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To evaluate the feasibility and the diagnostic efficacy of multislice diffusion-weighted and perfusion imaging in addition to FLAIR-TSE, T2w-GraSE and MR-angiography in the diagnosis of acute stroke.18 patients with acute stroke were examined at 1.5 Tesla (Gyroscan ACS-NT, Philips Medical Systems) within 6 (n = 9) and 6-48 (n = 9) hours, respectively, and followed at regular intervals. For diffusion imaging we used a multislice multishot EPI-SE sequence with navigator echo correction and cardiac gating. Perfusion imaging was done by means of a FFE-EPI sequence after bolus injection of Gd-DTPA.The diagnostic value of diffusion-weighted and perfusion imaging was significantly higher compared with FLAIR-TSE (p = 0.0023) and GraSE (p = 0.0012) during the first 6 hours. With FLAIR-TSE and GraSE first pathologic changes were seen after 4 hours. We detected perfusion deficit (rCBV10%) and a corresponding drop of the ADC in all infarcts larger than 1 cm in diameter. Within the area of low rCBV the combined analysis of diffusion and perfusion imaging allows to identify an infarct region with characteristics of a penumbra and one with characteristics of the infarct core. TTP was increased in the surrounding tissue. However, parts of this area were rarely included in the infarct. The final extension of the untreated infarct, as revealed by computed tomography, corresponded well to the perfusion deficit.Early ischaemic cerebral infarcts can be diagnosed with diffusion and perfusion imaging before pathological changes are visualized with other imaging modalities. The combined use may allow to distinguish the infarct core from surrounding, potentially salvageable tissue.

MRI is currently regarded as absolutely contraindicated in patients with implanted cardiac pacemakers. In this prospective study safety and feasibility of MRI in patients with new generation pacemakers (PM) was evaluated in vitro and in vivo.14 PM models in vitro and 18 patients with implanted new generation PM underwent a MRI exam at 0.5 Tesla with standard spin, turbo spin, and gradient echo (FFE) sequences under continuous ECG-monitoring. PM inquiry was performed before and after the MRI exam, including assessment of stimulation thresholds.In the static magnetic field all PM switched to the asynchronous mode due to activation of the Reed switch, resulting in continuous pacing at a fixed rate. In three PM models in vitro, however, after activation of the Reed switch, there was a software-induced switch back to the demand mode. In these PM inhibition and triggering were observed after starting the MRI scan due to influence of the pulsed magnetic fields. PM program changes, damage of PM components, dislocation/torque of the PM and rapid pacing of the PM were observed neither in vitro nor in vivo. Atrial and ventricular stimulation thresholds remained unchanged.MRI at 0.5 Tesla should not be regarded as absolutely contraindicated in patients with implanted new generation PM. However, knowledge of the behaviour of the specific PM model in static and pulsed magnetic fields is required, if necessary also changes of the PM program prior to the MRI exam, continuous ECG monitoring and cardiological stand-by.

To define the diagnostic efficacy of MR imaging, "time of flight" (TOF) and phase contrast (PC) MR angiography in craniocervical arterial dissections.The MR examinations of 16 patients with proven arterial dissections (n = 20) were retrospectively analysed by three independent readers. The MR protocol included T1w spin echo sequences with and without fat saturation (SPIR), T2w-turbo-spin echo, 2D- and 3D-TOF- and 3D-PC-MRA. The study was undertaken to assess the diagnostic sensitivity of each technique in detecting typical pathological features.The overall sensitivity was best in 3D-PC-MRA; reaching 88% of all possible points. Intramural haematoma could be easily detected with T1W spin echo with fat saturation (100%). Intimal flap and lumen narrowing was best defined with 3D-TOF-MRA in 86% resp. 96% and 3D-PC-MRA in 69% resp. 97%. 3D-PC-MRA was superior to all other sequences in 5 cases of aneurysmal dissection (100%).An accurate evaluation of craniocervical arterial dissections should rely on a combined protocol including T1w spin echo with fat saturation and an axial 3D-MR angiography (if possible 3D-phase contrast MRA).

