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Biomedical subjects

S J Riederer

Publications and source records attributed to S J Riederer.

At least 37 records · Page 2Linked to original sources

Simultaneous image acquisition utilizing hybrid body and phased array receiver coils.

In clinical MR imaging the design and selection of receiver coil is an important step in ensuring the highest image quality. Often this choice is based on selecting a receiver coil characterized by high spatial uniformity such as the body and head volume receiver coils or a surface coil (or array of coils) that provide high signal-to-noise ratio (SNR). In the past, it has been difficult to accomplish both high SNR and spatial uniformity as both coil types achieve one of these characteristics at the expense of the other. The purpose of this study was to achieve both high SNR and spatial uniformity through the simultaneous acquisition of the MR signal using the body and a surface coil array. Results indicate that this hybrid system can provide uniformity and SNR values comparable to those achieved by the body and surface coil arrays, respectively.

Humans↗

Determination of appropriate RF blocking impedance for MRI surface coils and arrays.

Surface and phased array receiving coils in MRI typically require that RF excitation be accomplished using the body coil. This process requires that the receiving coils contain blocking circuitry to increase the overall circuit impedance during RF excitation and withstand the electromotive force induced by the applied electromagnetic field. The aim of this study was to determine the optimal impedance range required during RF excitation based on an assessment of image quality. The experimental results are fit by an exponential model and establish criteria that can be applied for general receiver coil design.

Biophysical Phenomena↗

Dual-echo breathhold T(2)-weighted fast spin echo MR imaging of liver lesions.

The purpose of this study was to develop a multi-shot dual-echo breathhold fast spin echo technique (DFSE) and compare it with conventional spin echo (T2SE) for T(2)-weighted MR imaging of liver lesions. The DFSE acquisition (EffTE1/EffTE2/TR = 66/143/2100 ms) imaged 5 sections per 17 s breathhold. T2SE imaging (TE1/TE2/TR = 60/120/2500 ms) required 16:55 (min:s) for 14 sections. Both techniques used a receive-only phased-array abdominal multicoil and provided 192 x 256 effective resolution. The results showed first and second echo relative DFSE/T2SE contrast values for 27 representative lesions (15 consecutive patients) were 1.08 +/- 0.05 and 1.16 +/- 0.09 (mean +/- STD mean), respectively. Corresponding CNR values were 1.12 +/- 0.09 and 0.97 +/- 0.12. Overall DFSE was comparable-to-superior to T2SE for lesion sizing and image artifact. DFSE lesion detection was inferior to T2SE's in several patient studies because of decreased conspicuity of lesions located near multicoil edges and because of poor breathhold-to-breathhold reproducibility and lack of breathholding. However both DFSE (and T2SE) provided lesion detection rated to be of diagnostic quality for all patient studies. In conclusion, we found that DFSE provides diagnostically useful dual-echo T(2)-weighted MR liver images in a greatly decreased acquisition time.

Adult↗

Three-dimensional contrast-enhanced MR angiography with real-time fluoroscopic triggering: design specifications and technical reliability in 330 patient studies.

Technical reliability was determined for triggering three-dimensional (3D) contrast material-enhanced magnetic resonance (MR) angiography with MR fluoroscopy. Technical requirements for high reliability were also identified. Reliability was evaluated in 330 consecutive patient studies of the neck, thorax, abdomen, and pelvis. Contrast material arrival was detected fluoroscopically in 325 of the 330 studies (98.5%), and the 3D sequence was successfully triggered in 321 of 330 studies (97.3%). Fluoroscopic triggering of centrically encoded 3D MR angiographic acquisitions is a highly reliable means of obtaining 3D MR angiograms with high spatial resolution.

Abdomen↗

Interactive three-point localization of double-oblique sections using MR fluoroscopy.

Magnetic resonance imaging allows significant freedom in selecting the orientation and position of a tomographic section. However, it can nonetheless be challenging to determine quickly and efficiently the correct parameters required to image a targeted anatomic structure that may lie at an oblique angle in the imaging volume. We describe a three-point tool in which a) the user interactively selects three points from an anatomic structure of interest during live MR fluoroscopy; b) adjustments to pulse sequence are calculated to image the tomographic section defined by the three points; and c) the section is then immediately imaged fluoroscopically. The tool allows quick localization of, for example, longitudinal images of specific arterial structures.

Algorithms↗

A flexible view ordering technique for high-quality real-time 2DFT MR fluoroscopy.

