Optimization of spoiled gradient-echo phase imaging for in vivo localization of a focused ultrasound beam

The parameters of a spoiled gradient-echo (SPGR) pulse sequence have been optimized for in vivo localization of a focused ultrasound beam. Temperature elevation was measured by using the proton resonance frequency shift technique, and the phase difference signal-to-noise ratio (SNR_{δ ϕ}) was estimated in skeletal muscle and kidney cortex in 10 rabbits. Optimized parameters included the echo time equivalent to T^*₂of the tissue, the longest repetition time possible with a 20-s sonication, and the flip angle equivalent to the Ernst angle. Optimal SPGR phase imaging can detect a sonication beam with a peak phase difference of 0.55 radian, which corresponds to a temperature elevation of 7.3°C. The sonication beam can be localized within one voxel (0.6 × 0.6 × 5 mm³) at power levels that are below the threshold for thermal damage of the tissue.     

MR temperature mapping of focused ultrasound surgery

Deep lying soft tissue tumors may be treated by a nonincisional surgical procedure executed inside an MR imaging system using a thermal effect delivered by a focused ultrasound transducer. A prototype system is constructed to assess MRI thermal monitoring and the localization of the heat zone in muscle. The temperature distribution of the focal spot is imaged with MRI while mechanically moving the transducer with an hydraulic 3-axis positioner. Acoustic power is applied with a spherical shell transducer using 1- to 10-s duration pulses at frequencies of 1.5 MHz to selectively coagulate tissue at 60-70°C. The procedure is monitored with a series of fast second gradient echo, T₁-weighted, temperature sensitive MR sequences. Acquisitions are optimized for high temperature sensitive images that yield the thermal diffusivity, heat flow time constant and the focal spot size in muscle. MR temperature maps of muscle provide localization and dosimetry both in the focal region and near field.     

Time and temperature dependence of MR parameters during thermal coagulation of ex vivo rabbit muscle

Detailed measurements of the T₁-weighted, T₂-weighted, and MT-weighted signal were performed for ex vivo muscle samples heated to various temperatures for different times. Consistent, monotonic increases in signal intensity were observed with progressive thermal coagulation, corresponding to an increase in T₂ relaxation time and an increase in MT-weighted signal for temperatures above 60°C. The relationship for T₁ relaxation was more complex, showing a decrease in T₁ relaxation from 40 to 60°C and an increase above 60°C. These techniques provide a more direct measure of tissue thermal coagulation than that provided by MR thermometry and suggest MR imaging strategies for the optimization and monitoring of thermal coagulation therapy protocols that create thermal damage in target tissues.     

Magnetic resonance imaging: Comparison of four pulse sequences in assessing primary bone tumours

A prospective magnetic resonance imaging (MRI) study was carried out in 13 patients (19 examinations) with primary bone tumours to assess the relative value of each of four pulse sequences in showing the extent and nature of the lesion. The four pulse sequences used were a T₁-weighted spin-echo (SE544/44), a T₂-weighted spin echo (SE1500/80), a short TI inversion recovery (STIR) (IR500/100/44), and a partial saturation (PS) (PS500/22) with field echo data collection. For soft tissue disease the combination of PS and STIR gave better definition of the boundary of the tumour than the more conventional T₁ and T₂-weighted spin echo sequences. For the demonstration of bone cortex, periosteal change and calcification, T₁ and T₂-weighted spin echo sequences were better. However, for calcified tissues, plain radiographs were better than either MRI combination. On the assumption that plain films will be available in all cases, PS and STIR sequences could therefore be substituted for T1 and T2-weighted spin echo sequences allowing an increase in soft tissue detectability for lesions in both red and yellow marrow.     

MRI investigation of the threshold for thermally induced blood-brain barrier disruption and brain tissue damage in the rabbit brain.