To evaluate accuracy of needle localisation using and signal enhancement on a 1.OT MR imager for various needles for MR-guided biopsy.The differences between actual and virtual needle position of needles with different orientations were evaluated in a phantom for spin-echo including turbo-spin-echo sequences.Artifacts depended on the orientation of the needle relative to the field B0, frequency-encoding gradients (Gf) and slice orientation. This resulted in different artifact shapes and sizes for left or right and cranial or caudal biopsy access routes. Applying turbo spin echo sequences feasible for biopsy, the signal void of a 18 G needle (Cook) parallel to Gf reached between 0.3 and. 4.6 mm further into the medium than the real needle tip, depending on needle orientation relative to B0. The diameter of the signal void around the needle varied, the needle shaft was right in the centre of the signal void. With Gf orthogonal to the needle the offset of signal void to needle tip ranged from 2.7 to 3.3 mm, while the actual position of the needle shaft was up to 3.3 mm lateral of the signal void center. While nominal echo times did not influence the size of the artifact in turbo-spin-echo sequences, the artifacts increased with smaller matrix and larger water-fat shift. Material and mass of the needle determined the size of the artifacts as well.Localisation accuracy of the needle can be optimised by choosing optimal gradient directions depending on whether needle tip or shaft position should be displayed.

PURPOSE: To evaluate the usefulness of preoperative magnetic resonance (MR) imaging-guided stereotactic localization and core biopsy of suspicious breast lesions that are visible at breast MR imaging alone (ie, that are clinically, mammographically, and ultrasonographically occult), with the goal of integrating this technique into the diagnostic and therapeutic work-up of MR-suspicious lesions in a clinical setting. MATERIALS AND METHODS: A stereotactic breast biopsy device was used for needle placement in and guide wire localization of 97 lesions in 66 patients or core biopsy of five lesions in five patients; all lesions were visible at MR imaging. Interventions were performed with MR guidance on a 0.5- or 1.5-T system. RESULTS: Lesion localization and resection were successful in 95 of the 97 lesions; two of the lesions were not resected in spite of correct guide wire localization. In this series, 53 (55%) of 97 lesions proved malignant (11 [21%] in situ; 42 [79%] invasive). Lesions were 4-19 mm (mean, 8.7 mm); all invasive cancers corresponded to a pT1 tumor stage. Location of the lesion in the parenchyma (retroareolar or prepectoral) did not interfere with accessibility. CONCLUSION: MR imaging-guided stereotactic hook-wire placement and excisional biopsy are accurate and effective in managing lesions identified at only breast MR imaging. MR imaging-guided core biopsy holds promise for allowing a definite work-up of these lesions.

To evaluate the differentiation of benign from malignant breast tumors with T2*-weighted perfusion magnetic resonance (MR) imaging (blood volume imaging) versus that with dynamic T1-weighted contrast agent-enhanced MR imaging.Ten healthy adult volunteers and 18 adult patients with benign or malignant lesions underwent both conventional T1-weighted dynamic contrast-enhanced breast MR imaging and repetitive first-pass, single-section, dynamic T2*-weighted perfusion MR imaging. Images were obtained before, during, and after injection of 20 mL of gadopentetate dimeglumine; peak gadopentetate dimeglumine concentrations were calculated from the maximal signal intensity loss on T2*-weighted images.No perfusion effect was detectable in healthy breast parenchyma. A strong susceptibility-mediated signal intensity loss occurred in malignant breast tumors. No or only minor perfusion effects were seen in fibroadenomas, in spite of their rapid enhancement at T1-weighted dynamic imaging. Perfusion imaging was possible after conventional dynamic contrast-enhanced breast MR imaging.T2*-weighted perfusion imaging exploits the susceptibility-mediated signal intensity loss of a first-pass bolus of gadopentetate dimeglumine within the capillary bed. First-pass perfusion imaging of breast lesions is feasible. It is promising in the differentiation of benign from malignant, rapidly enhancing lesions.

Relaxation times of water were measured in human vertebral bodies by a fat-suppressed dual-echo turbo spin echo/turbo inversion recovery MRI sequence. Comparison was made with T1 and T2 values obtained by localized 1H-MR spectroscopy. The accuracy of the results and the diagnostic potential of the fast quantitative MRI technique were evaluated in 20 volunteers, 11 patients with osteoporosis, 6 patients with lymphoma, and 6 patients with bone marrow metastasis. No significant alterations of T1 and T2 relaxation times of water and fat were found in osteoporosis. With both methods, an increase in the T1 values of the water resonance by 16% was observed in lymphomas, which was highly significant (P.001) in the MRS measurements, and an elevation by the same amount was obtained by the MRI sequence for the metastases (P = .040). A strong reduction of fat fraction was quantified by MRS in the tumorous cases. T2 of the water resonance increased by more than 30% (P.003) in metastases. Water T2 values obtained by the MRI sequence showed systematic deviations from the MRS results, especially at short echo spacings.

