Fast Magnetic Resonance Imaging Techniques Research Articles

Tumor perfusion studies using fast magnetic resonance imaging technique in advanced cervical cancer: A new noninvasive predictive assay

Purpose : This study investigated sequential changes in tumor blood supply using magnetic resonance (MR) perfusion imaging and assessed their significance in the prediction of outcome of patients with advanced cervical cancer. The purpose of this project was to devise a simple, noninvasive method to predict early signs of treatment failure in advanced cervical cancer treated with conventional radiation therapy. Methods and Materials : Sixty-eight MR perfusion studies were performed prospectively in 17 patients with squamous carcinomas (14) and adenocarcinomas (3) of the cervix, Stages bulky IB (1), IIB (5), IIIA (1), IIIB (8), and IVA (1), and recurrent (1). Four sequential studies were obtained in each patient: immediately before radiation therapy (pretherapy), after a dose of 20–22 Gy/∼2 weeks (early therapy), after a dose of 40–45 Gy/∼4–5 weeks (midtherapy), and 4–6 weeks after completion of therapy (follow-up). Perfusion imaging of the tumor was obtained at 3-s intervals in the sagittal plane. A bolus of 0.1 mmol/kg of MR contrast material (gadoteridol) was injected intravenously 30 s after beginning image acquisition at a rate of 9 ml/s using a power injector. Time/signal-intensity curves to reflect the onset, slope, and relative signal intensity (rSI) of contrast enhancement in the tumor region were generated. Median follow-up was 8 months (range 3–18 months). Results : Tumors with a higher tissue perfusion (rSI ≥ 2.8) in the pretherapy and early therapy (20–22 Gy) studies had a lower incidence of local recurrence than those with a rSI of <2.8, but this was not statistically significant (13% vs. 67%; p = 0.05). An increase in tumor perfusion early during therapy (20–22 Gy), particularly to an rSI of ≥ 2.8, was the strongest predictor of local recurrence (0% vs. 78%; p = 0.002). However, pelvic examination during early therapy (20–22 Gy) commonly showed no appreciable tumor regression. The slope of the time/signal-intensity curve obtained before and during radiation therapy also correlated with local recurrence. Follow-up perfusion studies did not provide information to predict recurrence. Conclusion : These preliminary results suggest that two simple MR perfusion studies before and early in therapy can offer important information on treatment outcome within the first 2 weeks of radiation therapy before response is evident by clinical examination. Highg tumor perfusion before therapy and increasing or persistent high perfusion early during the course of therapy appear to be favorable signs. High perfusion suggests a high blood and oxygen supply to the tumor. The increase in tumor perfusion seen in some patients early during radiation therapy suggests improved oxygenation of previously hypoxic cells following early cell kill. Radiation tehrapy is more effective in eradicating these tumors, resulting in improved local control. Our technique may be helpful in identifying early—while more aggressive therapy can still be implemented—those patients who respond poorly to conventional radiation therapy.

Magnetic resonance fluoroscopy using spirals with variable sampling densities.

The imaging of dynamic processes in the body is of considerable interest in interventional examinations as well as kinematic studies, and spiral imaging is a fast magnetic resonance imaging technique ideally suited for such fluoroscopic applications. In this manuscript, magnetic resonance fluoroscopy pulse sequences in which interleaved spirals are used to continuously acquire data and reconstruct one movie frame for each repetition time interval are implemented. For many applications, not all of k-space needs to be updated each frame, and nonuniform k-space sampling can be used to exploit this rapid imaging strategy by allowing variable update rates for different spatial frequencies. Using the appropriate reconstruction algorithm, the temporal updating rate for each spatial frequency is effectively proportional to the corresponding k-space sampling density. Results from a motion phantom as well as in in vivo gadolinium diethylenetriaminopentaacetic acid (Gd-DTPA) bolus tracking studies in a rat model demonstrate the high temporal resolution achievable using these techniques as well as the tradeoffs available with nonuniform sampling densities. This paper focuses on the acquisition of real-time dynamic information, and all images presented are reconstructed retrospectively. The issues of real-time data reconstruction and display are not addressed.

The developing role of magnetic resonance contrast media in the detection of ischemic heart disease.

Recent developments in magnetic resonance (MR) imaging have opened up new avenues in the investigation of cardiovascular physiology. Inherent signal intensity of any tissue on MR images depends largely on proton concentration as well as longitudinal (T1) and transverse (T2) relaxation times. Myocardial contrast can be manipulated by using specific MR pulse sequences which are selectively sensitive to differences in any one of these parameters. Paramagnetic metal complexes are used as contrast media in MR imaging to enhance the inherent contrast. Contrast media in MR imaging are not directly visible but change the magnetic properties of other nuclei in close proximity, such as those of the water hydrogen. The signal of water can be altered by the contrast medium in different ways, either by changing the relaxation times or through bulb susceptibility effects, or both. The role of MR contrast media for quantitative characterization of ischemic heart disease has advanced considerably in the past 10 years. Conventional MR imaging techniques following the administration of contrast media are useful for identifying and sizing myocardial infarctions and for distinguishing between occlusive and reperfused myocardial infarctions as well as reversible (stunned) and irreversible injuries. Recent results suggest that contrast-enhanced MR imaging can also be used to identify dead cells in reperfused ischemically injured myocardium. The recently developed fast MR imaging techniques, with the aid of MR contrast media as a perfusion indicator, may be useful in estimating regional myocardial perfusion and blood volume. The assessment of capillary circulation or myocardial perfusion may be used for evaluating the extent of hypoperfusion and treatment efficacy. Experimental and clinical perfusion studies indicate that perfusion-sensitive MR imaging detects compromised myocardium (area at risk). Combining myocardial perfusion imaging with the anatomic and functional information provided by other MR imaging sequences could make MR imaging a comprehensive noninvasive technique for the evaluation of ischemic heart disease.

