Tissue specificity of low-field-strength magnetization transfer contrast imaging.

The time-dependent saturation transfer technique was used to measure the transfer of magnetization in several rat tissues at 0.1 T. The length of the saturation pulse was varied from 0 to 510 msec. The magnetization transfer contrast effect was characteristic for each type of tissue. A substantial reduction of image intensity was obtained in skeletal muscle (74%), heart (71%), spleen (64%), brain (65%), pancreas (64%), liver (64%), kidneys (62%), and lungs (56%) with the longest saturation pulse available. Much smaller declines occurred in stagnant blood and peritoneal fat. The potential of this imaging technique for clinical conditions is discussed.     

Pulsed magnetization transfer versus continuous wave irradiation for tissue contrast enhancement

Pulsed magnetization transfer and continuous wave irradiation techniques are analyzed and compared for saturation efficiency and radio-frequency (RF) power requirements at 1.5 and 0.5 T. Binomial RF pulses transmitted on resonance are a more power-efficient method of exciting saturation transfer and are easily implemented with any pulse sequence. Binomial pulses selectively excite all short T2 species and behave as 0° pulses for on-resonance, long T2 species.     

Hepatic tumors: Magnetization transfer MR imaging with gadolinium enhancement

Thirty patients with 15 hepatocellular carcinomas, 10 metastases, four hemangiomas, and one cholangiocarcinoma underwent magnetic resonance imaging at 1.5 T with T1-weighted, T2- weighted spin-echo (SE) images, gradient-echo (GRE) magnetization transfer (MT) images, and gadolinium-enhanced T1-weighted SE and MT- GRE images. The MT effect and lesion-liver contrast-to-noise ratio (C/N) were calculated and visual assessment (qualitative analysis) performed for unenhanced and enhanced MT-GRE images and enhanced Tl-weighted SE images. The C/N values for hepatic adenocarcinomas (seven metastases and one cholangiocarcinoma) and hemangiomas were larger for enhanced MT-GRE images (adenocarcinoma, 8.4 ± 2.3 [P < 0.01); hemangioma, 24 ± 2.1 [P < 0.05]) than for enhanced GRE images (5.0 ± 1.9 and 18 ± 2.7, respectively). These enhancing tumors had the highest scores in the qualitative analysis. Enhanced MT-GRE images showed no advantage for depiction of hepatocellular carcinomas relative to the other images.     

Magnetization transfer imaging with pulsed off-resonance saturation: Variation in contrast with saturation duty cycle

Magnetization transfer contrast (MTC) can be generated in magnetic resonance images of water-containing macromolecular structures. Results obtained with the pulsed off-resonance saturation technique for achieving MTC are presented, allowing comparison of this technique with others. The magnetization transfer effect is demonstrated in newborn piglet brain and bovine muscle tissue, as well as in human subjects. MTC increases nonlinearly with the duty cycle of the off-resonance irradiation, approaching a maximum value corresponding to continuous irradiation. However, approximately half of the available contrast can be obtained at a given saturation offset frequency, with 16% irradiation duty cycle. Examples of MTC in various human tissues demonstrate the specificity of this technique at power levels within U.S. Food and Drug Administration guidelines. Plots of MTC versus the irradiation offset frequency demonstrate the effects of magnetization transfer and are compared with the curve generated by a control phantom that does not show magnetization transfer. Magnetization transfer imaging with pulsed off-resonance saturation is effective and easily implemented and may be useful in the investigation of saturation transfer phenomena in tissue.     

Pulsed magnetization transfer spin-echo MR imaging

Cross relaxation between macromolecular protons and water protons is known to be important in biologic tissue. In magnetic resonance (MR) imaging sequences, selective saturation of the characteristically short T2 macromolecular proton pool can produce contrast called magnetization transfer contrast, based on the cross-relaxation process. Selective saturation can be achieved with continuous wave irradiation several kilohertz off resonance or short, intense 0° pulses on resonance. The authors analyze 0° binomial pulses for T2 selective saturation, present design guidelines, and demonstrate the use of these pulses in spin-echo imaging sequences in healthy volunteers and patients. Using the phenomenologic Bloch equations modified for two-site exchange, the authors derive the analytic expressions for water proton relaxation under periodic pulsed saturation of the macromolecular protons. This relaxation is shown to be monoexpo-nential, with a rate constant dependent on the saturation pulse repetition rate and the individual and cross-relaxation rates.     

