Biomedical subjects

Lambertus W Bartels

Publications and source records attributed to Lambertus W Bartels.

15 recordsLinked to original sources

PubMed2006

Analysis of preexistent vertebral rotation in the normal quadruped spine.

STUDY DESIGN: In this CT study, vertebral rotation was analyzed in the transverse plane of the normal, nonscoliotic canine spine with a computer-based measurement method. OBJECTIVES: To determine if a rotational pattern exists in the normal, nonscoliotic quadruped spine, similar to what is seen in humans. SUMMARY OF BACKGROUND DATA: Idiopathic scoliosis does not occur in quadrupeds. In humans, the normal, nonscoliotic spine shows a preexistent pattern of vertebral rotation, which corresponds to the most prevalent curve types of idiopathic scoliosis. Since this rotational tendency has only been demonstrated in humans, it is not clear if it can be considered as a part of the pathogenesis of idiopathic scoliosis or as a normal anatomic feature. METHODS: CT scans of the thorax of 42 dogs without clinical or radiologic evidence of scoliosis were used to measure axial vertebral rotation from T1-T13 with a previously developed computer-based CT measurement method. RESULTS: The results of this study demonstrated a predominant rotation to the right of the upper, mid, and lower thoracic vertebrae of the normal canine spine. The mean vertebral rotation angles differed significantly from zero degrees rotation at level T1, from level T4-T7, and from T11-T13. CONCLUSIONS: The normal spine of quadrupeds shows rotation of the thoracic vertebrae with a preferred direction to the right, similar to what is seen in humans. Since idiopathic scoliosis does not exist in quadrupeds, this preexistent rotation seems to be a physiologic process in normal spinal development, independent of the pathogenesis of scoliosis.

Animals↗

PubMed2006

The use of MR B+1 imaging for validation of FDTD electromagnetic simulations of human anatomies.

In this study, MR B(+)(1) imaging is employed to experimentally verify the validity of FDTD simulations of electromagnetic field patterns in human anatomies. Measurements and FDTD simulations of the B(+)(1) field induced by a 3 T MR body coil in a human corpse were performed. It was found that MR B(+)(1) imaging is a sensitive method to measure the radiofrequency (RF) magnetic field inside a human anatomy with a precision of approximately 3.5%. A good correlation was found between the B(+)(1) measurements and FDTD simulations. The measured B(+)(1) pattern for a human pelvis consisted of a global, diagonal modulation pattern plus local B(+)(1) heterogeneties. It is believed that these local B(+)(1) field variations are the result of peaks in the induced electric currents, which could not be resolved by the FDTD simulations on a 5 mm(3) simulation grid. The findings from this study demonstrate that B(+)(1) imaging is a valuable experimental technique to gain more knowledge about the dielectric interaction of RF fields with the human anatomy.

Computer Simulation↗

PubMed2006

Analysis of preexistent vertebral rotation in the normal spine.

STUDY DESIGN: A newly developed CT measurement method was used to investigate axial rotation from T2 to L5 in the normal, nonscoliotic spine. OBJECTIVES: To identify a preexistent rotational pattern in the normal, nonscoliotic spine. SUMMARY OF BACKGROUND DATA: The data available on axial rotation measurements in the normal spine are scant and limited to only a few vertebrae. Systematic analysis of the thoracic and lumbar vertebrae of the normal spine, based on computed tomography has, to our knowledge, not been performed. METHODS: CT scans of the thorax and abdomen of 50 persons without clinical or radiologic evidence of scoliosis were used to measure vertebral axial rotation from T2 to L5 with a newly developed semiautomatic computerized method. RESULTS: The results of the present study showed a predominant rotation to the left of the high thoracic vertebrae, and to the right of the mid and lower thoracic vertebrae in the normal, nonscoliotic spine, which differed significantly from an equal right-left distribution. This rotational pattern is present in both males and females. CONCLUSION: The normal, nonscoliotic spine demonstrates a preexistent pattern of vertebral rotation that corresponds to what is seen in the most prevalent types of thoracic idiopathic scoliosis.

Adolescent↗

PubMed2006

Fully MR-guided hepatic artery catheterization for selective drug delivery: a feasibility study in pigs.