A method to tailor the view order to the reconstruction cycle is introduced for real-time MRI. It is well known that view sharing and oversampling central k-space views can improve the temporal resolution of gradient-echo pulse sequences. By ordering phase-encodes to synchronize k-space acquisition with the reconstruction cycle, apparent temporal resolution can match the frame rate with as few as one-fourth of the phase-encodes sampled per reconstruction. Spatial resolution is maintained by periodically updating high spatial frequencies. In addition to apparent temporal resolution, three other criteria for real-time imaging are identified and evaluated: display latency, dispersion, and frame-to-frame consistency. Latency is minimized by ordering views in a reverse-centric manner within each reconstruction interval, sampling high-energy views immediately prior to beginning reconstruction. Dispersion is kept low and consistent by synchronizing acquisition and reconstruction, thus avoiding poorly timed reconstruction instances. Real-time implementation demonstrates pulsatile time-of-flight blood signal enhancement in humans.

Algorithms↗

Theoretical limits of spatial resolution in elliptical-centric contrast-enhanced 3D-MRA.

The point spread function (PSF) for contrast-enhanced three-dimensional (3D) MR angiography using the elliptical centric view order is derived. This view order has been shown previously to provide high venous suppression thereby enabling long acquisition times capable of high spatial resolution. The dependence of the PSF on TR, field of view (FOV), scan time, and trigger time are shown explicitly. Theoretical predictions are corroborated with experimental results in phantoms and in vivo. The PSF width decreases as the square root of the product of TR and the two phase encoding FOV's for fixed nominal voxel size. The PSF peak amplitude increases as the reciprocal of this product. Theory and experiment demonstrate that acquisition times over 40 sec provide superior resolution compared to shorter acquisitions, despite falling levels of contrast agent concentration. The analysis predicts that an isotropic spatial resolution of 1 mm before zero filling is possible in a FOV large enough to encompass the carotid and vertebral arteries bilaterally. Magn Reson Med 42:1106-1116, 1999.

Carotid Arteries↗

Contrast-enhanced 3D MR breathhold imaging of porcine coronary arteries using fluoroscopic localization and bolus triggering.

The purpose of this study was to develop cardiac-gated contrast-enhanced 3D MRA for imaging the coronary arteries of pigs. Each major coronary artery was imaged individually in a single 3D slab in one breathhold. To permit acquisition within a breathhold, a limited number of partitions (12-16) were collected in a single, oblique, thin 3D slab. Typical resolution of the acquisition was 0.8 (X) x 1.6 (Y) x 1.6 (Z) mm. MR fluoroscopic localization was used to establish the 3D double-oblique orientation. Real-time MR fluoroscopy was also used to instantaneously trigger the 3D scan after detection in the aortic root of the intravenously administered contrast bolus. Six pigs were used in the study. Each pig was scanned on two separate days. Images routinely show the majority of the length of the three principal coronary arteries. Magn Reson Med 42:1159-1165, 1999.

Animals↗

Real-time imaging and triggering of 3D contrast-enhanced MR angiograms using MR fluoroscopy.

Although a variety of timing techniques and data acquisition strategies have been used for three-dimensional contrast-enhanced MR angiography, many are still limited by inadequate overall reliability, limited spatial resolution, or complexity. A technique is presented in this work in which contrast arrival is detected in the targeted vasculature in real time using MR fluoroscopy. Upon detection the operator triggers a 3D MR angiographic acquisition which uses an elliptical centric view order. It is shown that the view order intrinsically provides a high degree of venous suppression which in turn allows acquisition times of 30 s or longer, permitting high spatial resolution. The reliability of fluoroscopic triggering in bolus detection is shown to be over 90%. The technique provides high quality contrast-enhanced MR angiograms for many vascular regions.

Carotid Arteries↗

3D contrast-enhanced MR angiography using fluoroscopic triggering and an elliptical centric view order.

Although a variety of timing techniques and data acquisition strategies have been used for three-dimensional contrast-enhanced MR angiography, many are still limited by inadequate overall reliability, limited spatial resolution, or complexity. A technique is presented in this work in which contrast arrival is detected in the targetted vasculature in real time using MR fluoroscopy. Upon detection the operator triggers a 3D MR angiographic acquisition which uses an elliptical centric view order. It is shown that the view order intrinsically provides a high degree of venous suppression which in turn allows acquisition times of 30 seconds or longer, permitting high spatial resolution. The reliability of fluoroscopic triggering in bolus detection is shown to be over 90%. The technique provides high quality contrast-enhanced MR angiograms for many vascular regions.

Fluoroscopy↗

Carotid arteries: maximizing arterial to venous contrast in fluoroscopically triggered contrast-enhanced MR angiography with elliptic centric view ordering.