Development of an improved MR sequence for examining the lung.T2 weighted ultra-short turbo spin echo sequences were used in five individuals with variations in echo times, delayed triggering and echo intervals. To reduce movement artifacts all examinations were carried out with ECG and respiratory triggering. The sequences giving optimal image quality were then employed in 19 patients having various pulmonary abnormalities. Image resolutions, artifacts, image contrasts and diagnostic value were then judged by two observers and compared with CT.In the first study, a diastole-triggered UTSE sequence with the shortest echo proved optimal (TE = 90 ms, TR = 2-4 s, echo = 9 ms, turbo factor = 19). In the patient series studied, MRT was inferior to CT with regard to resolution and number of artifacts, but better in respect of contrast and diagnostic value.Using UTSE of the lung, MRT can produce images of good quality. Compared with CT, contrast is better with MRT, offering diagnostic advantages for MRT.

To compare five pulse sequences in 60 patients with pancreatic disease by means of MRI at 0.5 T.T1 weighted Spin-Echo sequences (T1W SE) before and after i.v. contrast medium administration, fat-suppressed T1W SE sequences after i.v. contrast medium administration, T2 weighted Turbo-Spin-Echo sequences (T2W TSE), and fat-suppressed T2W TSE were evaluated within an ROC study.The standard sequences (T1W SE before and after i.v. contrast medium administration, T2W TSE) were superior to the fat-suppressed T1W and T2W (T)SE sequences in delineating the anatomy and for correct diagnosis. The T1W sequence with and without i.v. contrast media detected all anatomic structures best. In most instances the correct diagnosis and coexistent findings in the diseased pancreas were obtained with the T2W sequence.Fat-suppressed sequences are less useful in the diagnosis of pancreatic disease by MRI at medium field strength compared to the standard sequences.

A 3-D Scanning Particle Image Velocimetry (3-D SPIV) technique has been developed to overcome the resolution problem associated with conventional 3-D Particle Tracking Velocimetry (3-D PTV) without requiring the complexity of 3-D holography. The technique employs a high repetition rate Copper Vapor Laser synchronized with a scanning mirror drum and sheet forming optics to scan the volume of interest. 25 or 50 individual mirrors are adjusted to form an array relatively thick laser sheets parallel to each another, hence effectively covering the entire volume of interest (typically 100×100×100 mm3). This device is capable of recording the time evolution of a flow field in 3-D with a temporal resolution of 200 to 400 Hz, which is more than sufficient for most water flows. The digitized 16mm movies are then processed by a combination of techniques: conventional 2-D cross-correlation and 2-D PTV within each plane. Due to the 2D nature of the recorded information, relatively high seeding densities can be used to yield 20,000 to 50,000 vectors per scan, but yet provide the temporal information associated with successive scans at a rapid rate. The physical implementation of this technique, the overview of the processing and some sample images are presented in this paper.

Aim of the study was to evaluate FLAIR combined with Turbo-Spin-Echo (Turbo-FLAIR) at 0.5 tesla in comparison to conventional T1- and T2-weighted spin-echo images (SE) in MRI of the brain.A Turbo-FLAIR sequence was optimised for 0.5 Tesla (Philips Gyroscan T5-II) that provided seventeen 5 mm sections in 4:21 minutes (TR = 6075 ms, T1 = 1600 ms, TE = 120 ms and a turbofactor = 17). Images were compared with T1- (TR = 500 ms, TE = 15 ms) proton density and T2-weighted spin-echo (TR = 2500 ms, TE = 20/90 ms) studies in 10 healthy volunteers and 30 patients with various brain pathologies.Turbo-FLAIR could effectively eliminate the CSF signal in all studies except ventricular areas with CSF inflow. Contrast-to-noise ratios (C/N) for the contrast between lesions and CSF was superior in Turbo-FLAIR. C/N between lesions and grey matter was significantly higher in Turbo-FLAIR than in proton density (PD)-weighted SE. C/N between lesions and white matter was equal to PD-weighted SE but significantly smaller than on T2-weighted SE. Visual analysis showed greater lesions conspicuity with Turbo-FLAIR and a higher frequency of detection of cortical and subcortical lesions.Turbo-FLAIR is a reliable and practical technique on 0.5 Tesla, that is more sensitive than SE sequences especially for the detection of cortical, subcortical lesions and lesions surrounded by CSF.

