Biomedical subjects

D A Leung

Publications and source records attributed to D A Leung.

At least 19 recordsLinked to original sources

PubMed2005

Contrast-enhanced MR angiography at 1.5 T after implantation of platinum stents: in vitro and in vivo comparison with conventional stent designs.

OBJECTIVE: The objective of our study was to evaluate the in vitro and in vivo 3D contrast-enhanced MR angiography characteristics of a new platinum-based balloon-expandable stent system and compare this system with a variety of competing metallic stents. MATERIALS AND METHODS: All experiments were performed on 1.5-T scanners. In vitro experiments were performed using 10 stents implanted into a custom-built phantom. Different orientations of the stents along the magnetic field and multiple flip angles were examined. In addition, 19 patients underwent contrast-enhanced MR angiography after the implantation of 36 stents, including four patients with six platinum stents. Angiographic correlation was available for all 19 patients, and luminal patency and stent-induced artifacts were assessed quantitatively. RESULTS: Of the tested balloon-expandable stents, only the platinum-based stents created artifact causing luminal narrowing of 30% or less. All other balloon-expandable stents induced larger artifacts that resulted in higher degrees of narrowing. Thus, if patent, the platinum-based stents allow significant in-stent stenosis to be ruled out reliably. Selected nitinol- or tantalum-based self-expandable stents also are suitable in this regard. CONCLUSION: Of the tested devices, platinum-based stents are the only type of currently available balloon-expandable stent that creates 30% or less artifact-induced apparent stenosis and thus are suitable for MR angiography.

Aged↗

PubMed2004

Primary sarcoma of the distal abdominal aorta: CT angiography findings.

Primary aortic angiosarcomas are extremely rare. Clinically and radiographically, they mimic atherosclerosis and atheroembolic disease. For a definitive diagnosis, histologic evaluation of the tumor or of peripheral emboli is required. The imaging findings are frequently nonspecific and in most published cases did not allow a definitive preoperative diagnosis. This is the first report of the computed tomographic angiographic findings of a primary intimal abdominal aortic sarcoma and a review of previously described imaging findings in these tumors.

Aorta, Abdominal↗

PubMed2001

Determinants of postembolization syndrome after hepatic chemoembolization.

PURPOSE: Postembolization syndrome (PES) occurs in the majority of patients undergoing hepatic chemoembolization, and is the major reason for hospitalization after the procedure. The ability to identify which groups of patients are at increased or decreased risk of PES would be useful to better counsel patients, to minimize toxicity, and to plan inpatient versus outpatient therapy. MATERIALS AND METHODS: Seventy hepatic chemoembolization procedures were performed in 29 patients using cytotoxic drugs mixed with Ethiodol and polyvinyl alcohol. The following procedural variables were retrospectively assessed and evaluated for association with PES and length of postprocedural hospitalization: gallbladder embolization, lobe embolized, percentage liver volume embolized, percentage embolized volume occupied by tumor, previous embolization of the same territory, and dose of chemoembolic emulsion. Logistic regression was used to quantify the relative effect of each procedural variable. RESULTS: Gallbladder embolization and dose administered were associated with an increased risk of PES and an extended hospitalization, with odds ratios of 2.8 and 3.0, and 3.0 and 4.6, respectively. Previous embolization was associated with a decreased risk of both PES and extended hospitalization, with odds ratios of 0.5 and 0.4, respectively. There was a statistical trend toward significance for gallbladder embolization (P = .06), dose administered (P = .07), and previous embolization (P = .14). CONCLUSION: Clinically relevant predictors of the severity of PES and length of postprocedural hospitalization may exist. Avoiding embolization of the gallbladder reduces the risk of PES. Re-embolization of previously treated vessels is associated with decreased toxicity and may assist in selecting patients for treatment on an outpatient basis, especially when a reduced dose is required.

Antineoplastic Combined Chemotherapy Protocols↗

PubMed2001

Gadolinium contrast agents: their role in vascular and nonvascular diagnostic angiography and interventions.