PURPOSE: To obtain high-spatial-resolution, venous-suppressed, contrast material-enhanced, three-dimensional (3D) magnetic resonance (MR) angiograms of the carotid arteries and aortic arch by using an elliptic centric view ordering with MR fluoroscopic triggering. MATERIALS AND METHODS: Forty consecutive patients with cerebrovascular disease in the differential diagnosis were evaluated with fluoroscopically triggered 3D MR angiography (gadoteridol dose range, 0.1-0.3 mmol per kilogram of body weight; mean acquisition time, 40 second +/- 8 [SD]). The contrast-enhanced 3D MR angiograms were evaluated for overall quality, vascular signal intensity, venous suppression, and motion artifact. Twenty patients also underwent two-dimensional (2D) time-of-flight (TOF) MR angiography. The overall quality of the 2D TOF MR angiograms and comparative quality between the 2D TOF and contrast-enhanced 3D MR angiograms were determined. RESULTS: The contrast-enhanced 3D MR angiograms were of excellent or more than adequate quality for diagnosis in 36 of the 40 studies (90%). In 35 of the 38 contrast-enhanced 3D studies in which the contrast material bolus was detected fluoroscopically, the internal jugular vein signal intensity was either not detectable or barely visible. In 18 of the 20 patients who also underwent 2D TOF MR angiography, the quality of the contrast-enhanced 3D MR angiograms was graded as markedly superior or superior. CONCLUSION: Contrast-enhanced, elliptic centric 3D MR angiography with real-time MR fluoroscopic triggering offers high-spatial-resolution images of the carotid arteries and aortic arch with reliable venous suppression.

Aorta, Thoracic↗

Assessment of functional MR imaging in neurosurgical planning.

BACKGROUND AND PURPOSE: Presurgical sensorimotor mapping with functional MR imaging is gaining acceptance in clinical practice; however, to our knowledge, its therapeutic efficacy has not been assessed in a sizable group of patients. Our goal was to identify how preoperative sensorimotor functional studies were used to guide the treatment of neuro-oncologic and epilepsy surgery patients. METHODS: We retrospectively reviewed the medical records of 46 patients who had undergone preoperative sensorimotor functional MR imaging to document how often and in what ways the imaging studies had influenced their management. Clinical management decisions were grouped into three categories: for assessing the feasibility of surgical resection, for surgical planning, and for selecting patients for invasive functional mapping procedures. RESULTS: Functional MR imaging studies successfully identified the functional central sulcus ipsilateral to the abnormality in 32 of the 46 patients, and these 32 patients are the focus of this report. In epilepsy surgery candidates, the functional MR imaging results were used to determine in part the feasibility of a proposed surgical resection in 70% of patients, to aid in surgical planning in 43%, and to select patients for invasive surgical functional mapping in 52%. In tumor patients, the functional MR imaging results were used to determine in part the feasibility of surgical resection in 55%, to aid in surgical planning in 22%, and to select patients for invasive surgical functional mapping in 78%. Overall, functional MR imaging studies were used in one or more of the three clinical decision-making categories in 89% of tumor patients and 91% of epilepsy surgery patients. CONCLUSION: Preoperative functional MR imaging is useful to clinicians at three key stages in the preoperative clinical management paradigm of a substantial percentage of patients who are being considered for resective tumor or epilepsy surgery.

Adolescent↗

A prospective approach to correct for inter-image head rotation in fMRI.

Global head motion occurring between successive image acquisitions during a functional MRI time series can corrupt the signal of physiologic brain activation, potentially invalidating interpretation of the final activation map from that particular fMRI time series. By approximating the head as a rigid body, multiaxial global head motion can be decomposed into orthogonal linear and rotational components. This paper describes a method using orbital navigator echoes to provide prospective correction for both through-plane and in-plane inter-image head rotation in functional MRI. The dynamic detection and correction of rotation can be performed in <100 ms. Phantom experiments demonstrate accurate correction of rotational motion over a range of +/-0.36 degrees to +/-12 degrees. Imaging studies in volunteers document the feasibility of real-time prospective correction of rotational motion in vivo. Using a modified receiver operating characteristic method, motion-corrected functional MRI sensorimotor studies incorporating deliberate head rotations are shown to be superior to functional MRI time series acquired under similar conditions but without motion correction.

Artifacts↗

Arterial phase carotid and vertebral artery imaging in 3D contrast-enhanced MR angiography by combining fluoroscopic triggering with an elliptical centric acquisition order.

