Biomedical subjects

M E Ladd

Publications and source records attributed to M E Ladd.

At least 19 recordsLinked to original sources

PubMed2001

Rapid magnetic resonance angiography for detection of atherosclerosis.

BACKGROUND: Choice of treatment for atherosclerosis depends on various clinical factors and radiological techniques. We aimed to assess the diagnostic accuracy of a new three-dimensional magnetic resonance angiography (3D MRA) strategy for the display of arterial vasculature from supra-aortic arteries to distal runoff vessels in 72 s. METHODS: We examined five healthy volunteers and six patients over 6 weeks. Conventional digital subtraction angiography (DSA) was available as reference standard in all six patients. Magnetic resonance imaging was done on a commercially available 1.5 Tesla scanner. The imaging technique was based on the acquisition of five 3D data sets in rapid succession with an optimum single injection protocol. FINDINGS: Compared with conventional catheter angiography, according to the findings of two independent and masked readers, whole-body MRA had overall sensitivities of 91% (95% CI 0.76-0.98) and 94% (0.8-0.99), and specificities of 93% (0.85-0.97) and 90% (0.82-0.96) for the detection of substantial vascular disease (luminal narrowing >50%), interobserver agreement for assessment of whole-body magnetic angiograms was very good (kappa=0.94; 95% CI 0.9-0.98). INTERPRETATION: The technique provides a comprehensive non-invasive approach for morphological screening assessment of the arterial vasculature from supra-aortic arteries to the distal runoff arteries.

Adult↗

PubMed2001

Dynamic 3D MR angiography of the pulmonary arteries in under four seconds.

Although 3D MRA has been shown to provide excellent depiction of the pulmonary arterial tree, its clinical use has been limited due to lengthy breath-holding requirements. Employing the newest gradient generation (1.5 T MR system, amplitude of 40 mT/m and a slew rate of 200 mT/m/msec), we evaluated a technique permitting the dynamic acquisition of 3D data sets of the entire pulmonary tree in under 4 seconds. Coronal image sets were collected using a repetition time of 1.64 msec and an echo time of 0.6 msec, resulting in an acquisition time of 3.74 seconds. Three volunteers and eight dyspneic patients with known or suspected pulmonary embolism underwent MRI of the pulmonary arteries. The pulmonary arterial tree was visible to a subsegmental level in all examined subjects. Regarding the presence of pulmonary emboli in four patients, there was complete concordance between MR angiographic findings and those of corroborative studies. We conclude that diagnostic MRA of the pulmonary vasculature can be obtained even in patients with severe respiratory distress.

Dyspnea↗

PubMed2001

Improved multi-station peripheral MR angiography with a dedicated vascular coil.

Delineation of small branch vessels can be crucial for assessing the peripheral arterial system of patients requiring surgical grafting. Thus signal-to-noise needs to be maximized. We evaluated the performance of a dedicated peripheral vascular coil in four subjects by comparing it to the body coil using DSA as the standard of reference. SNR and CNR values of the dedicated peripheral coil exceeded those obtained with the body coil by a mean of 398%, thus permitting improved delineation of the infrapopliteal arterial morphology.

Adult↗

PubMed2001

Whole-body MR imaging in 30 seconds with real-time true FISP and a continuously rolling table platform: feasibility study.

A technique for whole-body magnetic resonance (MR) imaging in only 30 seconds was developed on the basis of a rolling table platform with integrated surface coils and real-time true fast imaging with steady-state precession. In five patients, all hepatic and pulmonary lesions with a diameter exceeding 8 mm were detected by using thoracic and abdominal helical computed tomography as the reference method. Whole-body MR imaging with real-time true fast imaging with steady-state precession is feasible and may be suitable for tumor screening and staging.

Adult↗

PubMed2000

Reduction of resonant RF heating in intravascular catheters using coaxial chokes.

The incorporation of RF coils into the tips of intravascular devices has been shown to enable the localization of catheters and guidewires under MR guidance. Furthermore, such coils can be used for endoluminal imaging. The long cable required to connect the coil with the scanner input inadvertently acts as a dipole antenna which picks up RF energy from the body coil during transmit. Currents are induced on the cable which can lead to localized heating of surrounding tissue. Cables of various lengths were measured to determine if a resonance in the heating as a function of cable length could be found. Coaxial chokes with a length of lambda/4 were added to coaxial cables to reduce the amplitude of the currents induced on the cable shield. A 0.7-mm diameter triaxial cable, small enough to fit into a standard intravascular device, was developed and measured both with and without a coaxial choke. It is demonstrated that resonant heating does occur and that it can be significantly reduced by avoiding a resonant length of cable and by including coaxial chokes on the cable.