PURPOSE: To demonstrate the feasibility of hepatic catheterization for selective delivery of therapeutic agents using a clinical MRI scanner for real-time image guidance. MATERIALS AND METHODS: Experiments were performed in three domestic pigs (70-80 kg) using a clinical 1.5-T MR scanner. After abdominal three-dimensional contrast-enhanced MR angiography (3D-CE-MRA) was performed, endovascular devices with susceptibility markers were tracked with passive tracking techniques. Catheters were maneuvered into the primary and secondary hepatic arteries. Selective catheterization was verified using selective time-resolved CE angiography. Paramagnetic microspheres were administered to a different region for each liver. The resulting biodistributions were investigated using MR images. RESULTS: Successful selective hepatic catheterization was repeatedly demonstrated using passive tracking techniques. 3D-CE-MRA significantly aided the interventional procedure by showing the vascular anatomy, and maximum-intensity projections (MIPs) were used as roadmaps during the interventions. In all cases, microspheres were successfully delivered to the selected regions. The catheters were visualized at a maximum frame rate of five frames per second, allowing a good depiction of the devices and a reliable catheterization of the hepatic arteries. CONCLUSION: Fully MR-guided real-time navigation of endovascular devices permits complex procedures such as selective intra-arterial delivery of therapeutic agents to parts of the liver.

Animals↗

PubMed2006

3D MR sialography protocol for postradiotherapy follow-up of the salivary duct system.

PURPOSE: To develop and evaluate an MR sialography protocol that reproducibly images the parotid and submandibular ducts over time, in 3D. Such a protocol is needed in order to investigate the possible radiation-induced changes to the salivary ducts in patients receiving radiotherapy to the head-and-neck. MATERIALS AND METHODS: MR sialography was performed on a 1.5-T MR scanner. Sequence parameters were optimized on 11 healthy volunteers. A 3D water-selective turbo spin echo (TSE) pulse sequence (TR/TE = 6000 msec/190 msec), using a two-element circular surface coil was applied twice in one MR session. In order to assess the reproducibility, the same procedure was repeated four to six months later. The quality of the MR sialograms was measured subjectively by developing a visibility scoring system and objectively by the means of contrast-to-noise ratio (CNR) of the ducts vs. fat (CNR(duct-fat)). RESULTS: High-quality, 3D MR sialographic images were obtained. The quality of the MR sialograms and the subjective visibility score of the salivary ducts were constant over time. The CNR(duct-fat) varied between volunteers (standard deviation, SD 26%) but it was relatively constant per volunteer (SD 5%). CONCLUSION: The MR sialography protocol presented in this study provides good quality 3D imaging of the major salivary ducts, submandibular duct, and the parotid duct and it can be used for the comparison of the salivary duct system of an individual over time.

Head and Neck Neoplasms↗

PubMed2006

Dynamic magnetic resonance angiography of the aneurysm neck: conformational changes during the cardiac cycle with possible consequences for endograft sizing and future design.

OBJECTIVE: Proper proximal fixation and stent-graft sealing within the aneurysm neck are critical for endovascular aneurysm repair (EVAR) durability. Computed tomography angiography (CTA) is the gold standard for preoperative sizing of endograft diameters, but the accuracy of these measurements is uncertain because they rely on static images of a dynamic process. The aortic configuration and diameter may change during the cardiac cycle. We studied these phenomena using dynamic electrocardiograph-triggered magnetic resonance angiography (MRA). METHODS: Eleven consecutive EVAR patients were included. Dynamic MRA was used to perform preoperative and postoperative measurements. Changes were measured in transverse aortic sections 10 mm below the lowest renal artery (level A), at the level of the renal arteries (level B), and 3 cm above the lowest renal artery (level C). Data were analyzed using image segmentation software. Aortic area and diameter changes along 256 axes were determined. RESULTS: Dynamic MRA demonstrated significant aortic area changes during the cardiac cycle before and after EVAR at all three measured levels. Pre-EVAR aortic area significantly increased per cardiac cycle: 8.4% at level A; 9.3% at level B; and 13.3% at level C (P < .001 for all levels). Post-EVAR aortic area increased 9.7% at level A, 9.6% at level B, and 15.8% at level C per cardiac cycle (P < .001 for all levels). Significant diameter changes during cardiac cycles were also observed at all three levels. Pre-EVAR mean diameter changed up to 8.9% (P < .001) compared with post-EVAR aortic changes of up to 11.5% (P < .001). EVAR had no effect on change in aortic area and diameter. Dynamic MRA also demonstrated that pulsatile aortic distension was not equal in all axes, but rather occurred as an asymmetrical expansion and contraction. CONCLUSION: In patients with (atherosclerotic) aneurysm disease, the aortic dimensions at the level of and proximal to the aneurysm neck change during the cardiac cycle. This phenomenon is preserved after EVAR. Therefore, maximum diameter using dynamic MRA may not be similar to the maximum diameter with static CTA in all patients, and a standard regimen of 10% to 15% oversizing of an endograft based on static CTA images may be inadequate for some patients. Further studies using dynamic MRA to evaluate effects of different endografts are anticipated, with possible consequences for endograft designs.