67 Tumor perfusion studies using fast magnetic resonance imaging technique in advanced cervical cancer — A new non-invasive predictive essay

67 Tumor perfusion studies using fast magnetic resonance imaging technique in advanced cervical cancer — A new non-invasive predictive essay

Identification of myocardial reperfusion with echo planar magnetic resonance imaging. Discrimination between occlusive and reperfused infarctions.

The current treatment of many cases of acute myocardial infarction involves the use of thrombolytic agents. Evaluation of this therapy requires determination of the success of reperfusion and assessment of the presence and extent of infarction in the reperfused territory. The present study was designed to simulate in rat models several possible outcomes of reperfusion therapy: (1) successful reperfusion and absence of myocardial infarction, (2) successful reperfusion and presence of myocardial infarction, and (3) unsuccessful reperfusion. The usefulness of contrast-enhanced fast magnetic resonance (MR) imaging in defining the success of reperfusion was investigated. The dynamic effects were examined of low and high doses of gadolinium-BOPTA/dimeglumine (Gd-BOPTA/dimeg) on myocardial signal using MR inversion recovery echo planar imaging (IR-EPI) and gradient recalled echo planar imaging (GR-EPI), respectively. Rats were subjected to one of the following regimens: reperfused reversible myocardial injury (n = 9), reperfused irreversible myocardial injury (n = 9), and occlusive infarction (n = 9). MR echo planar images were acquired every 1 or 2 seconds before, during, and after administration of Gd-BOPTA/dimeg. In all groups, normal myocardial signal was sharply increased on IR-EPI and decreased on GR-EPI at the peak of the bolus, followed by a gradual decline to baseline. In animals subjected to reperfused reversible myocardial injury, normal and previously ischemic regions were indistinguishable during and after the passage of Gd-BOPTA/dimeg. On the other hand, enhancement of reperfused irreversibly injured myocardium was delayed but increased steadily to a higher level than normal myocardium on IR-EPI. The reperfused irreversibly injured myocardium was identified on IR-EPI as a zone of high signal (hot spot). On GR-EPI, signal loss in reperfused irreversibly injured myocardium was significantly less compared with normally perfused myocardium. In animals with occlusive infarctions, there was no change in signal intensity over the ischemic region on either IR-EPI or GR-EPI. Occlusive infarction was identified as zones of either low (cold spot) or high (hot spot) signal compared with normal myocardium, depending on MR pulse sequence and dose of the contrast medium. The transit of Gd-BOPTA/dimeg monitored by fast MR imaging techniques can be used to distinguish between reperfused reversibly and reperfused irreversibly injured myocardium and between occlusive and reperfused infarctions.

Contrast media for MR imaging of the heart.

The role of contrast media for quantitative characterization of ischemic myocardial events with magnetic resonance (MR) imaging has advanced considerably in the past few years. Contrast material-enhanced MR imaging is useful for identifying and sizing myocardial infarcts and for distinguishing between occlusive and reperfused myocardial infarcts. Recent results suggest that contrast-enhanced MR imaging can also be used to identify areas of cell death in regions of reperfused myocardial infarction. With the aid of MR contrast media, fast MR imaging techniques may be useful in estimating regional myocardial perfusion. Although no simple relationship between signal intensity and concentration exists, contrast-enhanced MR perfusion imaging can demonstrate the presence and relative severity of hypoperfused myocardium. Combining myocardial perfusion imaging with the anatomic and functional information provided by other MR imaging techniques could make MR imaging a comprehensive noninvasive means of evaluating ischemic cardiac disease.

Interleaved Magnetic Resonance and Ultrasound by Electronic Synchronization

Increasing attention has been directed toward using magnetic resonance imaging (MRI) to assess blood flow velocity. Complete acceptance of this application requires validation of MRI-derived flow measurements against an accepted flow measurement technique such as Doppler ultrasound in an in vivo situation. To provide an accurate correlation in the presence of rapid changes in blood flow, the MR acquisition should be made nearly simultaneously with the ultrasonic measurements. Unfortunately, standard ultrasound equipment generates radio frequency signal which interferes with MRI. Near-simultaneous acquisition of MR data and ultrasonic blood flow data should be possible if the two measurements are properly synchronized. In the technique presented, ultrasound is made to peacefully coexist with MRI by gating the ultrasound so that it is disabled during the time of MR data acquisition. Phantom and animal experiments confirm the use of this procedure. Although we did not specifically test new fast-scan MR techniques, our technique is completely general and should work equally well with spin-echo as well as newer fast scanning MRI techniques.

Use of Gd-DTPA and fast gradient-echo and spin-echo MR imaging to demonstrate renal function in the rabbit.

The paramagnetic magnetic resonance (MR) imaging contrast agent gadolinium diethylenetriaminepentaacetic acid (DTPA) is freely filtered at the glomerulus and is neither secreted nor reabsorbed by the renal tubules. Fast MR imaging techniques, either gradient-echo or spin-echo, can be used to document the passage of Gd-DTPA through the renal tubules, as reflected by alteration in the MR signal intensity within the different anatomic regions of the kidney. Gradient-echo (repetition time of 35 msec, echo time of 7 msec, flip angles of 10 degrees-100 degrees) and spin-echo (repetition time of 35 msec, echo time of 8 msec) pulse sequences were used to acquire 20 consecutive images, one every 12 seconds, of the rabbit kidney. Both pulse sequences depicted the time course of Gd-DTPA distribution through the kidney but with distinctly different patterns of MR signal change. These dynamic MR images provide an MR nephrogram that directly demonstrates renal morphology and indirectly reflects the functional status of the renal vasculature, renal perfusion, and tubular concentrating ability.