The value of T2 weighted turbo-spin-echo sequences with selective fat suppression (TSE-SPIR) in the detection and differential diagnosis of focal liver lesions was compared to TSE sequences and conventional T1 weighted spin-echo (T1 SE) sequences at 0.5T.In 50 patients with focal liver lesions T1 SE-TSE-, and TSE-SPIR sequences were performed. Two independent readers evaluated the sequences with regard to the image quality, fat suppression, and the number and conspicuity of liver lesions. Quantitative analysis was performed by tumour-liver contrast-to-noise (CNR) measurements.From a total of 118 liver lesions the T1 SE sequence, the TSE sequence, and the TSE-SPIR sequence showed 85.6%, 83.9%, and 91.5%, respectively (p0.05). The tumour-liver CNR of malignant liver lesions was 30.2-46.6% higher on the TSE-SPIR sequence compared to the TSE sequence (p0.01). With the exception of fat-containing lesions there was no significant difference of CNR in benign lesions on both TSE sequences.With regard to lesion detection the TSE-SPIR sequence is superior to the TSE sequence without fat suppression, whereas it is inferior to the TSE sequence in the differential diagnosis of benign and malignant lesions.

To evaluate the range of normal contrast enhancement behaviour in dynamic MR mammography of healthy breast parenchyma, and to reduce false-positive results caused by incidental contrast enhancing lesions.We performed a prospective two-armed study on 20 healthy premenopausal volunteers who were examined four times within one month (Group 1) or during four consecutive months (Group 2). Incidence, prevalence, morphology, and contrast enhancement pattern of lesions were evaluated.82% (9/11 of Group 1) and 78% (7/9 of Group 2) showed at least one of a total of 60 contrast enhancing lesions. In 54% or 33% (Group 1 or 2), lesions with enhancement velocity beyond the malignancy threshold were found. 72% or 75% of lesions resolved completely during follow-ups. 69% of lesions were focal with irregular border. Time/signal intensity diagrams were almost always not suspicious; no wash-out phenomenon was seen.In MR mammography of premenopausal breast parenchyma, focal contrast enhancing lesions-even with enhancement beyond the malignancy threshold-may occur without any underlying pathology.

The value of turbo-spin-echo (TSE) sequences was compared with conventional spin-echo (SE) and inversion-recovery (IR) sequences for cerebral MRT in 70 children at 0.5 T and 1.5 T. In addition we evaluated whether proton weighted sequences (PD) were diagnostically important and in what proportion of cases.Conventional T1 and T2 weighted SE and T2 weighted TSE sequences were used in all children. An IR sequence was performed in 39 patients. The various sequences were analysed semiquantitatively with regard to image quality, artifacts and the demonstration of normal and anatomical structures and pathological findings.By any criteria, TSE sequences were superior to conventional T2 weighted SE sequences at 0.5T and 1.5 T, requiring a shorter examination time (35-53%). In 8.6% the pathological finding was best seen on PD-SE sequences (5 glial scars, 1 tumor).Although TSE sequences are better than T2 SE sequences with regard to image quality and the demonstration of abnormalities, conventional double-echo SE sequences (with PD and T2 weighted images) cannot be entirely replaced by T2 weighted TSE sequences in children.

To determine whether medium field strength (0.5 T) MR mammography is able to differentiate between benign and malignant lesions in the same way as a 1.5 T standard technique.In 40 consecutive female patients with nodular lesions, examinations were carried out at 1.5 T (2D-FFE, TR/TE/FA 200/3.9/80) and 0.5 T (3D-FFE, 24/3.4/40).There was wide spread of the speed of enhancement of malignant tumours for both techniques (44%-145% with signal increase in the first minute of 42%-189%). In 15 out of 17 carcinomas, the rapidity of uptake and final degree of enhancement after contrast was higher during 0.5 T/3D measurements than it was for 1.5 T/2D images. Fibroadenomas and mastopathies showed similar enhancement characteristics for both techniques. Sharply defined "wash-out" was seen only in malignant lesions; it appeared ten times more frequently in the 0.5 T/3D examinations than at 1.5 T/2D.The distinction between benign and malignant lesions can be made with more certainty using medium field strength and 3D-FFE sequence than using the 2D high-field standard technique.