Catheterization, Central Venous↗

PubMed2000

Interventional MRA and intravascular imaging.

Several attributes make magnetic resonance imaging (MRI) attractive for guidance of intravascular therapeutic procedures, including high soft tissue contrast, imaging in arbitrary oblique planes, lack of ionizing radiation, and the ability to provide functional information, such as flow velocity or flow volume per unit time, in conjunction with morphologic information. For MR guidance of vascular interventions to be safe, the interventionalist must be able to visualize catheters and guidewires relative to the vascular system and surrounding tissues. A number of approaches for rendering instruments visible in an MR environment have been developed, including both passive and active techniques. Passive techniques depend on contrast agents or susceptibility artifacts that enhance the appearance of the catheter in the image itself, whereas active techniques rely on supplemental hardware built into the catheter, such as a radiofrequency (RF) coil. Additionally, the ability to introduce an RF coil mounted on a catheter presents the opportunity to obtain high-resolution images of the vessel wall. These images can provide the capability to distinguish and identify various plaque components. The additional capabilities of MRI could potentially open up new applications within the purview of vascular interventions beyond those currently performed under X-ray fluoroscopic guidance.

Angioplasty, Balloon↗

PubMed2000

Interventional and intravascular MR angiography.

Magnetic resonance imaging (MRI) has a number of characteristics which make it attractive for guidance of intravascular therapeutic procedures, including high soft tissue contrast, imaging in any arbitrary oblique plane, lack of ionizing radiation, and the ability to provide functional information, such as flow velocity and volume per unit time. For MR guidance of vascular interventions to be safe, catheters and guidewires must be visualized relative to the vascular system and surrounding tissues. A number of approaches for making instruments visible in an MR environment are presented, including both passive and active techniques. Passive techniques depend on contrast agents or susceptibility artifacts, whereas active techniques, including MR tracking, MR profiling, and active field inhomogeneity, use some form of electrical coil built into the instrument. The potential for obtaining high-resolution images of the vessel wall using coils built into a catheter is also discussed. These images provide the capability to distinguish and identify various plaque components. The additional capabilities of MRI could potentially open up new applications beyond those currently performed under X-ray fluoroscopic guidance.

Angioplasty, Balloon↗

PubMed2000

[Whole-body MRA on a rolling table platform (AngioSURF)].

PURPOSE: Development of a technique for whole-body MR angiography based on a rolling table platform and integration of a surface coil. MATERIAL AND METHODS: The developed rolling table platform AngioSURF (System for Unlimited Rolling Field-of-view) with integrated surface coil can be mounted on top of the original patient table of a Siemens Symphony System. Data acquisition was performed with a standard body array surface coil. The system was tested on three volunteers and one patient with angiographically documented vascular pathology. Data acquisition was performed with a 3D-FLASH-sequence (TR/TE 2.1/0.7 ms, flip angle: 20 degrees, FOV 40 x 40 cm, 80 partitions, matrix 512 x 420 with zero interpolation). Five data sets were collected in immediate succession during continuous injection of a paramagnetic contrast agent. Time of acquisition per data set was 10 seconds. Table repositioning was performed manually within 3 seconds. Thus the total acquisition time amounted to 72 seconds. RESULTS: No problems with handling occurred in any of the four cases. The excellent image quality enables detailed assessment of the displayed vascular territories. CONCLUSIONS: The rolling table platform with integrated surface coil (AngioSURF) allows diagnostic display of the arterial vascular system from supraaortic vessels to the distal trifurcation arteries in only 72 seconds.

Angiography↗

PubMed2000

Ultrafast MR imaging of the fetus.

OBJECTIVE: We examined the capability of ultrafast single-shot fast spin-echo imaging to assess different fetal organ systems compared with prenatal sonography, using autopsy or postpartum imaging as a standard of reference. SUBJECTS AND METHODS: Thirty women with complicated pregnancies (mean age of gestation, 190 +/- 54 days) underwent T2-weighted ultrafast MR imaging. MR images were analyzed with regard to diagnostic confidence in assessing abnormalities of fetal organ systems, and data were correlated with postpartum findings or necropsy. Results were compared with those of prenatal sonography. RESULTS: Using receiver operating characteristic curve analysis, diagnostic confidence of MR imaging was best for assessing the brain (area under the curve [Az] = 0.96) and spinal canal (Az = 1.0), uteroplacental unit (Az = 0.93), and lungs (Az = 0.91). Results for the heart (Az = 0.63) and extremities (Az = 0.77) were significantly lower than that of other organs (p < 0.001). Diagnostic accuracy increased with gestational age. No statistically significant difference between sonography and MR imaging was found for the detection of abnormality in any organ system. In three fetuses, MR imaging was superior to sonography in characterizing cerebral abnormalities. MR imaging was inferior to sonography in characterizing abnormalities of the heart and extremities. CONCLUSION: Our results indicate that ultrafast MR imaging can be used for in vivo fetal imaging, especially in assessing cerebral abnormalities. However, MR imaging should be restricted to situations in which sonographic findings are ambiguous or impaired.