The ability of MRI-derived thermometry to predict thermally induced tissue changes in the brain was tested, and the thermal thresholds for blood-brain barrier (BBB) disruption and brain tissue damage were estimated. In addition, the ability of standard MRI to detect threshold-level effects was confirmed. These safety thresholds are being investigated to provide guidelines for clinical thermal ablation studies in the brain. MRI-monitored focused ultrasound heating was delivered to 63 locations in 26 rabbits. Tissue changes were detected in T(2)-weighted imaging and T(1)-weighted imaging (with and without contrast) and with light microscopy. The probability for tissue damage as a function of the accumulated thermal dose, the peak temperature achieved, the applied acoustic energy, and the peak acoustic power was estimated with probit regression. The discriminative abilities of these parameters were compared using the areas under the receiver operator characteristic (ROC) curves. In MRI, BBB disruption was observed in contrast-enhanced T(1)-weighted imaging shortly after the ultrasound exposures, sometimes accompanied by changes in T(2)-weighted imaging. Two days later, changes in T(2)-weighted imaging were observed, sometimes accompanied by changes in T(1)-weighted imaging. In histology, tissue damage was seen at every location where MRI changes were observed, ranging from small (diameter <1.0 mm) areas of tissue necrosis to severe vascular damage and associated hemorrhagic infarct. In one location, small (diameter: 0.8 mm) damage was not detected in MRI. The thermal dose and peak temperature thresholds were between 12.3-40.1 equivalent min at 43 degrees C and 48.0-50.8 degrees C, respectively, and values of 17.5 equivalent min at 43 degrees C and 48.4 degrees C were estimated to result in tissue damage with 50% probability. Thermal dose and peak temperature were significantly better predictors than the applied acoustic energy and peak acoustic power (P < 0.01). BBB disruption was always accompanied by tissue damage. The temperature information was better than the applied acoustic power or energy for predicting the damage than the ultrasound parameters. MRI was sensitive in detecting threshold-level damage.     

MR-guided focused ultrasound surgery.

Magnetic resonance guided focused ultrasound surgery provides a minimally invasive controlled method for selectively destroying deep-lying tissue. A thermal analysis of focused ultrasound provides an estimate of the time-dependent temperature distribution and thermal dose required for ultrasound surgery. The temperature distribution is estimated by accumulating heat sources, considering the effects of thermal conductivity, heat content, and perfusion. In this study, both gel phantoms and excised in vitro bovine muscle specimens were imaged in a 1.5 T MR system while heated with a 5 cm diameter, 10 cm focal length, 1.1 MHz transducer. During sonication, the thermal effects were observed with T1-weighted pulse sequences. Below a critical temperature, the heat zone appeared as a dark spot that moved with the focal spot. Above a critical thermal dose, the in vitro tissue was irreversibly altered and the focal lesion was observed on both the MR image and the specimen slice.     

Studying neonatal bilirubin encephalopathy with conventional MRI,MRS, and DWI

Introduction  The purpose of this study was to evaluate the diagnostic value of conventional magnetic resonance imaging (MRI), proton magnetic resonance spectroscopy (¹H-MRS), and diffusion-weighted imaging (DWI) for neonatal bilirubin encephalopathy. Methods  We collected conventional MRI in 24 neonates with neonatal bilirubin encephalopathy. We performed ¹H-MRS and DWI sequences to nine of the 24 patients and seven age-matched healthy control subjects. Multiple-voxel ¹H-MRS data were acquired using PRESS pulse sequence with TE = 135 ms and TR = 1500 ms. The spectroscopic regions of interest were the bilateral basal ganglia and thalamus with a 1.0 mL spatial resolution. The data from DWI were collected by using a single shot-spin echo-echo planar imaging sequence with TR/TE: 2900/98, and imaging regions were also focused on the bilateral basal ganglia and thalamus. Results  Nineteen of the 24 patients had abnormal T₁-weighted image hyperintensity in the globus pallidus, but these lesions appeared as normal T₂-weighted image intensity in the same region. Ten of the 24 patients had T₁-weighted image high signal intensity in the subthalamic nucleus and appeared as normal intensity in the region for the T₂-weighted images. The peak area ratios of NAA/Cho and NAA/Cr were significantly decreased (t-test, P < 0.05) in the patients compared to the controls in the basal ganglia. Conclusion  Conventional MR imaging and ¹H-MRS are important complementary tools in the diagnosis of neonatal bilirubin encephalopathy. The study provides important information for applying these MR modalities to evaluate neonates with bilirubin encephalopathy.     

Off-resonance proton T1p dispersion imaging of 17O-enriched tissue phantoms

Proton T_1p dispersion imaging is a recently described method for indirect detection of ¹⁷O. However, clinical implementation of this technique is hindered by the requirement for a high-amplitude spin-locking field (γB₁ > 1 kHz) that exceeds current limitations in specific absorption rate (SAR). Here, a strategy is offered for circumventing high SAR in T_1p dispersion imaging of ¹⁷O through the use of low-amplitude off-resonance spin-locking pulses (γB₁ < 300 Hz). Proton spin-lattice relaxation times in the off-resonance rotating frame were measured in H₂¹⁷O-enriched tissue phantoms. On- and off-resonance T_1p dispersion imaging was implemented at 2 T using a spin-locking preparatory pulse cluster appended to a standard spin-echo sequence. On- and off-resonance dispersion images exhibited similar ¹⁷O-based image contrast. Magnetization transfer effects did not depend on ¹⁷O concentration and had no effect on image contrast. In conclusion, off-resonance proton T_1p dispersion imaging shows promise as a safe, sensitive technique for generating ¹⁷O-based T_1p contrast without exceeding SAR limitations.     