Aged↗

PubMed2006

Aortic compliance following EVAR and the influence of different endografts: determination using dynamic MRA.

PURPOSE: To utilize dynamic magnetic resonance angiography (MRA) to characterize aortic stiffness (beta) and elastic modulus (Ep) as indexes of wall compliance during the cardiac cycle and determine any influence of different endograft designs or the presence of endoleaks on these indexes. METHODS: Eleven consecutive patients (11 men; median age 74 years, range 63-78) with abdominal aortic aneurysm (AAA) selected for endovascular repair were scanned pre- and postoperatively. Aortic area and diameter changes during the cardiac cycle were determined using dynamic MRA at 4 levels: 3 cm above the renal arteries, between the renal arteries, 1 cm below the renal arteries, and at the level of maximum aneurysm sac diameter. Ep and beta were calculated. Data are presented as median (range); p<0.05 was considered significant. RESULTS: Preoperatively, Ep and beta were significantly higher at the level of the aneurysm sac compared to all other levels (p<0.05). Following EVAR, stiffness increased at this level (p<0.05). After implantation, patients with an Excluder endograft demonstrated Ep and beta measurements at the aneurysm neck that were 94% and 60% higher, respectively, compared to those with a Talent (p<0.05) endograft. The presence of an endoleak had no effect on Ep or beta. CONCLUSION: This study introduces the feasibility of dynamic MRA imaging-based calculations of aortic elastic modulus and stiffness. AAA patients demonstrate increased Ep and beta at the level of the aneurysm sac. EVAR results in increased aneurysm sac Ep and beta. Stent-graft design seems to alter Ep and beta within the aneurysm neck, which may have consequences for endograft durability. The presence of an endoleak does not seem to have an effect on Ep or beta.

Aged↗

PubMed2005

Internal radiation therapy of liver tumors: qualitative and quantitative magnetic resonance imaging of the biodistribution of holmium-loaded microspheres in animal models.

In internal radiation therapy of unresectable liver tumors, microspheres containing a radionuclide are injected in the hepatic artery to achieve a preferential deposition of microspheres in the lesions. In this study, MR imaging techniques for qualitative and quantitative assessment of the biodistribution of holmium-loaded microspheres (HoMS) were investigated for their use in selective internal radiation therapy of liver tumors. To achieve this goal, the relaxivity of HoMS was first investigated in gel experiments. The resultant calibration curve was subsequently employed to quantify the biodistribution of HoMS administered to 13 excised rabbit livers and to the livers of 3 live rabbits with an implanted tumor. Finally, the feasibility of MR imaging of the biodistribution during treatment of a large animal was investigated by MR imaging of hepatic administration of HoMS to a live pig. Overall, the study showed that MRI can clearly depict the biodistribution of HoMS, but that quantification by means of the gel calibration curve yields an underestimation that increases for higher amounts of HoMS. The observed underestimation is tentatively attributed to accumulations of HoMS in larger liver vessels. The exploratory quantification experiments suggest the feasibility of MR dosimetry.

Animals↗

PubMed2005

The effect of B1 field inhomogeneity and the nonselective inversion profile on the kinetics of FAIR-based perfusion MRI.

Perfusion imaging with pulsed arterial spin labeling techniques, like flow-sensitive alternating inversion recovery (FAIR), may suffer from inflow of fresh, i.e., unlabeled, spins. Inflow of fresh spins is caused by the arrival of unlabeled spins in the image slice and can lead to underestimation of the perfusion if not taken into account. In this study it was shown that a decrease in B(1) field strength toward the edge of the transmit coil and the consequent reduction in the inversion efficiency leads to a narrowing of the arterial delivery function and a reduction in FAIR signal. Increasing the B(1) amplitude of the adiabatic inversion pulse from 2.3 to 5.7 times its minimum amplitude requirement resulted in an observed increase of 40 to 80% in the rat brain FAIR signal at inflow times longer than 0.65 s. For coils with limited dimensions and significant B(1) inhomogeneity over the perfusion labeling slab, the application of an excessively large B(1) amplitude in combination with adiabatic inversion is recommended to optimize the FAIR perfusion contrast.