Congenital Abnormalities↗

PubMed1999

Autoperfused balloon catheter for intravascular MR imaging.

An intravascular magnetic resonance (MR) imaging catheter for high-resolution imaging of vessel walls was developed. The catheter design is based on an autoperfusion balloon catheter that allows passive perfusion of blood during balloon inflation. The blood enters a central lumen through multiple sideholes of the catheter shaft proximal to the balloon. A remotely tuned, matched, and actively decoupled, expandable single-loop radiofrequency coil was mounted onto the balloon to receive intravascular MR signals. The autoperfusion rate through the catheter was determined experimentally relative to perfusion pressure. The catheter concept was evaluated in vitro on human femoral artery specimens and in vivo in the internal carotid artery of two pigs. The proposed catheter design allowed for maintained blood perfusion during the acquisition of high-resolution intravascular images. During perfusion, image quality remained unaffected by flow, motion, and pulsatility artifacts. The availability of an autoperfused intravascular catheter design can be considered an important step toward high-resolution atherosclerotic plaque imaging in critical vessels such as the carotid and coronary arteries.

Animals↗

PubMed1999

Single-loop coil concepts for intravascular magnetic resonance imaging.

Compared with other coil designs that have been investigated for intravascular use, the single-loop coil can be designed with a very small diameter for insertion into small vessels and with a longitudinal extent over several centimeters for multislice imaging. If it designed to be expandable inside the target vessel, then it combines these features with increased signal-to-noise ratio (SNR) and penetration depth. Expandable single-loop coils that are capable of meeting these requirements were developed and integrated into two different commercial catheter-based delivery systems: a self-expandable, single-loop made from NiTinol and a single-loop coil mounted on an inflatable balloon. The influence of a small-diameter coaxial cable for remote tuning and matching on the coil performance was investigated. Calculations showed the dependence of the signal on the separation between the conductors. The comparison of both catheter approaches in in vitro flow experiments and in an in vivo pig experiment revealed the influence of pulsatile flow on image quality during intravascular imaging with these designs.

Animals↗

PubMed1999

Vascular stents as RF antennas for intravascular MR guidance and imaging.

Stent deployment is used to improve the immediate and long-term results of vascular interventions in various vascular sites. X-ray angiography as an imaging modality is often limited in providing an accurate assessment with regard to vessel size, plaque calcification, or stent deployment. In this study, the potential of using the stent endoprothesis as a radiofrequency (RF) receive-only probe for MR guidance and lesion imaging was investigated. Three different principles were developed to visualize stents actively, the first employing the stent as a loop antenna, the second employing the stent in an electrical dipole configuration, and the third employing the stent in a hybrid configuration as a coaxial line antenna. The three configurations resulted in different signal characteristics. Based on two of these antenna configurations, stent deployment devices were built and evaluated in in vitro as well as in vivo sheep experiments. Active stent visualization allows real-time MR guidance through the vessel tree and monitoring of stent deployment. In addition, the stent antenna may become useful for high resolution imaging of the vessel wall. Magn Reson Med 42:738-745, 1999.

Animals↗

PubMed1998

Prenatal diagnosis of fetal malformations by ultrafast magnetic resonance imaging.

Prenatal ultrasonography is the primary imaging modality in pregnancy as it allows direct real-time fetal examination. Antenatal magnetic resonance imaging (MRI) has so far been of limited clinical value owing to poor image quality. This was due to the long acquisition times that were needed to achieve a high enough spatial resolution for assessment of the small fetal anatomic structures resulting in severe motion artefacts. This problem has now been overcome by recent technical improvements. We present a case of fetal malformation (MURCS association), where an ultrafast T2-weighted RARE sequence (single shot fast spin echo sequence) provided images of excellent quality, which led to the correct antenatal diagnosis. MRI using this sequence can now be used for prenatal diagnosis and is thus likely to become an important adjunct to ultrasonography, especially in cases in which ultrasound findings are unclear of sonographic images are impaired by maternal obesity or oligohydramnios.

Adult↗

PubMed1998

Tissue-independent MR tracking of invasive devices with an internal signal source.

