Diffusion-weighted echo planar MR imaging of the neck at 3 T using integrated shimming: comparison of MR sequence techniques for reducing artifacts caused by magnetic-field inhomogeneities.

OBJECTIVE: Our objective was to compare available techniques reducing artifacts in echo planar imaging (EPI)-based diffusion-weighed magnetic resonance imaging MRI (DWI) of the neck at 3 Tesla caused by B0-field inhomogeneities. MATERIALS AND METHODS: A cylindrical fat-water phantom was equipped with a Maxwell coil allowing for additional linear B0-field variations in z-direction. The effect of increasing strength of this superimposed gradient on image quality was observed using a standard single-shot EPI-based DWI sequence (sEPI), a zoomed single-shot EPI sequence (zEPI), a readout-segmented EPI sequence (rsEPI), and an sEPI sequence with integrated dynamic shimming (intEPI) on a 3-Tesla system. Additionally, ten volunteers were examined over the neck region using these techniques. Image quality was assessed by two radiologists. Scan durations were recorded. RESULTS: With increasing strength of the external gradient, marked distortions, signal loss, and failure of fat suppression were observed using sEPI, zEPI, and rsEPI. These artifacts were markedly reduced using intEPI. Significantly better in vivo image quality was also observed using intEPI compared with the other techniques. Scan time of intEPI was similar to sEPI and zEPI and shorter than rsEPI. CONCLUSION: The use of integrated 2D shim and frequency adjustment for EPI-based DWI results in a significant improvement in image quality of the head/neck region at 3 Tesla. Combining integrated shimming with rsEPI or zEPI can be expected to provide additional improvements.

Resolution adapted finite element modeling of radio frequency interactions on conductive resonant structures in MRI.

Prediction of interactions between the radiofrequency electromagnetic field in magnetic resonance scanners and electrically conductive material surrounded by tissue plays an increasing role for magnetic resonance safety. Testing of conductive implants or instruments is usually performed by standardized experimental setups and temperature measurements at distinct geometrical points, which cannot always reflect worst-case situations. A finite element method based on Matlab (The Mathworks, Natick, MA) and the finite element method program Comsol Multiphysics (Stockholm, Sweden) with a spatially highly variable mesh size solving Maxwell's full-wave equations was applied for a comprehensive simulation of the complete geometrical arrangement of typical birdcage radiofrequency coils loaded with small conductive structures in a homogenous medium. Conductive implants like rods of variable length and closed and open ring structures, partly exhibiting electromagnetic resonance behavior, were modeled and evaluated regarding the distribution of the B(1)- and E-field, induced currents and specific absorption rates. Numerical simulations corresponded well with experiments using a spin-echo sequence for visualization of marked B(1)-field inhomogeneities. Even resonance effects in conductive rods and open rings with suitable geometry were depicted accurately. The proposed method has high potential for complementation or even replacement of common experimental magnetic resonance compatibility measurements.

RF tissue-heating near metallic implants during magnetic resonance examinations: an approach in the ac limit.

PURPOSE: State of the art to access radiofrequency (RF) heating near implants is computer modeling of the devices and solving Maxwell's equations for the specific setup. For a set of input parameters, a fixed result is obtained. This work presents a theoretical approach in the alternating current (ac) limit, which can potentially render closed formulas for the basic behavior of tissue heating near metallic structures. Dedicated experiments were performed to support the theory. METHODS: For the ac calculations, the implant was modeled as an RLC parallel circuit, with L being the secondary of a transformer and the RF transmission coil being its primary. Parameters influencing coupling, power matching, and specific absorption rate (SAR) were determined and formula relations were established. Experiments on a copper ring with a radial gap as capacitor for inductive coupling (at 1.5 T) and on needles for capacitive coupling (at 3 T) were carried out. The temperature rise in the embedding dielectric was observed as a function of its specific resistance using an infrared (IR) camera. RESULTS: Closed formulas containing the parameters of the setup were obtained for the frequency dependence of the transmitted power at fixed load resistance, for the calculation of the resistance for optimum power transfer, and for the calculation of the transmitted power in dependence of the load resistance. Good qualitative agreement was found between the course of the experimentally obtained heating curves and the theoretically determined power curves. Power matching revealed as critical parameter especially if the sample was resonant close to the Larmor frequency. CONCLUSIONS: The presented ac approach to RF heating near an implant, which mimics specific values for R, L, and C, allows for closed formulas to estimate the potential of RF energy transfer. A first reference point for worst-case determination in MR testing procedures can be obtained. Numerical approaches, necessary to determine spatially resolved heating maps, can be supported.