Animals↗

PubMed2004

Experimental validation of hyperthermia SAR treatment planning using MR B1+ imaging.

In this paper the concept of using B1+ imaging as a means to validate SAR models for radiofrequency hyperthermia is presented. As in radiofrequency hyperthermia, in common clinical MR imaging which applies RF frequencies between 64 and 128 MHz, the RF field distribution inside a patient is largely determined by the dielectric distribution of the anatomy. Modern MR imaging techniques allow measurement of the RF magnetic field component B1+ making it possible to measure at high resolution the dielectric interaction of the RF field with the patient. Given these considerations, we propose to use MR imaging to verify the validity of our dielectric patient model used for SAR models of radiofrequency hyperthermia. The aim of this study was to investigate the feasibility of this concept by performing B1+ measurements and simulations on cylindrical split phantoms consisting of materials with dielectric properties similar to human tissue types. Important topics of investigation were the accuracy and sensitivity of B1+ measurements and the validity of the electric model of the MR body coil. The measurements were performed on a clinical 1.5 T MR scanner with its quadrature body coil operating at 64 MHz. It was shown that even small B1+ variations of 2 to 5% could be measured reliably in the phantom experiments. An electrical model of the transmit coil was implemented on our FDTD-based hyperthermia treatment planning platform and the RF field distributions were calculated assuming an idealized quadrature current distribution in the coil. A quantitatively good correlation between measurements and simulations was found for phantoms consisting of water and oil, while highly conductive phantoms show considerable deviations. However, assuming linear excitation for these conductive phantoms resulted in good correspondence. As an explanation it is suggested that the coil is being detuned due to the inductive nature of the conductive phantoms, breaking up the phase difference of pi/2 between the two quadrature modes. It is concluded that B1+ imaging is an accurate and sensitive method for obtaining quantitative information about the RF field in phantoms. The electrical model of the body coil is inadequate for highly conductive phantoms. It is expected that for experiments on human bodies the inductive coupling is also significant, demonstrating the need for a full resonant FDTD model of the transmit coil. This will be pursued in the near future.

Computer Simulation↗

PubMed2004

Adaptive subtraction as an aid in MR-guided placement of catheters and guidewires.

PURPOSE: To demonstrate the utility of mask subtraction optimization in magnetic resonance (MR)-guided placement of catheters and guidewires. MATERIALS AND METHODS: MR-guided positioning of magnetically prepared catheters and guidewires was done by dynamically imaging a single thick slab at two frames per second. Selective visualization of the prepared parts of the devices was achieved by the use of a conventional baseline subtraction technique and by the use of an adaptive subtraction technique. In the latter, the best reference image is automatically selected from a fixed or a sliding subset of hitherto acquired dynamic images. The efficacy of both approaches was compared by tracking experiments in a flow phantom and in the aortoiliac arteries of a pig. RESULTS: Baseline subtraction produced adequate visualization of paramagnetic markers in the absence of subject motion and for fixed scan conditions. The sensitivity to subject motion and interactive modification of the scan parameters was greatly reduced by using adaptive subtraction. Adaptive subtraction images, other than conventional subtraction images, appeared to be insensitive to slow periodic motion, e.g., respiratory motion, and were only transiently affected by gross subject motion and interactive alterations of the scan parameters. CONCLUSION: Adaptive subtraction is superior to baseline subtraction for guiding the manipulation of catheters and guidewires in the presence of gross and periodic subject motion and whenever scan parameters are modified in the course of a procedure.

Animals↗

PubMed2004

Suitability of 7 aortic stent-graft models for MRI-based surveillance.

PURPOSE: To evaluate the magnetic resonance imaging (MRI) characteristics of commercially available stent-grafts used for abdominal aortic aneurysm repair. METHODS: Seven endovascular grafts (AneuRx, Lifepath, Talent, Excluder, Zenith, Quantum LP, and Ancure) were suspended in a water bath containing gadolinium and scanned using a 1.5-T clinical MRI scanner. Two different scan techniques (T(1)-weighted spoiled gradient echo and spin echo) based upon a clinical MRI endograft surveillance protocol were used for each stent-graft. The scans were evaluated for susceptibility artifacts and radiofrequency (RF) shielding and caging artifacts. RESULTS: For most endografts, the lumen and structures surrounding the endograft were well visualized. However, the ferromagnetic properties of the Zenith and Lifepath devices resulted in large susceptibly artifacts that obliterated the endograft lumen as well as adjacent structures. All fully supported grafts showed some amount of signal loss from the graft lumen caused by RF caging. For the Ancure graft, evaluation around the attachment sites might be problematic. CONCLUSIONS: An MRI-based surveillance protocol appears to be a viable option for the AneuRx, Talent, Excluder, and Quantum LP stent-grafts.