We aimed both at improving image quality and diagnostic value of pancreatic MRI by using fast multislice imaging techniques and at demonstrating normal and pathologic signal enhancement in the pancreas after i.v. gadopentate-dimeglumine.MR imaging at was performed in 30 patients with suspicion of pancreatic neoplasm or chronic disease and in 15 patients without pancreatic abnormalities.SNR, CNR, mean signal enhancement, and time/signal-intensity curves were calculated. Signal enhancement after i.v. Gd-DTPA averaged 34.4% in neoplasms (n = 14) vs 58.8% in pancreatic tissue not involved by tumour (p0.05) in T1-w SE and was also significant in the T1-w GE sequence (p0.005). CNR (pancreas vs lesion) and SNR improved significantly (p0.005; pancreas: p0.001; lesion: p0.05) after i.v. Gd-DTPA administration. In patients without pancreatic disease enhancement curves of pancreas demonstrate a rapid signal increase (79.0% after 34 s). Patients with adenocarcinoma or chronic pancreatitis demonstrated flattened signal intensity curves and higher peak signal intensity values (90.1, 111.3% after 68 s). The slope of signal increase of neoplastic tissue was significantly different (p0.01) from unaffected parenchyma (48.3% after 51 s, 90.1% after 68 s).MR imaging with fast T2-w SE and T1-w SE sequences provides excellent visualisation of the pancreas and adequate conspicuity of pancreatic adenocarcinoma. Following i.v. Gd-DTPA significant enhancement of tumour/parenchyma contrast is found.

The aim of this study was the evaluation of spiral computed tomography (Spiral-CT), multiplane transesophageal echocardiography (multiplane TEE) and magnetic resonance imaging (MRI) in the diagnosis of thoracic aortic dissection. 41 patients were examined: 30 with clinically suspected acute aortic dissection (14 Stanford A, 7 Stanford B), 11 with aortic repair (7/11 with persisting distal dissection). In 14 patients there was involvement of the supraaortic vessels. Sensitivity of Spiral-CT, multiplane TEE and MRI in the detection of aortic dissection was 100%, specificity was 100, 92, and 91%. In the assessment of involvement of aortic arch vessels sensitivity was 100, 67, and 60%, specificity was 100, 95, and 90%. The new imaging modalities Spiral-CT and multiplane TEE are equal to MRI in the detection of aortic dissection. In the assessment of the supraaortic branches Spiral-CT is superior to multiplane TEE and MRI and might become the method of choice.

Turbo-spin-echo (TSE) imaging is a fast technique, which provides true spin-echo contrast in a definitely shorter scan time than conventional spin-echo (SE). Clinical studies at 1.5 T show that TSE is a valuable alternative to SE and will ultimately replace time-consuming T2-weighted SE imaging for examinations of the brain and brainstem. Besides the reduction of scan time, TSE is less sensitive to motion artefacts. A slight decrease of susceptibility effects of 1.5 T can be a disadvantage in the detection of old haemorrhagic lesions and calcifications. In these cases the gradient-echo sequence will remain the most sensitive. TSE allows a greater flexibility of examination strategies, including several orientations, strongly T2-weighted images and high spatial resolution (HR) MRI. HR MRI using thin slices (1-3 mm), a small field of view and a 512 image matrix yields detailed information about small anatomical regions (e.g. the pituitary fossa, cavernous sinus, internal auditory canal and brainstem) and permits the delineation of specific cranial nerves. In this study TSE was utilized at 0.5 T field strength in order to assess whether the advantages of TSE hold for midfield systems. Preliminary results in imaging of the brain and brainstem are very promising.

3D-phase-contrast MR angiography (PCA) was utilised in 58 patients with suspected cerebral sinus (ST) or venous thrombosis. ST was diagnosed in 16 patients, demonstrating both intraluminal thrombus (on Tl-weighted gradient-echo images) and the absence of flow (on phase-subtraction images). In a patient with a thrombosis of the cortical veins only (proven by arterial angiography), asymmetry of the cortical veins was visible, without detection of the thrombus. Of the patients with ST 47% presented with a combination of general symptoms, e.g. headache, and focal neurological signs, e.g. motor and/or sensory deficits, whereas 41% only had one or more general symptoms. A semiquantitative analysis of the demonstration of normal vessels showed that the best results were obtained using a velocity sensitivity of 20 cm/s covering the flow range of both small veins and sinuses. However, assessment of cortical veins and basal sinuses (petrosal and cavernous remains difficult, so that arterial angiography should be used as a supplementary measure in such cases.