Magnetic resonance perfusion imaging without contrast media.

PURPOSE: Principles of magnetic resonance imaging techniques providing perfusion-related contrast weighting without administration of contrast media are reported and analysed systematically. Especially common approaches to arterial spin labelling (ASL) perfusion imaging allowing quantitative assessment of specific perfusion rates are described in detail. The potential of ASL for perfusion imaging was tested in several types of tissue. METHODS: After a systematic comparison of technical aspects of continuous and pulsed ASL techniques the standard kinetic model and tissue properties of influence to quantitative measurements of perfusion are reported. For the applications demonstrated in this paper a flow-sensitive alternating inversion recovery (FAIR) ASL perfusion preparation approach followed by true fast imaging with steady precession (true FISP) data recording was developed and implemented on whole-body scanners operating at 0.2, 1.5 and 3 T for quantitative perfusion measurement in various types of tissue. RESULTS: ASL imaging provides a non-invasive tool for assessment of tissue perfusion rates in vivo. Images recorded from kidney, lung, brain, salivary gland and thyroid gland provide a spatial resolution of a few millimetres and sufficient signal to noise ratio in perfusion maps after 2-5 min of examination time. CONCLUSIONS: Newly developed ASL techniques provide especially high image quality and quantitative perfusion maps in tissues with relatively high perfusion rates (as also present in many tumours). Averaging of acquisitions and image subtraction procedures are mandatory, leading to the necessity of synchronization of data recording to breathing in abdominal and thoracic organs.

Biopsy needle tips with markers--MR compatible needles for high-precision needle tip positioning.

Needle tip visualization is of high importance in magnetic resonance imaging (MRI) guided interventional procedures, for example for taking biopsies from suspicious lesions in the liver or kidney. The exact position of the needle tip is often obscured by image artifacts arising from the magnetic properties of the needle. The authors investigated two special biopsy needle tip designs using diamagnetic coatings. For common interventional MR sequences, the needle tip can be identified in the MR image by several equidistant dark spots arranged along a straight line. A dotted instead of a solid line allows for an improved control of the movement of the needle, not only if the needle is tilted toward the imaging plane, but also if the needle leaves an empty canal with signal extinction, which cannot be distinguished from the needle material itself. With the proposed design the position of the needle tip can be estimated with a precision of approximately 1 mm using conventional FLASH, FISP, and TSE sequences, as used for interventional MR. Furthermore, the size of the biopsy probe can be estimated from the artifact. In using needles with a properly designed tip coating, taking biopsies under MR control is beginning to be greatly simplified. The approach to design artifacts using diamagnetic material in combination with paramagnetic material paves the way toward new instruments and implants, suitably tailored to the needs of the interventional radiologist.

Relaxivity of Gadopentetate Dimeglumine (Magnevist), Gadobutrol (Gadovist), and Gadobenate Dimeglumine (MultiHance) in human blood plasma at 0.2, 1.5, and 3 Tesla.