Aortic Aneurysm, Abdominal↗

PubMed2003

Magnetic resonance imaging measurement of blood volume flow in peripheral arteries in healthy subjects.

OBJECTIVE: Peripheral arterial disease results in insufficient blood supply to the leg. Assessment of blood flow may provide information about severity of the disease. Magnetic resonance imaging (MRI) has potential use for simple, fast quantitative blood flow measurement. We investigated normal blood flow values for age and sex in the common femoral artery and popliteal artery in 50 healthy volunteers. In addition, we examined reproducibility and determinants of blood flow. METHODS: We performed cardiac-triggered phase-contrast quantitative flow measurements in the common femoral artery and popliteal artery, and MRI of the calves in 50 healthy volunteers (age, 26-80 years). Ten persons underwent MRI three times, to analyze reproducibility. RESULTS: All measurements were technically successful. Mean blood flow was 353 mL/min in the femoral artery and 61.9 mL/min in the popliteal artery. Coefficient of variation of femoral measurements was 16%, and of popliteal measurements was 19%. Femoral blood flow was significantly related to age and sex. Popliteal blood flow was significantly related to calf muscle volume, adjusted for age and sex. CONCLUSION: Normal values for blood flow to the legs are presented. The measurements have reasonable reproducibility. Blood flow to the legs depends on age, sex, and calf muscle volume.

Adult↗

PubMed2002

Improved lumen visualization in metallic vascular implants by reducing RF artifacts.

In this study, a method is proposed for MRI of the lumen of metallic vascular implants, like stents or vena cava filters. The method is based on the reduction of artifacts caused by flow, susceptibility, and RF eddy currents. Whereas both flow artifacts and susceptibility artifacts are well understood and documented, RF artifacts are not. Therefore, the present study comprises an in-depth theoretical explanation of the factors governing the severity of these RF artifacts. It is explained that the RF caging inside cage-like implants is caused by disturbances of the send and receive sensitivities due to coupling between the loops in the implant and the MR scanner's send and receive coils. A scaled excitation angle model describing the behavior of the signal intensity inside the implants as a function of the applied nominal excitation angle is introduced. This theoretical model was validated in phantom experiments. Reduced signal from within implants due to the caging problem could be restored by increasing the applied RF power in the excitation pulse, without exceeding the generally accepted SAR safety limits. The method was tested in vitro and in vivo in a pig model and allowed adequate depiction of the interior of a nitinol stent and that of a vena cava filter in contrast-enhanced MR angiograms. Magn Reson Med 47:171-180, 2002.

Alloys↗

PubMedDate not supplied

Time-of-flight MR angiography of carotid artery stenosis: does a flow void represent severe stenosis?

BACKGROUND AND PURPOSE: Time-of-flight (TOF) magnetic resonance angiography (MRA) is commonly used to visualize the carotid arteries; however, flow void artifacts can appear. Our purpose was to determine the frequency and diagnostic meaning of flow voids by using real patient data, as part of a larger study of MRA compared with the criterion standard, digital subtraction angiography (DSA). METHODS: In 1997-2000, 390 consecutive patients with sonographic findings suggestive of carotid artery stenosis were included in this study. All patients subsequently underwent three-dimensional (3D) TOF MRA and conventional DSA. The frequency of flow void artifacts on 3D TOF MRA images were compared with stenosis measurements on DSA images. RESULTS: We recorded 107 flow voids (16%) during 3D TOF MRA of 662 carotid arteries. DSA images were available for comparison in 102 cases. The median percentage of stenosis in this subgroup of flow voids on MRA images was 80%, compared with measurements on DSA images according to the North American Symptomatic Carotid Endarterectomy Trial (NASCET) criteria. Stenoses ranged from 36% to 100% (occlusion). Three flow voids (2.9%) were in the 0-49% range; 11 (10.8%), in the 50-69% range; and 86 (84.3%), in the 70-99% range. Two flow voids (2.0%) represented occlusions. The positive predictive value of a flow void artifact for the presence of severe (70-99%) stenosis was 84.3% (95% CI: 77.3%, 91.4%). CONCLUSION: Flow void artifacts represented severe stenosis in most of the arteries. According to our data, the assumption that flow voids on 3D TOF MRA images represent severe stenosis is justified.

Adult↗