The value of rapid T1- and T2-weighted fast field (T1-FFE, T2-FFE) gradient-echo sequences and T2 weighted turbo-spin echo (TSE) sequences in the diagnosis of focal liver lesions using 0.5 Tesla was compared with conventional spin-echo sequences (SE). Amongst 88 liver lesions T1-SE sequences showed 98%, T2-TSE sequences showed 92%, T2-SE sequence showed 84%, T1-FFE sequence showed 78% and T2-FFE sequence showed 69%. Direct comparison has shown that TSE sequences are able to demonstrate significantly more lesions (p0.05) than T2-SE sequences. Quantitative assessment has shown that the tumor/liver contrast/noise ratio in the T1-SE sequence was 118% higher than in the T1-FFE sequence (p0.001). Comparison of T2 weighted sequences has shown that the tumour/liver contrast/noise ratio was 45% higher in the TSE sequence than in the T2-SE and 239% higher than in the T2-FFE sequence (p0.001). The results indicate that the TSE sequence should be used instead of the conventional T2-SE sequence because of the reduced time (about 40%), due to improved image quality and increased demonstration rate of liver lesions. Neither T1-FFE nor T2-FFE are suitable, when using moderate field strength, to replace T1-SE or TSE sequences.

Segmented 2D times-of-flight (TOF) magnetic resonance angiography (MRA) with ECG gating is a suitable adjunct to conventional magnetic resonance tomography. Based on our experience with 106 patients, cardiovascular MRA primarily aids in the diagnosis of congenital heart disease, but is of minor importance for the evaluation of acquired cardiovascular disease, such as aortic aneurysms. Changes in flow dynamics as a result of spin dephasing may render partial signal loss. Due to flow turbulence and resulting signal loss distal to stenoses, difficulties may arise in correctly determining the degree of stenosis and differentiating between partial thrombosis and slowly flowing blood. The most promising approach for optimizing 2D TOF MRA is enhancement of the intravascular signal and suppression of background signal. For the equipment we use with a 0.5 Tesla scanner, the most powerful combination to enhance intravascular signal and suppress back-ground noise was applying a flip angle of 65 degrees and a short inversion prepulse. The influence of the flip angle and prepulse on the intravascular signal varies, dependent on the flow velocities and heart rates. Secondary reconstructions, such as the MIP technique, contribute to better spatial orientation by using rotating projection views. To avoid overprojection artifacts and loss of diagnostic information, 2D single slices should be the primary source of image analysis.

Magnetic resonance (MR) angiography of the cardiovascular system was evaluated in 41 patients with congenital heart disease by using a two-dimensional (2D) inflow technique based on a magnetization-prepared gradient-echo pulse sequence with segmented k-space data acquisition and electrocardiographic gating at 0.5 T. Inversion and saturation prepulses were used to suppress stationary tissue and enhance intravascular signal. Presaturation slabs were applied where certain vascular structures had to be suppressed. Sequence parameters were optimized by evaluating signal intensity and contrast characteristics for various flip angles and inversion and saturation delay times. The heart and intrathoracic vasculature were encompassed with 40-50 overlapping sections. Both 2D angiograms and maximum-intensity-projection images were evaluated. Combining data sets acquired in the sagittal and transverse orientations provided the most satisfactory information about the pulmonary arteries. The highest signal-to-noise ratios were obtained with a flip angle of 65 degrees and short prepulse delay times. Two-dimensional MR angiography can provide useful diagnostic information but requires a thorough understanding of in-plane and hemodynamically induced signal intensity changes.

One disadvantage of the STIR sequence in MRI is its long acquisition time. A TR shortening for acceleration requires shortening of the inversion time depending upon the field strength. We optimized this "fast-STIR" technique at 0.5 T using calculations, phantom, volunteer, and patient measurements. This optimization procedure is transferable to other field strengths. The resulting sequence takes 4.5 minutes, fat is visualised signal-free. In the second part of our study we compared quantitatively and qualitatively the signal intensities, contrast, and sensitivity towards artifacts of this sequence with that of conventional SE and GRE sequences in 21 patients with neoplastic, inflammatory and traumatic disorders of the musculoskeletal system. The fast-STIR sequence showed similar or better lesion contrast as SE in 100% and as GRE in 88%. Decreased spatial resolution and higher sensitivity towards pulsatile artifacts of the fast-STIR sequence were only minor drawbacks.