OBJECTIVES: We sought to determine the relaxivity and accurate relaxation rates of Gd-DTPA, Gd-BT-DO3A, and Gd-BOPTA at 0.2, 1.5, and 3 T in human blood plasma. MATERIALS AND METHODS: Contrast media concentrations between 0.01 and 16 mM in human plasma were used for relaxation measurements. The R1 and R2 relaxation rates and r1 and r2 relaxivities were determined. RESULTS: Gd-BOPTA produced the highest relaxation rates and relaxivities at all field strengths. The r1 and r2 values for Gd-BOPTA were 107-131% and 91-244% higher than for Gd-DTPA, respectively, and 72-98% and 82-166% higher than for Gd-BT-DO3A. Higher field strengths resulted in lower values of R1, R2, and r1 for all contrast agents tested and of r2 for Gd-DTPA and Gd-BT-DO3A. A linear dependence of R1 and R2 on concentration was found for Gd-DTPA and Gd-BT-DO3A and a nonlinear dependence for Gd-BOPTA for concentrations larger than 1 mM. The r1 and r2 relaxivity of Gd-BOPTA increased with decreasing concentration. CONCLUSIONS: Gd-BOPTA demonstrates the highest longitudinal r1 at all field strengths, which is ascribable to weak protein interaction. The R2/R1 ratio increases at higher field strength only for Gd-BOPTA, hence very short echo times are required for Gd-BOPTA to benefit from the higher longitudinal relaxivity.

Effects on MRI due to altered rf polarization near conductive implants or instruments.

In magnetic resonance imaging near metal parts variations in radio frequency (rf)-amplitude and of receive sensitivity must be considered. For loop structures, e.g., vascular stents, B1 produces rf eddy currents in accordance to Faraday's law; the B1-related electrical rf field E1 injects directly to elongated structures (e.g., wires). Locally, the rf magnetic field Bl,ind (induced B1) is superimposed onto the rf field from the transmitter coil, which near the metal can dominate spin excitation. Geometry and arrangement of the parts determine the polarization of B(1,ind). Components parallel to B0 are of special interest. A copper sheet (100 mm x 15 mm, 3 mm thick) and a 27 cm long copper wire were examined in a water phantom using the spin-echo (SE) technique. In addition to rf-amplitude amplification, rf-phase shift due to z components of B(1,ind) could be detected near the metallic objects. Periodic rf-amplitude instabilities had an amplified effect for phase-shifted regions. Phase-encoding artifacts occurred as distinct ghosts (TR=200 ms) or band-like smearing (TR=201 ms) from affected spin ensembles. SE phase imaging can potentially be used in interventional magnetic resonance imaging for background-free localization of metallic markers.

Magnetic resonance-guided percutaneous radiofrequency ablation of renal cell carcinomas: a pilot clinical study.

OBJECTIVE: The objective of this study was to assess the feasibility and efficacy of magnetic resonance imaging-(MRI) guided percutaneous radiofrequency (RF) ablation of renal cell carcinomas (RCC). SUBJECTS AND METHODS: Twelve patients with RCC (63 to 82 years old) were treated with RF ablation in an interventional 0.2-Tesla open MR unit. Tumor sizes varied from 1.6 cm to 3.9 cm in maximum diameter (tumor volumes 1.9 cm3 to 28.7 cm3). RF procedures were entirely performed in the MR suite. For positioning of the MR-compatible RF-electrode, near real-time MR fluoroscopy by means of rapid gradient echo sequences (acquisition time approximately 2 seconds) was used. Monitoring of ablation was obtained by intermittent imaging with T1- and T2-weighted spin echo sequences. RESULTS: Accurate placement of the RF electrodes was possible in all cases using near real-time MR fluoroscopy. Eleven of 12 patients were successfully treated within 1 single session; 1 patient had to be retreated for tumor relapse at 13 months follow up. Mean number of electrode repositionings under MR guidance during 1 session was 1.7; ablation time ranged between 12 and 28 minutes. Mean duration of 1 treatment session was 5 hours. Coagulation volumes ranged from 7.3 cm3 up to 30.2 cm3. All patients now appear to be disease-free with a mean follow up of 10.3 months (range, 3-23 months). CONCLUSION: MRI-guided RF ablation of RCC in an interventional MR unit is safe and feasible. Fast MR imaging is a convenient method for rapid positioning of MR-compatible RF electrodes. MR monitoring of ablation procedure with T2-weighted imaging allows for immediate assessment of coagulation extent.