Dynamic contrast-enhanced MR imaging of the pituitary gland with fast spin-echo technique

Preliminary work has demonstrated that dynamic contrast material—enhanced magnetic resonance imaging improves the detection sensitivity for pituitary microadenomas. The authors present a new method of obtaining dynamic contrastenhanced pituitary images with a short TR/TE fast spin-echo technique. This approach allows acquisition of contrast-enhanced spin-echo images with high temporal and spatial resolution. The new technique is applied in a small group of patients and control subjects.     

High-dose gadoteridol in MR imaging of intracranial neoplasms.

Twelve patients with a high suspicion of brain metastases by previous clinical or radiologic examinations were studied in a phase III investigation with magnetic resonance (MR) imaging at 1.5 T after a bolus intravenous injection of 0.1 mmol/kg gadoteridol followed at 30 minutes by a second bolus injection of 0.2 mmol/kg gadoteridol. All lesions were best demonstrated (showed greatest enhancement) at the 0.3-mmol/kg (cumulative) dose, with image analysis confirming signal intensity enhancement in the majority of cases after the second gadoteridol injection. More lesions were detected with the 0.3-mmol/kg dose than with the 0.1-mmol/kg dose, and more lesions were detected with the 0.1-mmol/kg dose than on precontrast images. In this limited clinical trial, high-dose gadoteridol injection (0.3-mmol/kg cumulative dose) provided improved lesion detection on MR images specifically in intracranial metastatic disease.     

Tissue characterization of intracranial tumors by magnetization transfer and spin-lattice relaxation parameters in vivo

T1s and magnetization transfer (MT) parameters of 36 intracranial tumors were determined in vivo at 0.1 T to assess their use in tissue characterization. The mobile water relaxation times (T1_w) did not differ between tumor groups, whereas the T1s, the apparent MT relaxation times (T1_a), and the parameters MT contrast (MTC) differed significantly between several tumor types. The MT rates (R_wm) demonstrated the most significant differences; R_wm values could reliably separate high grade and low grade gliomas. T1_ws of the tumors were commonly in the same range as that of normal gray matter, whereas other parameters differed from those of normal brain. The results indicate that MT rates are superior to other parameters in the characterization of intracranial tumors and may be also useful clinically in the grading of gliomas.     

Efficacy of nonionic low-osmolar gadodiamide injection in animals with intracranial mass lesions.

Gadodiamide injection is a nonionic, low-osmolar formulation of a paramagnetic metal chelate complex consisting of gadodiamide and caldiamide sodium. The efficacy of gadodiamide injection as a magnetic resonance (MR) imaging enhancement medium was evaluated by imaging intracranial 9L-glioma lesions induced in rats and naturally occurring lesions in dogs. T1- and T2-weighted spin-echo images were obtained before and after administration of gadodiamide injection at doses of 0.1 and 0.2 mmol/kg. On the precontrast T1-weighted images, the intracranial lesions were not well seen, appearing isointense to normal brain parenchyma. Although the presence of disease was shown unequivocally on the T2-weighted images, the margins of the masses could not be delineated. Postcontrast T1-weighted images were characterized by marked enhancement of the tumor, with no change in signal intensity in the surrounding edematous brain tissue. Gadodiamide injection was efficacious in identifying areas of blood-brain barrier breakdown associated with intracranial masses.     