Subdural strip electrodes or intracerebral electrodes are required in order to precisely localize epileptic foci, which is mandatory for epilepsy surgery. The subdural electrodes are inserted via a borehole and thus precise positioning cannot be directly monitored. Therefore, the location of the electrodes in relation to the gyri and sulci has to be determined by imaging techniques. In this study, a three-dimensional magnetic resonance imaging (3D MRI) technique was used for visualization of intracranial EEG electrodes. A 3D gradient echo sequence with thin slices of 1 mm is required for the assessment of the gyral anatomy of the brain surface. Using a short repetition time of 30 ms good image quality within an acceptable acquisition time of about 15 min was possible. Post-processing with 3D surface reconstruction was performed using a Sun workstation. Three-dimensional reconstruction allows a more precise delineation of the localization of the EEG electrodes and the simultaneous display of the adjacent brain structures. Misplacement of intracranial electrodes is readily detected. 3D MRI with 3D surface reconstruction proved superior to 2D imaging and CT for the assessment of EEG electrode localization.

Signal intensities, signal-to-noise ratios (S/N), and contrast-to-noise ratios (C/N) in dependence on the echo distance and echo number of the new turbo (fast) spin-echo technique are analysed by phantom, volunteer and patient measurements. With increasing echo number (3 to 15 echoes), signal intensities increase (25 to 50%), S/N varies between different tissues, and C/N of fat to water decreases (70%). A high echo number, although it shortens the imaging time, is thus not always the best choice since the fat/water contrast is lost. However, a high fat/water contrast is advantageous for several MR applications.

The stochastic estimation technique, originally proposed by Adrian[1] and applied by Adrian[2], Guezennec[3], and others, is a method by which conditional turbulent velocity fields can be estimated. Velocity “conditions” are imposed at a few locations within a reconstruction space and velocities around the conditions are computed using simple functions of the conditions in space and time. The conditions can be set, to evaluate typical flow structures associated with a specific phenomenon, or they can be measured experimentally, to estimate physically realized flow fields. The mathematical basis for the procedure lies in multiple regressions analysis which requires detailed space-time correlation measurements between all involved variables, usually the perturbation velocities, .

Identifying coherent structures in turbulent flows has been successfully accom plished in the past using several techniques. Among them are flow visualization, proper orthogonal decomposition (POD), conditional averaging, pattern recognition, and stochastic estimation. Each of these methods has advantages, but the stochastic estimation is exceptional in its flexibility, objective nature, and ease of validation. As an illustrative problem and application of the stochastic estimation, a turbulent wake at Re d =5000 is examined at x / d=100. Single and multiple point conditionally averaged structures will be estimated and a quantitative three-dimensional flow-reconstruction technique will be used to study the flow dynamics.

Since the introduction of magnetic resonance imaging (MRI) into the clinical diagnosis of focal liver disease, Tl- and T2-weighted spin-echo (SE), Tl-weighted inversion recovery (Reinig et al. 1989), short Tl inversion recovery (Dousset 1989), relaxation time measurements (Steudel et al. 1989), and breath-hold gradient-echo (GE) sequences (Winkler 1989) have been used to detect metastatic lesions and primary liver tumors and to differentiate them from benign focal lesions such as hemangioma or focal nodular hyperplasia (FNH).

A sonar quieting system for a forward-looking sonar array is provided. The sonar quieting system includes a cavitator for forming an envelope, a strut for supporting the cavitator to a marine platform, such as a ship, a forward-looking sonar array mounted to a forward face of the cavitator, and a ventilation system for pumping a gas, such as air, into the envelope to create a gas cavity which envelops the supporting strut and equipment downstream of and during a forward movement of the cavitator.

Preliminary experiments performed to measure the radiated noise from subsonic high‐speed underwater projectiles indicated the presence of a characteristic disturbance in the mean pressure field during the approach of the projectile to the location of the sensing hydrophone in the test tank. A numerical model based on the potential flow field surrounding the projectile was developed to explain this behavior. A series of 10 projectile shots were fired at speeds ranging from 700–900 m/s to investigate this phenomenon. The underwater range in the tank was instrumented with 5 PCB narrow span pressure transducers. The transducer pressure ranges and locations in the tank were selected using the potential flow model predictions. Results from these experiments are presented. The mean pressure field disturbances show steep cross range gradients. Passage of the projectile at a fixed location in the tank was determined by observing self‐similar features of the pressure field at different transducer locations. Projectile velocity estimates were determined from time delays between transducers and are compared with break screen data. The cross range gradients and velocity measurement techniques will be used to develop an open range system to track underwater projectiles. [Work supported by ONR.]