Two-point Dixon technique for water-fat signal decomposition with B0 inhomogeneity correction

To separate water and lipid resonance signals by phase-sensitive MRI, a two-point Dixon (2PD) reconstruction is presented in which phase-unwrapping is used to obtain an inhomogeneity map based on only in-phase and out-of-phase image data. Two relaxation-weighted images, a “water image” and a “fat image,” representing a two-resonance peak model of proton density, are output. The method is designed for T₁- or density-weighted spin-echo imaging; a double-echo scheme is more appropriate for T₂-weighted spin-echo imaging. The technique is more time-efficient for clinical fat-water imaging than 3PD schemes, while still correcting for field inhomogeneity.     

Magnetic resonance imaging of peripheral soft tissue hemangiomas

Ten patients with soft tissue hemangiomas outside the central nervous system were studied with MR imaging. Eight patients were studied at 1.5 Tesla (T) with T₁-weighted and triple echo T₂-weighted sequences. Two additional patients were imaged on a 0.5-T system. The MR images were correlated with images from other modalities. Histologie diagnosis was obtained in all cases. It was found that prolonged T₂-weighted imaging together with standard spin echo T₁ and T₂ pulse sequences is a good substitute for contrast-enhanced CT and arteriographic evaluation of soft tissue hemangiomas.     

High-Field MR microscopy using fast spin-echoes

Fast spin-echo imaging has been investigated with attention to the requirements and opportunities for high-field MR microscopy. Two-and three-dimensional versions were implemented at 2.0 T, 7.1 T, and 9.4 T. At these fields, at least eight echoes were collectable with a 10 ms TE from fixed tissue specimens and living animals, giving an eightfold improvement in imaging efficiency. To reduce the phase-encoding gradient amplitude and its duty cycle, a modified pulse sequence with phase accumulation was developed. Images obtained using this pulse sequence exhibited comparable signal-to-noise (SNR) to those obtained from the conventional fast spin-echo pulse sequences. Signal losses due to imperfections in RF pulses and lack of phase rewinders were offset in this sequence by reduced diffusion losses incurred with the gradients required for MR microscopy. Image SNR, contrast, edge effects and spatial resolution for three k-space sampling schemes were studied experimentally and theoretically. One method of sampling k-space, 4-GROUP FSE, was found particularly useful in producing varied T₂ contrast at high field. Two-dimensional images of tissue specimens were obtained in a total acquisition time of 1 to 2 min with in-plane resolution between 30 to 70 μm, and 3D images with 256³ arrays were acquired from fixed rat brain tissue (isotropic voxel = 70 μm) and a living rat (isotropic voxel = 117 μm) in∼4.5 h.     

Proton MR spectroscopy of the normal human prostate with an endorectal coil and a double spin-echo pulse sequence

This report describes the use of an endorectal coil and a double spin-echo pulse sequence for localized ¹H MR spectroscopy of the normal prostate in volunteers. The spectra showed well-resolved signals for citrate, (phospho)choline, and creatine protons. Additional signals were assigned to taurine and myoinositol protons. J modulation of the main and outer peaks of citrate could be monitored in vivo. Apparent relaxation times T₁ and T₂ have been estimated for the methyl protons of cholines and creatine. An effective T₁ relaxation time was estimated for the main peaks of the citrate multiplet. Ratios of the integrals of these resonances have been evaluated, and tissue contents of choline and creatine were estimated using the H₂O signal as an internal reference. Spectro-scopic imaging experiments revealed a lower relative citrate signal in central parts of the prostate than in peripheral parts.     

Invited. Brain edema development after MRI-guided focused ultrasound treatment

The aim of this study was to investigate a potential technique for image-guided minimally invasive neurosurgical interventions. Focused ultrasound (FUS) delivers thermal energy without an invasive probe, penetrating the dura mater, entering through the cerebrospinal fluid (CSF) space, or harming intervening brain tissue. We applied continuous on-line monitoring by MRI to demonstrate the effect of the thermal intervention on the brain tissue. For this, seven rabbits had a part of their skull removed to create access for the FUS beam into the brain through an acoustic window of 11 mm in diameter. Dura was left intact and skin was sutured. One week later, the rabbits were sonicated for 3 seconds with 21 W acoustic power, and the FUS focus was visualized with a temperature-sensitive T1-weighted MRI pulse sequence. The tissue reaction was documented over 7 days with T2-weighted images of the brain. The initial area of the central low signal intensity in the axial plane was .4 ± .3 mm², and for the bright hyperintensity surrounding the lesion, it was 2.3 ± .6 mm² (n = 7). In the coronal plane, the corresponding values were .4 ± .1 mm² and 3.4 ± .9 mm² (n = 5). The developing brain edema culminated 48 hours later and thereafter diminished during the next 5 days. Histology revealed a central necrosis in the white matter surrounded by edematous tissue with inflammatory cells. In summary, the image-guided thermal ablation technique described here produced a relatively small lesion in the white matter at the targeted location. This was accomplished without opening the dura or the need for a stereotactical device. MRI allowed on-line monitoring of the lesion setting and the deposition of thermal energy and demonstrated the tissue damage after the thermal injury.     