Brain MR post-gadolinium contrast in multiple sclerosis: the role of magnetization transfer and image subtraction in detecting more enhancing lesions

Our purpose was to evaluate the role of magnetization transfer and image subtraction in detecting more enhancing lesions in brain MR imaging of patients with multiple sclerosis (MS). Thirty-one MS patients underwent MR imaging of the brain with T1-weighted spin echo sequences without and with magnetization transfer (MT) using a 1.5 T imager. Both sequences were acquired before and after intravenous injection of a paramagnetic contrast agent. Subtraction images in T1-weighted sequences were obtained by subtracting the pre-contrast images from the post-contrast ones. A significant difference was found between the numbers of enhanced areas in post-gadolinium T1-weighted images without and with MT (p=0.020). The post-gadolinium T1-weighted images with MT allowed the detection of an increased (13) number of enhancing lesions compared with post-gadolinium T1-weighted images without MT. A significant difference was also found between the numbers of enhanced areas in post-gadolinium T1-weighted images without MT and subtraction images without MT (p=0.020). The subtraction images without MT allowed the detection of an increased (10) number of enhancing lesions compared with post-gadolinium T1-weighted images without MT. Magnetization transfer contrast and subtraction techniques appear to be the simplest and least time-consuming applications to improve the conspicuity and detection of contrast-enhancing lesions in patients with MS.     

Pulsed magnetization transfer contrast for MR imaging with application to breast

The relative populations and transverse relaxation times of the solid-like hydrogen pool (P_B and T_2B) and the magnetization transfer (MT) rates between the solid-like and liquid-like hydrogen pools (K) have been determined for three different agar gel concentrations (2%, 4%, and 8% by weight) as well as excised fibroglandular breast tissue specimens. P_B was determined to be .003(.001), .01(.002), .02(.01), and .06(.01); T_2B was determined to be 13.0(.2), 14.0(.1), 14.5(.1) and 15.2(1.3) μs; and K was determined to be 0.78(.01), 1.15(.02), 2.00(.02), and 3.55(1.5) sec^?1 for the 2%, 4%, and 8% agar gels and the fibroglandular tissue, respectively. The image signal intensities of a pulsed MTC-prepared gradient-echo imaging technique are predicted using these MT parameters and are shown to agree well with experimental data obtained from a clinical MR imaging system. This technique is shown to suppress signal intensity of fibroglandular breast tissue by 40%–50% without exceeding SAR limits (≤ 8W/kg) and is helpful for visualizing lesions and silicone implants.     

Analysis of on- and off-resonance magnetization transfer techniques

Three methods of performing magnetization transfer (MT) MR imaging ere emlyzed: (a) off-resonance continuous wave, (b) off-rceonance shaped pulses, end (c) on-resonance binomial pulses. With two-pool Bloch-model simulations., signal levels from “MT active” spin systems were calculated. with reference to direct saturation of “MT inactive” systems. allowing calculation of contrast due to MT. Simulations demonstrate several trends with variation of excitation amplitude and Offret fiequency for the off-resoname methods and with variation of excitation emplitude end pulse shape ?order”? for binomial pulses. The simulations show that nominally optimized versions of each of these approaches provide essentially equivalent contrast at a given level of applied MT power, contrary to previous claims. Experiments with an MT-inactive phentom, with a whole-body syetem, show results with off- rewnence pulses to be in good agreement with simulations. whereas binomiel- pulse experiments show anomalously large direct saturation.     

Gadolinium-enhanced high-resolution MR angiography with adaptive vessel tracking: preliminary results in the intracranial circulation.

To overcome problems associated with poor contrast between vessels and background tissue in time-of-flight magnetic resonance angiography, the role of intravenous gadopentetate dimeglumine in conjunction with a postprocessing adaptive vessel tracking scheme was studied. Vessel tracking makes it possible to discriminate arteries from veins, to prevent problems associated with other bright tissues on maximum-intensity projections, and to increase the signal-to-noise ratio. Short, asymmetric, velocity-compensated field echoes were used in conjunction with high-resolution imaging techniques to spatially discriminate between adjacent vessels and stationary background tissue. Gadopentetate dimeglumine was shown to be useful for visualization of small vessels, aneurysms, and regions of slow flow, when used with this post-processing scheme.     

