and perfusion in initial preclinical studies.

T_{2}

In this work the generalized autocalibrating partially parallel acquisition (GRAPPA) technique was implemented with modified reconstruction and applied to in vivo high‐resolution (HR) magnetic resonance imaging (MRI) of the trabecular bone microarchitecture at 3 Tesla (T) with a multiple‐acquisition balanced steady‐state free precession (b‐SSFP) sequence. Trabecular bone is made up of a network of microstructures (80–140 μm), and its structural deterioration is associated with the skeletal metabolic disorder osteoporosis. HR‐MRI is a promising noninvasive tool for assessing the trabecular microarchitecture in vivo, but it involves long acquisition times. Using partially parallel imaging (PPI) to accelerate the acquisition may help mitigate this shortcoming and allow more flexibility in protocol design. In this study the effects of GRAPPA‐based reconstruction on image characteristics and the measurement of trabecular bone structural parameters were evaluated. Initial studies showed that image quality and depiction of microstructure were preserved in the GRAPPA‐based reconstruction, indicating the feasibility of PPI in HR‐MRI of trabecular bone. The results also demonstrated the potential of PPI for increasing the signal‐to‐noise ratio (SNR) efficiency of multiple‐acquisition b‐SSFP imaging protocols. Magn Reson Med, 2006.

Mapping and

A three‐axis uniplanar gradient coil was designed and built to provide order‐of‐magnitude increases in gradient strength of up to 500 mT/m on the x‐ and y‐axes, and 1000 mT/m for the z‐axis at 640 A input over a limited FOV (∼16 cm) for superficial regions, compared to conventional gradient coils, with significant gradient strengths extending deeper into the body. The gradient set is practically accommodated in the bore of a conventional whole‐body, cylindrical‐geometry MRI scanner, and operated using standard gradient supplies. The design was optimized for gradient linearity over a restricted volume while accounting for the practical problems of torque and heating. Tests at 320 A demonstrated up to 420‐mT/m gradients near the surface at efficiencies of up to 1.4 mT/m/A. A new true 2D gradient‐nonlinearity correction algorithm was developed to rectify gradient nonlinearities and considerably expand the imageable volumes. The gradient system and correction algorithm were implemented in a standard 1.5T scanner and demonstrated by high‐resolution imaging of phantoms and humans. Magn Reson Med 58:134–143, 2007.

^{15}

Urea functions as a key osmolyte in the urinary concentrating mechanism of the inner medulla. The urea transporter UT-A1 is upregulated by antidiuretic hormone, facilitating faster equilibration of urea between the lumen and interstitium of the inner medullary collecting duct, resulting in the formation of more highly concentrated urine. New methods in dynamic nuclear polarization, providing ∼50,000-fold enhancement of nuclear magnetic resonance signals in the liquid state, offer a novel means to monitor this process in vivo using magnetic resonance imaging. In this study, we detected significant signal differences in the rat kidney between acute diuretic and antidiuretic states, using dynamic (13)C magnetic resonance imaging following a bolus infusion of hyperpolarized [(13)C]urea. More rapid medullary enhancement was observed under antidiuresis, consistent with known upregulation of UT-A1.

N Labeling Effects

A new method was developed for simultaneous spatial localization and spectral separation of multiple compounds based on a single echo, by designing the acquisition to place individual compounds in separate frequency encoding bands. This method was specially designed for rapid and robust metabolic imaging of hyperpolarized (13)C substrates and their metabolic products, and was investigated in phantom studies and studies in normal mice and transgenic models of prostate cancer to provide rapid metabolic imaging of hyperpolarized [1-(13)C]pyruvate and its metabolic products [1-(13)C]lactate and [1-(13)C]alanine at spatial resolutions up to 3mm in-plane. Elevated pyruvate and lactate signals in the vicinity of prostatic tissues were observed in transgenic tumor mice. The multi-band frequency encoding technique enabled rapid metabolic imaging of hyperpolarized (13)C compounds with important advantages over prior approaches, including less complicated acquisition and reconstruction methods.