Lambertus W. Bartels

Heating Around Intravascular Guidewires by Resonating RF Waves

We examined the unwanted radiofrequency (RF) heating of an endovascular guidewire frequently used in interventional magnetic resonance imaging (MRI). A Terumo guidewire was partly immersed in an oblong saline bath to simulate an endovascular intervention. The temperature rise of the guidewire tip during an FFE sequence [average specific absorption rate (SAR) = 3.9 W/kg] was measured with a Luxtron fluoroscopic fiber. Starting from 26°C, the guidewire tip reached temperatures up to 74°C after 30 seconds of scanning. Touching the guidewire may cause sudden heating at the point of contact, which in one instance caused a skin burn. The excessive heating of a linear conductor like the guidewire can only be explained by resonating RF waves. The capricious dependencies of this resonance phenomenon on environmental factors have severe consequences for predictability and safety guidelines. J. Magn. Reson. Imaging 2000;12:79–85.

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 scanners 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.

Simultaneous B 1+ homogenization and specific absorption rate hotspot suppression using a magnetic resonance phased array transmit coil

In high‐field MRI severe problems with respect to B  1+ uniformity and specific absorption rate (SAR) deposition pose a great challenge to whole‐body imaging. In this study the potential of a phased array transmit coil is investigated to simultaneously reduce B  1+ nonuniformity and SAR deposition. This was tested by performing electromagnetic simulations of a phased array TEM coil operating at 128 MHz loaded with two different homogeneous elliptical phantoms and four dielectric patient models. It was shown that the wave interference of a circularly polarized RF field with an ellipse and a pelvis produces largely identical B  1+ and electric field patterns. Especially for obese patients, this results in large B  1+ nonuniformity and global areas with elevated SAR deposition. It is demonstrated that a phased array transmit coil can reduce these phenomena. The technique was especially successful in suppressing SAR hotspots with a decrease up to 50%. The application of optimized settings for an ellipse to the patient models leads to comparable results as obtained with the patient‐specific optimizations. This suggests that generic phase/amplitude port settings are possible, requiring no preinformation about patient‐specific RF fields. Such a scheme would, due to its simultaneous B  1+ homogenization and extra SAR margin, have many benefits for whole‐body imaging at 3 T. Magn Reson Med 57:577–586, 2007.

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.

7 T body MRI: B1 shimming with simultaneous SAR reduction

The high frequency of the radiofrequency (RF) fields used in high field magnetic resonance imaging (MRI) results in electromagnetic field variations that can cause local regions to have a large specific absorption rate (SAR) and/or a low excitation. In this study, we evaluated the use of a B1 shimming technique which can simultaneously improve the B+1 homogeneity and reduce the SAR for whole body imaging at 7 T. Optimizations for four individual anatomies showed a reduction up to 74% of the peak SAR values with respect to a quadrature excitation and a simultaneous improvement of the B+1 homogeneity varying between 39 and 75% for different optimization parameters. The average SAR was reduced with approximately 50% for all optimizations. The optimized phase and amplitude settings from an elliptical phantom model were applied to four realistic human anatomy models to evaluate whether a generic application without prior knowledge of the detailed human anatomy is possible. This resulted in an average improvement of the B+1 homogeneity of 37% and an average reduction of the maximum and average SAR of 50 and 55%, respectively. It can be concluded that this generic method can be used as a simple method to improve the prospects of 7 T body imaging.

Toward endografting of the ascending aorta: Insight into dynamics using dynamic cine-CTA

PURPOSE To evaluate pulsatility and movement along the ascending thoracic aorta using dynamic electrocardiographically-gated 64-slice cine computed tomographic angiography (CTA). METHODS Diameter and area change and center of mass (COM) movement of the ascending thoracic aorta was determined per cardiac cycle in 15 patients at surgically relevant anatomical levels: (A) 5 mm distal to the coronary arteries, (B) 5 mm proximal to the innominate artery, and (C) halfway up the ascending aorta. Additionally, COM movement was determined 1 cm (level P) and 2 cm (level Q) distal from the origins of the innominate, left carotid, and left subclavian arteries. Eight gated datasets covering the cardiac cycle were used to reconstruct images at each level perpendicular to the aortic lumen. The distance between important anatomical landmarks was determined. RESULTS All levels showed significant cardiac cycle-induced diameter and area changes (p<0.001), with the largest pulsatility 5 mm distal to the coronary arteries. Mean maximum diameter changes were (A) 17.4%+/-4.8% (range 7.5%-27.5%), (B) 13.9%+/-3.5% (range 10.6%-25.0%), and (C) 12.9%+/-3.4% (8.3%-19.6%). Mean area changes were (A) 12.7%+/-5.5% (range 4.3%-21.8%), (B) 7.5%+/-2.0% (range 4.1%-11.0%), and (C) 5.6%+/-2.2% (range 1.9%-11.4%). Mean maximum COM movements were (A) 6.1+/-2.0 mm (range 2.7-9.0), (B) 2.3+/-1.1 mm (range 1.1-5.6), and (C) 3.6+/-1.5 mm (range 1.4-6.5). Mean COM movements of the innominate, left carotid, and left subclavian arteries, respectively, were (P) 1.9+/-0.7 mm (range 0.9-3.7), 2.4+/-0.6 mm (range 1.4-3.3), and 1.9+/-0.6 mm (range 0.8-2.8), and (Q) 1.8+/-0.7 mm (range 0.8-3.5), 1.8+/-0.6 mm (range 0.8-2.7), 1.9+/-0.6 mm (range 1.1-3.4). CONCLUSION The dynamics of the ascending thoracic aorta and the arch vessels are impressive, showing a wide range of 3-dimensional motions. Future ascending arch branched and fenestrated thoracic endograft designs must consider this active local environment, as it may have implications for durability, sealing, and ultimate clinical success.