Dual contrast GRASE (gradient-spin echo) imaging using mixed bandwidth

Equal time spacing of RF pulses in the CPMG sequence imposes a constraint of equal signal read periods in spin-echo train imaging. GRASE imaging differs by using multiple read gradients in each π-π time interval, which are not constrained to be equal in number or duration. This additional degree of freedom is developed in dual contrast imaging. Closely spaced read periods are used for the PDW image to reduce T₂ decay effects, while fewer low-bandwidth read periods in each of several π-π intervals are used to raise the signal-to-noise ratio and avoid signal averaging in the T₂-weighted image. Key words: magnetic resonance imaging; gradient-spin echo; fast imaging.     

Magnetic resonance imaging of disseminated bone marrow disease in patients treated for malignancy

Magnetic resonance imaging (MRI) is a sensitive method for the diagnosis of bone marrow abnormalities, but its usefulness in detecting active disseminated cancer in this tissue in treated patients has not been determined. We therefore examined 14 children who had been treated for disseminated bone marrow involvement by neuroblastoma (n=6), lymphoma (n=3), Ewing's sarcoma (n=3), osteosarcoma (n=1), and leukemia (n= 1). MRI studies were performed at 21 marrow sites to evaluate residual or recurrent tumor and were correlated with histologic material from the same site. T₁- and T₂-weighted sequences were employed in 21 and 14 studies, respectively; short tau inversion recovery (STIR) in 18; and static gadolinium diethylene triamine pentaacetic acid (Gd-DPTA)-enhanced, T₁-weighted sequences in 13. All MRI studies showed an altered bone marrow signal. Technetium 99m methylene diphosphonate (^99mTc-MDP) bone scintigraphy was also performed (19 studies). On histologic examination, 7 marrow specimens contained tumor, and 14 did not. Of the 7 tumor-positive lesions, all T₁-weighted, 4 of 6 T₂-weighted, and all 6 STIR sequences showed abnormal signal; all 5 GdDTPA-enhanced, T₁-weighted sequences showed enhancement of the lesion. However, abnormal signals were also observed on all T₁-weighted, 6 of 8 T₂-weighted, 11 of 12 STIR, and 5 of 8 Gd-DTPA-enhanced, T₁-weighted images of the tumor-negative sites. In this clinical setting, MRI did not consistently differentiate changes associated with treatment from malignant disease.     

Reperfused ischemia of the rat intestine: Detection by MR imaging with polylysine-Gd-DTPA enhancement

To detect reperfused ischemia of the rat intestine, T₂-weighted spin-echo images were acquired, followed by T₁-weighted images before and after administration of polylysine-Gd-DTPA or Gd-DTPA. Before administration of the contrast agent, the reperfused intestine was hyperintense on T₂-weighted images, and to a lesser extent on T₁-weighted images. After administration of polylysine-Gd-DTPA, the reperfused intestine enhanced more than the normal one, giving a significantly better contrast-to-noise (CNR) ratio than on unenhanced images. Gd-DTPA induced the same enhancement of the reperfused and the normal intestine and the CNR was lower than on unenhanced T₂-weighted images. Reperfused intestinal ischemia could thus be better detected on polylysine-Gd-DTPA-enhanced MR images than on unenhanced images or on Gd-DTPA- enhanced images.     