Incidental magnetization transfer contrast in fast spin-echo imaging of cartilage

In this article, the authors illustrate the contributions of incidental magnetization transfer contrast (MTC) to the signal characteristics of patellar cartilage on routine, multiplanar fast spin-echo (FSE) images. Incidental MTC diminishes the signal of patellar cartilage by 30% on routine FSE scans. Spectral fat saturation does not significantly increase incidental MTC on multislice FSE acquisitions. By increasing the contrast between synovial fluid and patellar cartilage, incidental MTC may account for greater conspicuousness of chondromalacia on FSE as compared to conventional spin-echo images. Incidental MTC may also alter or obscure the normal, laminar appearance of hyaline cartilage on short-TE, FSE images.     

Use of three-dimensional segmented flash sequence with magnetization transfer contrast to improve gd-dtpa-enhanced intrahepatic MR portography

Twenty healthy volunteers underwent gadopentetate dimeglumine (gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA))-enhanced MR angiography (MRA) using three-dimensional-segmented fast low angle shot images (FLASH) with magnetization transfer contrast (MTC) pulses. MRA was obtained at 75 seconds (early phase) and 135 seconds (late phase) after bolus injection of Gd-DTPA (MTC+ group) during one period of breath-holding. Within 1 week, MRA without MTC was performed under the same scanning conditions. Visualization of intrahepatic portal branches with these methods was compared in both phases. Portal vein-liver contrast-to-noise ratios were significantly higher in the MTC+ group in both phases. For third-and fourth-order portal branches, visualization was significantly better in the MTC+ group in both phases. Use of three-dimensional-segmented FLASH shortened acquisition time and facilitated imaging during breath-holding and also reduced whole-body average specific absorption rate values. Visualization of intrahepatic portal vein branches was improved by MTC pulses, and effective imaging time was prolonged.     

Effects of contrast dose,delayed imaging,and magnetization transfer saturation on gadolinium-enhanced MR imaging of brain lesions

This paper discusses the types of paramagnetic agents available for clinical brain imaging and reviews investigations that have sought to optimize the use of these agents by varying the administered dose, delaying the imaging time after contrast administration, and altering image contrast by using magnetization transfer saturation pulses.     

Use of magnetization transfer contrast to improve single breath-hold three-dimensional MR-portography with bolus injection of gadopentetate dimeglumine: A preliminary report

We investigated whether better visualization of the intrahepatic portal system could be obtained by adding magnetization transfer contrast (MTC) to three-dimensional MR portography (3D-MRP). In 15 healthy volunteers, 3D fast low-angle shot images combined with an off-resonance pulse were obtained after bolus injection of gadopentetate dimeglumine during one period of breath-holding. Within 1 week after injection, MR images without MTC were obtained using the same scanning condition. MRPs with and without the MTC pulse were compared quantitatively and qualitatively. The contrast-to-noise ratio of the portal vein–liver was significantly higher in the group imaged with the addition of MTC. Qualitatively, the use of MTC improved peripheral portal branch depiction on maximum-intensity projection images. Even when the MTC pulse was added to the conventional method of 3D data collection, it was possible to obtain images within one period of breath-holding. In conclusion, the use of MTC can substantially enhance the quality of 3D-MRPs.     

Early detection of regional myocardial ischemia in ex vivo piglet hearts: MR imaging with magnetization transfer

The effects of regional myocardial ischemia and reperfusion on magnetization transfer (MT) contrast were investigated in an ex vivo perfused piglet heart model. The extent of the ischemic area was defined with perfusion magnetic resonance (MR) studies performed with use of extracellular contrast agents. Relative MT contrast was calculated for a total of 106 regions of interest in nine hearts. In the areas defined as being severely ischemic in the perfusion studies, a small but significant increase in the MT contrast of 18% ± 9 (standard deviation) (n = 35) was found as early as 10 minutes after the start of ischemia. This contrast difference was reduced to 11% ± 10 after 2 hours of total occlusion. The contrast between normal and ischemic tissue can be explained in part by the effect of inflowing blood, which leads to changes in both equilibrium magnetization and the apparent T1 of the perfused tissue. However, theoretical estimation suggests that these flow-related changes would produce a maximal relative change in MT contrast of approximately 4%. The most likely explanation for the rest of the observed changes is alteration in the distribution of cellular water related to the so-called intracellular edema that is known to be associated with the acute phase of myocardial ischemia.