Arterial-phase three-dimensional (3D) contrast-enhanced MR angiograms of the carotid and vertebral arteries from their origins through the carotid bifurcations were obtained in 20 patients using acquisition times over 30 sec by using an MR fluoroscopy-triggered pulse sequence with elliptical centric view order. The typical pixel size was 0.8 mm (x) x 1.6 mm (y) x 1.5 mm (z), and 32-48 coronal slices were acquired. The fluoroscopic monitoring of bolus arrival was effective in 18 of the 20 cases; two failures were attributed directly to a poor choice of RF coil. To exploit peak arterial-to-venous contrast, the central 3D views were acquired first in the most compact time period possible for the given TR. For the 18 successfully triggered cases, arterial-phase 3D images were obtained with excellent venous suppression as demonstrated by an average internal jugular vein to common carotid signal enhancement ratio of only 0.05 +/- 0.04.

Adolescent↗

Two-dimensional multishot echo-planar coronary MR angiography.

This work presents a two-dimensional (2D) multishot echo-planar imaging (EPI) technique for magnetic resonance angiography (MRA) of individual coronary arteries in a 17-heartbeat breath-hold. Conventional 2D and 3D segmented gradient-echo (GRE) coronary MRA requires repetitive excitation of the same slice or slab within each cardiac cycle, which can result in reduced blood signal and in motion artifacts. Two-dimensional multishot EPI can address these limitations by eliminating multiple excitations per cardiac cycle, using large flip-angle excitations, markedly reducing the data acquisition window, and performing oblique multislice 2D imaging. The goal of this study was to assess the feasibility of breath-hold 2D multishot EPI for multislice coronary MRA and to demonstrate its reliability by consistently acquiring high-quality images of the coronary arteries in a series of 16 volunteers.

Adult↗

Mapping of the central sulcus with functional MR: active versus passive activation tasks.

PURPOSE: Our purpose was to assess the pattern of functional MR activation obtained with a passive sensory versus an active sensorimotor hand stimulus paradigm. METHODS: Eight functional MR runs, four with an active sensorimotor (sponge-squeezing) task and four with a passive sensory (palm-finger brushing) reception, were acquired for each of 10 healthy volunteers. Activation maps were generated by thresholding cross-correlation maps. Regions of interests (ROIs) were drawn around the precentral and postcentral gyri on T1-weighted images according to established anatomic criteria, and the number of activated pixels inside the ROIs was ascertained. Displacement of the sensorimotor and sensory activation centroids within the ROIs from the central sulcus as well as from each other was measured. RESULTS: Active sensorimotor stimulation produced a significantly greater number of activation pixels than did passive sensory stimulation. Run-to-run variability was equivalent between sensorimotor and sensory activation tasks. On average, the sensorimotor and sensory activation centroids were located in the postcentral gyrus, and their spatial locations were not significantly different. CONCLUSION: Active and passive activation tasks produce largely equivalent results. Presurgical mapping of the sensorimotor area can be performed with functional MR imaging using a passive palm-finger brushing task in patients who are physically unable to perform active finger-tapping or hand-squeezing sensorimotor activation tasks.

Adult↗

Real-time reconstruction and high-speed processing in functional MR imaging.

Access to fully processed activation maps in near real time during a functional MR examination enables run-to-run assessment of results. This is particularly useful in clinical studies, since the results of the functional MR examination can be ascertained before the patient leaves the MR suite, permitting interactive tailoring of the functional MR study. We describe how a real-time MR system can be customized to complete the following tasks in less than 3 minutes: obtain an 81-second acquisition of a multisection functional MR imaging time series using single-shot echo-planar imaging, perform image reconstruction, extract functional MR activation maps using cross-correlation and thresholding, and superimpose activation maps on previously acquired T1-weighted anatomic images.

Algorithms↗

Three-dimensional magnetization-prepared time-of-flight MR angiography of the carotid and vertebral arteries.

Three-dimensional magnetization-prepared (MP) MR angiography (MRA) was applied to the carotid and vertebral arteries by using ECG triggering, a slab selective RF inversion pulse, centric phase encoding and acquisition during diastole. Both theoretically and experimentally, the MP MRA sequence was shown to perform well in cases where there was substantial blood replenishment during the inversion time TI (>330 ms). In comparison with standard, ungated, steady-state 3D time-of-flight MRA in 13 consecutive volunteers, the MP MRA images demonstrated better background suppression with less artifact and generally had more uniform vessel depiction. The MP MRA sequence was generally superior for portraying vessels exhibiting high pulsatility such as the carotid siphon. However, in distal vessels with slow flow and incomplete blood replenishment, the MP MRA technique was inferior due to increased loss of vessel signal. The reasons for this increased signal loss are identified and improvements to the MP MRA sequence are suggested.

Adult↗