Gadolinium-based contrast agents can be used for diagnostic and interventional angiographic procedures to reduce contrast-related nephrotoxicity or in patients with a history of severe allergic reaction to iodine-containing contrast material. These agents are best used in conjunction with CO(2) to complete nondiagnostic CO(2) angiograms and to monitor the progress of a percutaneous intervention. However, the total volume of gadolinium that can be administered, the reduced quality of gadolinium digital subtraction angiography images, and the increased cost of the gadolinium-based agents can limit their use.

Contrast Media↗

PubMed2001

Angiographic evaluation and treatment of transplant renal artery stenosis.

Transplant renal artery stenosis is an uncommon but important complication of renal transplantation. It is a potentially reversible cause of patient morbidity and allograft dysfunction, which can present both early and late in the post-transplant period. Although transplant renal artery stenosis can be detected using noninvasive imaging, definitive diagnosis and percutaneous treatment typically require the use of invasive angiographic techniques. In experienced hands, these studies can be performed safely, effectively and with a low risk of contrast induced nephrotoxicity when alternative contrast agents are used.

Algorithms↗

PubMed2000

Acute mesenteric ischemia caused by spontaneous isolated dissection of the superior mesenteric artery: treatment by percutaneous stent placement.

Spontaneous and isolated dissection of the superior mesenteric artery is a rare and often fatal event which has been successfully treated by surgery in several reported cases. We present a patient with acute mesenteric ischemia due to superior mesenteric artery dissection who was successfully treated by percutaneous endovascular placement of a Wallstent.

Acute Disease↗

PubMed1999

Characterization of renal artery stenoses based on magnetic resonance renal flow and volume measurements.

BACKGROUND: The benefits of percutaneous angioplasty of renal artery stenosis is not known. In an attempt to better define the patient subgroup most likely to benefit from a revascularization procedure, the relationship between renal arterial morphology, blood flow volume, and renal volume was investigated using magnetic resonance (MR) techniques. METHODS: Analysis was based on arterial flow volume and renal volume measurements of 130 kidneys in 65 patients with clinically suspected renal vascular disease. Significant renal arterial stenosis (RAS), documented by contrast-enhanced three-dimensional MR angiography, compromised blood supply to 31 kidneys. Renal volume measurements were related to the body mass index to derive the renal volume index (RVI). The RVI of 72 kidneys in 36 patients without evidence of RAS or severe renal insufficiency was used as a standard of reference to differentiate normal-volume from reduced-volume kidneys. RESULTS: Eighteen out of 31 RAS kidneys were significantly reduced in volume (3.08 +/- 0.75); the volume of the remaining 13 kidneys was within one standard deviation of the normal reference value. Flow volumes in kidneys with RAS were significantly reduced compared with kidneys without RAS (91.56 vs. 279.15 ml/min). Based on the RFI values (RFI = flow volume/renal volume), there was only minimal overlap between normal volume kidneys with RAS (0.73 +/- 0.34) and kidneys without RAS (2.02 +/- 0.59). RFI values of small volume kidneys with RAS (1.55 +/- 0.47), on the other hand, overlapped with both groups. CONCLUSIONS: Normal volume kidneys with impaired arterial flow caused by RAS can be differentiated from those without based on a flow index (RFI). These data suggest the existence of a critical cut-off value (flow index <1.2 ml/min per cm3 of renal tissue) beyond which the renal parenchyma starts shrinking.

Adult↗

PubMed1999

Magnetic resonance angiography versus duplex sonography for diagnosing renovascular disease.