An improved MR tracking coil design is described that provides more robust tracking performance. It is shown theoretically and experimentally that a coil equipped with an internal spin source increases the signal-to-noise ratio in comparison to a coil system without internal source. The tracking behavior is stable even in air, which is important for laparoscopic work, in which the device has to be placed inside the inflated abdomen. Two types of coil windings were tested: one with the windings perpendicular to the coil axis and the other one with the windings tilted relative to this axis. The experiments showed no significant effect on the signal-to-noise ratio between the two types. The improved MR-tracking coil design with internal source was successfully used in cholecystostomies and in laparoscopic interventions; both procedures were performed on swine.

Animals↗

PubMed1997

Guidewire antennas for MR fluoroscopy.

Before MRI can be used for guiding vascular interventions, the problem of dependably visualizing guidewires must be solved. Ideally, the position and curvature of the tip of the guidewire should be visible to facilitate steering through the vascular system. In this paper, various antennas are described that can be incorporated into a guidewire tip. These antennas allow the guidewire to be visualized with high contrast. Computer simulation and experimental evidence are presented showing the value of adding a passive MR signal source, with a short T1, internal to the coil. The internal source increases the available signal, narrows the apparent width of the guidewire, and allows the use of fast imaging sequences.

Blood Vessels↗

PubMed1996

Biopsy needle susceptibility artifacts.

Understanding the appearance of thin metallic structures in magnetic resonance imaging is important for evaluating the potential role of MRI in guiding and monitoring percutaneous interventions. As most MR compatible instruments are made from materials with a susceptibility different from water, their visibility is enhanced beyond what is expected on the grounds of displaced water alone. Unfortunately, this artifactually enhanced visibility is not constant, but instead depends on a variety of factors. This article presents computer simulations of the image distortion resulting from magnetic susceptibility differences between a needle and the surrounding tissue. The simulations show not only an artifact size that is dependent on needle composition, orientation, and pulse sequence, but also a corresponding shift of the artifact center away from the actual center of the needle. These effects place limits on the accuracy of MRI-guided needle tip placement.

Artifacts↗

PubMed1996

[Virtual intravascular endoscopy in the renal arteries: a new way of observing 3-D-MIR angiography data sets].

PURPOSE: To determine the potentials and limitations of a recently developed algorithm for virtual intravascular endoscopy (VIE) based on 3-dimensional (3-D) magnetic resonance (MR) data sets. METHOD: The data from 6 patients derived from gadolinium-enhanced MR angiography of the renal arteries were reviewed. All patients had unilateral or bilateral renal arterial pathologies (stenoses n = 2, aneurysms n = 4). Imaging was performed on a 1.5-T scanner using a 3-D gradient-echo MR sequence. The imaging parameters were as follows: TR/TE/flip angle 3.9/1.9/40 degrees, matrix 256 x 192, slice thickness 1.5 to 2 mm, FOV 32 cm, and 48 slices. Image acquisition time was 28 seconds under apnoeia conditions. A total of 60 ml 0.5 molar Gd-DTPA was injected during the scan. RESULTS: 3-D data sets could be obtained in all patients and postprocessed for VIE. The intraluminal vessel wall was depicted with high clarity in all cases. All pathologies that were not intraparenchymal could be easily seen. Limitations to the technique include the image quality of the original data set, use of the ideal threshold to minimise intraluminal artifacts, and a complicated prescription sequence. CONCLUSIONS: We have shown that VIE can be consistently performed in the renal arteries using MR data sets acquired with a contrast enhanced 3-D gradient-echo technique. It provides a hitherto unused approach to viewing 3-D vascular MR data sets.

Adolescent↗

PubMed1996

Human aorta: preliminary results with virtual endoscopy based on three-dimensional MR imaging data sets.

PURPOSE: To determine the feasibility of use of magnetic resonance (MR) imaging data sets to perform virtual intraarterial endoscopy (VIE). MATERIALS AND METHODS: Seven female and 14 male patients (aged 9-86 years [mean, 42 years]) with various pathologic aortic conditions underwent three-dimensional gadolinium-enhanced spoiled gradient-echo MR imaging. With prototype software, VIE postprocessing algorithms (based on ray casting) were applied to the imaging data sets. Findings at conventional angiography were used as the standards of reference. RESULTS: The vessel wall was seen from the inside in each case, and the following pathologic conditions were depicted clearly: the two lumina in a congenial double aortic arch and the single lumen after correction, vessel narrowing in coarctations, characteristics of Leriche syndrome, stenoses, occlusions, and the true and false lumina of dissections. CONCLUSIONS: Limitations of VIE include the image quality of the original data set, the threshold chosen to minimize intraluminal artifacts, and the inherent smoothing of vessel walls.

Adult↗