Feasibility of post mortem cardiac proton density weighted fast field echo imaging in two cases of sudden death

ObjectiveThe aim of this work is to investigate and compare cardiac proton density (PD) weighted fast field echo (FFE) post-mortem magnetic resonance (PMMR) imaging with standard cardiac PMMR imaging (T₁-weighted and T₂-weighted turbo spin-echo (TSE)), postmortem CT (PMCT) as well as autopsy.Materials and methodsTwo human cadavers sequentially underwent cardiac PMCT and PMMR imaging (PD-weighted FFE, T₁-weighted and T₂-weighted TSE) and autopsy. The cardiac PMMR images were compared to each other as well as to PMCT and autopsy findings.ResultsFor the first case, cardiac PMMR exhibited a focal region of low signal in PD-weighted FFE and T₂-weighted TSE images, surrounded by a signal intense rim in the T₂-weighted images. T₁-weighted TSE and PMCT did not appear to identify any focal abnormality. Macroscopic inspection identified a blood clot; histology confirmed this to be a thrombus with an adhering myocardial infarction.In the second case, a myocardial rupture with heart tamponade was identified in all PMMR images, located at the anterior wall of the left ventricle; PMCT excluded additional ruptures. In PD-weighted FFE and T₂-weighted TSE images, it occurred hypo-intense, while resulting in small clustered hyper-intense spots in T₁-weighted TSE. Autopsy confirmed the PMMR and PMCT findings.ConclusionsPresented initial results have shown PD-weighted FFE to be a valuable imaging sequence in addition to traditional T₂-weighted TSE imaging for blood clots and myocardial haemorrhage with clearer contrast between affected and healthy myocardium.     

A pitfall associated with determination of transverse relaxation times of the 31P NMR signals of ATP using the Hahn spin-echo

T₂ measurements of ³¹P NMR signals of ATP using the Hahn 90°–180° spin-echo sequence imply difficulties whenever the 180° pulse angle is not completely perfect. The reason for this finding is the crucial influence of the J -couplings of the ATP signals which result in intensity modulations and consequently in false T₂ values even when the echo times are chosen to TE = n/J. Examinations on the calf muscles of healthy volunteers were performed to demonstrate this effect and its influence on in vivo T₂ determinations. The T₂ relaxation times evaluated with the Hahn spin-echo in combination with a Helmholtz coil are far shorter than the true T₂ values.     

Multisection T1-weighted hybrid-rare: A pulse sequence for MR imaging of the entire liver during suspended respiration

It is shown that the maximum average-data-collection-speed (ADCS) of multisection 2D hybrid-RARE sequences is independent of TR and TE_eff, and a monotonically increasing function of echo-train-length (ETL). This result was used in the design of an optimized T₁-weighted hybrid-RARE sequence that produces 20 images of the abdomen in 31 s divided into four breath-hold periods. The resulting ADCS is 58 lines in k-space per second. Twenty-four subjects (2 healthy volunteers and 22 patients) were imaged with a protocol that also included: (a) breath-hold T₁-weighted FLASH which acquires data at 34 lines in k-space per second (49 s scan time), and (b) T₁-weighted conventional spin-echo (9:44 minutes scan time) with respiratory compensation. The experiments show that this T₁-weighted-hybrid-RARE sequence has: (1) a level of T₁ weighting that is comparable with the conventional sequences, (2) very low vulnerability to susceptibility artifacts, (3) high data acquisition efficiency, and (4) higher SNR than T₁-weighted-FLASH. In conclusion, the T₁-weighted-hybrid-RARE sequence described herein is an efficacious and reproducible technique for rapid imaging of the upper abdomen during suspended respiration.     

High-resolution echo-planar imaging of the brain: is it suitable for routine clinical imaging?

To assess the value of echo-planar imaging (EPI) for ultra-fast routine clinical diagnostic MRI, we compared four different EPI sequences with conventional T₂-weighted spin-echo images on a commercial clinical imager. We examined 25 randomly selected patients who posed different clinical questions. The images were interpreted by two experienced neuroradiologists blinded as to the sequence used. Image quality and diagnostic certainty were evaluated and the main diagnosis established from the EPI study was compared to that obtained from the T₂-weighted images. Finally, EPI- and T₂-based diagnoses were compared with the diagnosis resulting from a complete MRI examination. Apart from one sequence that was generally rated low as regards both diagnostic certainty and image quality, the EPI sequences were comparable to each other, but inferior to the T₂-weighted images. However, two EPI sequences gave better diagnostic results than T₂-weighted images compared to the full MRI examination. Gradient-echo EPI was particularly sensitive to haemorrhagic lesions. All normal cases were correctly identified on EPI studies. Only two pathological cases were missed; both had isolated cranial nerve lesions. The absence of false-positive results and the high sensitivity to ischaemic and mass lesions mean that EPI can be used for ultra-fast screening. However, from these initial studies, EPI seems unsuitable for neuroradiological investigation of patients who may have subtle lesions whose detection requires either special sequences or administration of contrast medium. EPI can nevertheless be used in addition to high-resolution T₁-weighted images and may replace T₂-weighted spin-echo sequences for special indications. Received: 21 February 1997 Accepted: 18 April 1997