31 examinations of temporo-mandibular joints, using T1-weighted multislice gradient echo sequences have been performed. The short scan time of one minute and 50 seconds ensured that there were no movement artefacts in 28 cases. The position of the disc was visible in more than 90% of examinations. Degenerative changes in the cartilage and arthroses of the joint cannot be demonstrated adequately by the rapid sequences.

MR tomography provides images equal to, or superior to, CT of lymph node involvement in the thorax or abdomen. It may be possible to achieve tissue characterisation by means of T1 and T2 images. Chronic inflammatory diseases causing lymph node enlargement, such as sarcoidosis, show characteristic T1 intervals (about 650 msecs), whereas lymphomas show intervals of 900 msecs. Enlarged lymph nodes of low malignancy show lower T2 values (78 to 94 msecs), highly malignant show higher T2 values (83 to 145 msecs). Already we believe MR to be the method of choice in evaluating lymphoma and sarcoidosis.

31 examinations of temporo-mandibular joints, using T1-weighted multislice gradient echo sequences have been performed. The short scan time of one minute and 50 seconds ensured that there were no movement artefacts in 28 cases. The position of the disc was visible in more than 90% of examinations. Degenerative changes in the cartilage and arthroses of the joint cannot be demonstrated adequately by the rapid sequences.

ECG-triggered cardio-MRT, using T1-weighted SE sequences, provides images of the heart showing very high anatomical resolution. So far, however, it has not been possible to demonstrate the morphology of the valves and coronary vessels. By using multiplanar sections, it is possible to obtain cuts which are perpendicular to the portion of myocardium or valve under consideration. This provides optimal imaging for morphological diagnosis and for evaluating the ventricular myocardium and its function.

Contrast performance in MR tomography is influenced by the choice of technical parameters such as pulse repetition time (TR), echo time (TE) and inversion time (TI) in the same manner as by differences in the tissue parameters relaxation time and proton density. To optimise image contrast it is therefore suggested to preselect the imaging sequence. Such presselection is possible on the basis of isosignal visualisations S(T1, T2). In the inversion recovery sequence (IR) the isosignal images obtained with the parameters TE, TI, TR enable determination of relaxation time T1 by graphic representation, from the time of zero passage of the longitudinal tissue magnetisation.

In 62 patients spinecho or gradientecho sequences compensated for motion were compared with corresponding non-compensated sequences. Compensation during MRT reduces those artifacts which are caused by regular motion. This is particularly effective during examination of the brain stem, spinal canal and the heart. The improved demonstration of the morphology of normal and diseases organs simplifies the analysis of the types of movement.

A new method for correcting the signal intensity from surface coil (homomorphic filters) was evaluated in 40 examinations. With this filter it is possible to compensate for signal loss when using surface coils for deep tissues and to avoid excessive signals from the surface. The method does not, however, provide any additional information. Properly used, there were no false positive findings, nor was there any loss of abnormal findings. The use of homomorphic filters is therefore safe and sensible.

By using high field strengths and surface coils, MRT achieves a resolution comparable with CT in the orbita. The advantages of MRT are good contrast resolution and imaging in several planes. Twenty-six patients have been examined by MRT, which has shown high sensitivity and good detail for the demonstration of pathological changes. In spite of this, MRT at present is not a realistic alternative to ultrasound and CT, because it is unable to demonstrate bone and calcification; its specificity is low, but the time and cost of the examination is high. It is indicated only for problems involving the optic nerve and chiasma.

The fast-field-echo (FFE) sequence provides a new rapid imaging method for MR tomography. By using gradient echoes with shorter high frequency pulse repetitions than conventional sequences with 180 degrees echo pulses, it is possible to produce single cuts in 10 to 20 s with good signal-to-noise ratio. In this way the liver can be examined while respiratory movement artifacts are reduced. The possibility of obtaining T1- and T 2-weighted FFE images with good contrast resolution and the ability to determine relaxation times suggests a wide application of this rapid imaging procedure in the diagnosis of liver disease.