MR-guided high-intensity focused ultrasound ablation of breast cancer with a dedicated breast platform.

Optimizing the treatment of breast cancer remains a major topic of interest. In current clinical practice, breast-conserving therapy is the standard of care for patients with localized breast cancer. Technological developments have fueled interest in less invasive breast cancer treatment. Magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU) is a completely noninvasive ablation technique. Focused beams of ultrasound are used for ablation of the target lesion without disrupting the skin and subcutaneous tissues in the beam path. MRI is an excellent imaging method for tumor targeting, treatment monitoring, and evaluation of treatment results. The combination of HIFU and MR imaging offers an opportunity for image-guided ablation of breast cancer. Previous studies of MR-HIFU in breast cancer patients reported a limited efficacy, which hampered the clinical translation of this technique. These prior studies were performed without an MR-HIFU system specifically developed for breast cancer treatment. In this article, a novel and dedicated MR-HIFU breast platform is presented. This system has been designed for safe and effective MR-HIFU ablation of breast cancer. Furthermore, both clinical and technical challenges are discussed, which have to be solved before MR-HIFU ablation of breast cancer can be implemented in routine clinical practice.

Quantitative diffusion weighted imaging for differentiation of benign and malignant breast lesions: The influence of the choice of b‐values

To evaluate the influence of the choice of different combinations of b‐values on the ADC and on the diagnostic performance of quantitative diffusion weighted imaging (DWI) in breast lesions.

Use of dynamic computed tomography to evaluate pre- and postoperative aortic changes in AAA patients undergoing endovascular aneurysm repair

Purpose: To utilize dynamic computed tomographic angiography (CTA) on pre- and postoperative endovascular aneurysm repair (EVAR) patients to characterize cardiac-induced aortic motion within the aneurysm neck, an essential EVAR sealing zone. Methods: Electrocardiographically-gated CTA datasets were acquired utilizing a 64-slice Philips Brilliance CT scanner on 15 consecutive pre- and postoperative AAA patients. Axial pulsatility measurements were taken at 2 clinically relevant levels within the aneurysm neck: 2 cm above the highest renal artery and 1 cm below the lowest renal artery. Changes in aortic area and diameter were determined. Results: Significant aortic pulsatility exists within the aneurysm neck during the cardiac cycle. Preoperative aortic area increased significantly, with a maximum increase of up to 12.5%. The presence of an endograft did not affect aortic pulsatility (p = NS). Postoperative area also changed significantly during a heart cycle, with a maximum increase of up to 14.5%. Diameter measurements demonstrated an identical pattern, with significant pre- and postoperative intracardiac pulsatility within and above the aneurysm neck (p<0.05). An increase in maximum diameter change up to 15% was evident. Conclusion: Patients undergoing EVAR experience aortic diameter changes within and above the aneurysm neck. The presence of an endograft does not abrogate this response to intracardiac pressure changes. Static CT imaging may not adequately identify patients with large aortic pulsatility, potentially resulting in endograft undersizing, stent-graft migration, intermittent type I endoleaks, and poor patient outcomes. The current standard regime of 10% to 15% oversizing based on static CT may be inadequate for some patients.

Endovascular interventional magnetic resonance imaging

Minimally invasive interventional radiological procedures, such as balloon angioplasty, stent placement or coiling of aneurysms, play an increasingly important role in the treatment of patients suffering from vascular disease. The non-destructive nature of magnetic resonance imaging (MRI), its ability to combine the acquisition of high quality anatomical images and functional information, such as blood flow velocities, perfusion and diffusion, together with its inherent three dimensionality and tomographic imaging capacities, have been advocated as advantages of using the MRI technique for guidance of endovascular radiological interventions. Within this light, endovascular interventional MRI has emerged as an interesting and promising new branch of interventional radiology. In this review article, the authors will give an overview of the most important issues related to this field. In this context, we will focus on the prerequisites for endovascular interventional MRI to come to maturity. In particular, the various approaches for device tracking that were proposed will be discussed and categorized. Furthermore, dedicated MRI systems, safety and compatibility issues and promising applications that could become clinical practice in the future will be discussed.