Noninvasive testing for renovascular disease is required to identify patients who may benefit from revascularization procedures without exposing an unnecessary amount of patients to the risks of catheter angiography. All available methods of diagnosing renal artery stenosis have significant limitations. We compared a new technique, contrast-enhanced magnetic resonance angiography, with an established technique, duplex ultrasonography, for the detection of renal artery stenosis using catheter angiography as the standard of reference. Eighty-nine patients with clinically suspected renovascular disease underwent duplex renal scanning and contrast-enhanced magnetic resonance angiography. Sixty of these also underwent catheter angiography. All studies were interpreted for the presence of renal artery stenosis blinded to the results of the other imaging modalities. For detection of hemodynamically significant (>/=60% diameter reduction) main renal artery stenosis, sensitivity and specificity were 90% and 86%, respectively, for magnetic resonance angiography and 81% and 87% for duplex sonography. Most false readings involved differential grading of stenoses detected with all 3 techniques. When patients with fibromuscular dysplasia were excluded from the analysis, the sensitivity of magnetic resonance angiography increased to 97%, with a negative predictive value of 98%. Magnetic resonance angiography detected 96% and duplex 5% of accessory renal arteries seen at catheter angiography. Contrast-enhanced magnetic resonance angiography is a useful technique for diagnosing atherosclerotic renovascular disease. It overcomes the major limitations of duplex renal scanning. However, duplex has the advantage of providing hemodynamic information and appears better suited for the assessment of patients with suspected fibromuscular dysplasia.

Adult↗

PubMed1998

[Active instrumental guidance in interventional MR tomography: introduction to a new concept].

PURPOSE: An active MR-based guidance system for visualisation of invasive instruments is described. METHODS: The principle of MR tracking is based on the integration of a miniaturised coil into the tip of the instrument itself. A phantom experiment was designed to demonstrate the localising accuracy of this technique. In addition, biocompatibility and warming effects were evaluated. Preliminary intravascular applications that were performed in animal experiments under MR guidance included embolisation, vascular occlusion as well as transjugular intrahepatic punctures. Percutaneous biopsies, cholecystostomies and laparoscopic applications were also evaluated with MR tracking. RESULTS: Phantom experiments confirmed an excellent localisation accuracy of MR tracking compared top conventional radiography. At a field strength of 0.5 T, the temperature increase remained below 2 degrees C. Results of phantom experiments revealed a potential of significant heating dependent on the sequence parameters employed. MR tracking allowed a robust, simultaneously biplanar visualisation of the instrument tips in real time. Based on MR "road map" images, various intravascular and percutaneous interventions were successfully performed in vivo under MR guidance. CONCLUSIONS: MR tracking is a flexible concept permitting monitoring in the guidance of instruments in an MR environment. Various preliminary in vitro and in vivo experiments demonstrate safety, localisation accuracy and feasibility of this biplanar localisation technique in real time.

Biopsy, Needle↗

PubMed1998

Contrast-enhanced magnetic resonance angiography of the renal arteries. Original investigation.

RATIONALE AND OBJECTIVES: The authors determine the value of contrast-enhanced, three-dimensional (3D) magnetic resonance angiography (MRA) in the assessment of the renal arteries in comparison with conventional arteriography (CA). METHODS: One hundred three patients (71 m, 32 f) were evaluated with both CA and 3D MRA. The 3D MRA data set consisted of 44 contiguous sections, acquired in apnea (23-28 seconds) using the following parameters: TR/TE 3.9/1.5 milliseconds, flip angle 40 degrees to 50 degrees, 3/4 k-space acquisition. A bolus of 0.3 mmol/kg BW gadolinium-DTPA was administered intravenously, using an automated injector. A test bolus method was used for timing of the bolus relative to the beginning of the data acquisition. Intra-arterial CA was used as the standard of reference in all patients. Separate interpretations of the CA and MRA results were made by two different pairs of radiologists, who were each blinded to the results of the other exam. RESULTS: In all, 31 of 33 accessory renal arteries were correctly identified. All 205 main renal arteries were seen with MRA. Of 65 significant stenoses identified on CA, 61 were correctly identified and graded by MRA. Sensitivity and specificity values for the assessment of significant renal arterial lesions were 93% and 90%, respectively. CONCLUSIONS: Breath-hold, contrast-enhanced 3D MRA allows for the reliable assessment of renal arterial morphology and pathologic states.

Aged↗

PubMed1997

[Value of contrast-enhanced 3D magnetic resonance angiography of the renal arteries].

PURPOSE: To determine the value of gadolinium-enhanced, three-dimensional breath-hold Magnetic Resonance Angiography (MRA) in the assessment of the aorta and renal arteries in comparison to conventional arteriography (CA). PATIENTS AND METHODS: 49 patients were evaluated with both CA and 3D MRA. 0.3 mmol/kg BW gadolinium-DTPA was administered intravenously in a bolus, using an automated injector. A test bolus method was used for timing of the bolus and beginning of the data acquisition. The intraarterial CA was used as the gold standard. RESULTS: MRA-based assessment of renal artery stenosis was identical with CA in 31 of 45 stenoses (68.8%). Sensitivity and specificity for assessment of renal arterial disease by MRA were 84% and 96%; for clinically relevant lesions they amounted to 90% and 98%. CONCLUSION: The presented contrast-enhanced 3D MRA technique allows for the reliable assessment of renal arterial morphology and pathology.

Adult↗

PubMed1997

Three-dimensional contrast-enhanced magnetic resonance angiography of the thoracic vasculature.

Magnetic resonance angiography (MRA) has become a useful non-invasive imaging technique for the assessment of vascular disease. Due to limitations such as respiratory motion artefacts, saturations problems, and long acquisition times, applications of MRA in the thorax have largely been restricted to imaging of the aorta. The recent introduction of breath-hold three-dimensional (3D) contrast-enhanced MRA promises not only to enhance conventional MR protocols for aortic imaging, but to extend the clinical indications of MRI to diseases affecting other vascular structures of the thorax, most notably the pulmonary arteries. This article describes the technical aspects of contrast-enhanced 3D MRA and reviews existing and potential future clinical applications.

Aorta, Thoracic↗

PubMed1997

[Ultrafast contrast-enhanced 3D MR angiography of the aorta and renal arteries in apnea].

PURPOSE: To determine the value of ultrafast, gadolinium-enhanced, three-dimensional breathhold magnetic resonance angiography (MRA) in the assessment of the aorta and renal arteries in comparison to conventional arteriography (CA). PATIENTS AND METHODS: 49 patients (31 m, 18 f) were evaluated with both CA and 3D MRA. The 3D MRA data set consisted of 44 continuous sections, acquired in apnoea (23-28 s) using the following parameters: TR/TE 3.9/1.5 ms, flip angle 40 degrees, 3/4 k-space acquisition. 0.3 mmol/kg BW gadolinium-DTPA were administered intravenously in a bolus, using an automated injector. A test bolus method was used for timing of the bolus and beginning of the data acquisition. Intraarterial CA was used as the gold standard in 47 patients; in two patients the intraoperative findings were employed as the standard of reference. CA and MRA were interpreted separately by two different radiologists, who were blinded to the results of the other examiner. RESULTS: All 11 accessory renal arteries were visualised on MRA. MRA-based assessment of renal artery stenosis was identical with CA in 31 of 41 (75%) stenoses, Sensitivity and specificity values for assessment of renal arterial disease were 84.4% and 96.1%, for haemodynamically significant lesions they amounted to 90% and 98.9%, respectively. CONCLUSION: The presented ultrafast contrast-enhanced 3D MRA technique allows for the reliable assessment of aortic and renal arterial morphology and pathology.

Adult↗

PubMed1997

MR imaging-guided intravascular procedures: initial demonstration in a pig model.

With use of an open 1.5-T magnetic resonance (MR) imager and a tracking catheter, the authors successfully placed the catheter into the left or right sacral artery in pigs. The tracking catheter comprised a 5.3-F percutaneous transluminal angioplasty catheter with a small copper radio-frequency coil in its tip. With use of the coil as an antenna, the catheter tip position was projected in real time onto MR angiography road maps in two planes. Guidance of placement of the catheter with the MR angiography road maps allowed successful embolization, balloon occlusion, and transjugular intrahepatic puncture of the portal system. Specialized catheters can be tracked in vivo to allow MR guidance in intravascular interventional procedures.

Animals↗

PubMed1997

Evaluation of the aortoiliac and renal arteries: comparison of breath-hold, contrast-enhanced, three-dimensional MR angiography with conventional catheter angiography.

PURPOSE: To determine the diagnostic accuracy of breath-hold, contrast material-enhanced, three-dimensional (3D) magnetic resonance (MR) angiography in the assessment of the aortoiliac inflow tract and renal arteries with conventional catheter angiography as the standard of reference. MATERIALS AND METHODS: In 39 patients with symptomatic peripheral vascular disease, both catheter and contrast-enhanced, 3D MR angiography were performed. In a prospective blinded analysis, each vascular segment (23 segments per arterial tree) was evaluated for the presence of obstructive stenosis, graded as mild (<50%), severe (50%-99%), or occlusion (100%), as well as for aneurysmal disease. RESULTS: Obstructive (n = 2) and aneurysmatic aortic (n = 6) lesions and 10 of 11 accessory renal arteries were identified and graded correctly with MR angiography. For detection of hemodynamically significant stenotic lesions (>50%) or occlusions with MR angiography, sensitivity and specificity, respectively, were 93% and 98% for the renal arteries, 96% and 100% for the common iliac arteries, 93% and 93% for the external iliac arteries, and 96% and 94% for the internal iliac arteries. CONCLUSION: The diagnostic performance of contrast-enhanced 3D MR angiography is comparable with that of catheter angiography and thus may contribute to assessment of the aorta, the renal arteries, and the pelvic arterial system in patients with peripheral vascular disease before they undergo therapy.

Angiography, Digital Subtraction↗

PubMed1996

Towards active guidewire visualization in interventional magnetic resonance imaging.

Improving the visibility of interventional devices is of paramount importance if MRI-guided fluoroscopy is to become a reality. Passive visualization is problematic in that the susceptibility-induced artifacts are material- and orientation-dependent. Here a concept is presented for making interventional devices visible. It involves fitting a device with a straight-wire antenna. As the sensitivity of such an antenna is highest for signal sources in the immediate neighborhood, using the antenna for reception gives an outline image. In this manner a guidewire or other interventional device could be made MRI-visible. The image appearance of a straight-wire antenna depends on the orientation of the device with respect to the main magnetic field and imaging plane. This phenomena is discussed theoretically and documented with MR images.

Catheterization↗

PubMed1996

Breath-hold, contrast-enhanced, three-dimensional MR angiography.

Breath-hold gadolinium-enhanced three-dimensional magnetic resonance (MR) angiography was performed with a high-performance gradient MR imaging system in 10 volunteers with no history of vascular abnormalities (breath-hold interval, 28 seconds; repetition time, 4.0 msec; echo time, 1.9 msec; 0.4 mmol gadoterate meglumin per kilogram of body weight). High-resolution three-dimensional time-of-flight angiograms of the aorta, renal arteries, pulmonary arteries, pelvic arteries, and portal venous system were successfully acquired in all cases.

Adolescent↗

PubMed1995

Real-time biplanar needle tracking for interventional MR imaging procedures.

PURPOSE: To evaluate real-time biplanar tracking of a specially designed needle with magnetic resonance (MR) imaging guidance. MATERIALS AND METHODS: The needle is made of polyetheretherketone and has a miniature radio-frequency coil incorporated into the tip. Tracking software on two workstations is used to compute three-dimensional coordinates of the coil and to display the position as a moving symbol in two imaging planes. Validation of needle tracking was performed in a harvested human liver. T2-weighted fast spin-echo images were used to target a 1-cm cyst. Success of needle placement was confirmed with aspiration and with updated gradient-recalled-echo images. RESULTS: The cyst was successfully targeted from different approaches. Tracking procedures were monitored in real time simultaneously on two separate images. CONCLUSION: Real-time biplanar needle tracking may prove to be useful for both diagnostic and therapeutic interventional MR imaging procedures.

Artifacts↗