Compensation of magnetic field distortions from paramagnetic instruments by added diamagnetic material: measurements and numerical simulations.

In minimally invasive procedures guided by magnetic resonance (MR) imaging instruments usually are made of titanium or titanium alloys (e.g., nitinol), because other more MR-compatible materials often cannot provide sufficient mechanical properties. Artifacts depending on susceptibility arise in MR images due to incorrect spatial encoding and intravoxel dephasing and thereby hamper the surgeon's view onto the region of interest. To overcome the artifact problem, compensation of the paramagnetic properties by diamagnetic coating or filling of the instruments has been proposed in the literature. We used a numerical modeling procedure to estimate the effect of compensation. Modeling of the perturbation of the static magnetic field close to the instruments reflects the underlying problem and is much faster and cost efficient than manufacturing prototypes and measuring artifact behavior of these prototypes in the MR scanner. A numerical model based on the decomposition of the susceptibility distribution in elementary dipoles was developed by us. The program code was written object oriented to allow for both maximum computational speed and minimum random access memory. We used System International units throughout the modeling for the magnetic field, allowing absolute quantification of the magnetic field disturbance. The field outside a simulated needlelike instrument, modeled by a paramagnetic cylinder (out of titan, chi =181.1) of length 8.0 mm and of diameter 1.0 mm, coated with a diamagnetic layer (out of bismuth, chi=-165.0) of thickness 0, 0.1, 0.2, 0.3, and 0.4 mm, was found to be best compensated if the cross-sectional area of the cylinder, multiplied by the absolute susceptibility value of the cylinder material, is equal to the cross-sectional area of the coating, multiplied by the absolute susceptibility value of the coating material. At the extremity of the coated cylinder an uncompensated field distortion was found to remain. We studied various tip shapes and geometries using our computational model: Suitable diamagnetic coating or filling of paramagnetic instruments clearly reduced tip artifacts and diminished the dependency of artifact size on orientation of the instrument with respect to B0 in the numerical studies. We verified the results of the simulations by measuring coated and uncoated titanium wires in a 1.5 T MR scanner.

rf enhancement and shielding in MRI caused by conductive implants: dependence on electrical parameters for a tube model.

Radio frequency (rf) eddy-currents induced in implants made of conductive material might cause significant image artifacts in magnetic resonance imaging (MRI) such as shielding of the lumen of vascular stents. rf alteration near metal parts was assessed theoretically in the approximation of alternating current electrodynamics: The implant was modeled as tube with diameter d(o), resistance R, and reactance Y, constituting the secondary winding of a transformer. The transmitter coil of the scanner acted as primary winding and generated the linearly polarized rf field B1,app. Tube axis was assumed parallel to B1,app. The results of the calculations were as follows: Ninety percent of the applied rf-field amplitude is reached in the lumen at a ratio chi=R/Y approximately 2. A rapid drop occurs with the reduction of chi, whereas a further increase of chi causes only a small effect. With chi approximately 1/d(o)(Y approximately d2o,R approximately d(o)), conditions for rf alteration clearly depend on the diameter of the tube. Inside tubes with smaller diameter, rf shielding is less pronounced. rf alteration increases in good approximation with the square root of the strength of the static field B0. The following experiments were carried out: Tubes of similar diameter (d(o) approximately 8 mm) made of material of different conductivity (Cu, Nitinol, carbon fiber reinforced plastic with three different fiber structures) were examined at B0=0.2 and 1.5 T in water phantoms. Tube axis was aligned perpendicular to B0 and spin-echo technique was applied. Local rf enhancement near the outer surface of the metal tubes was detected applying manual reduction of the transmitter amplitude. Shielding inside a carbon fiber tube with d(o) approximately 8 mm and inside a smaller tube with d(o)=3.3 mm was compared. Both tubes showed the same wall structure and thickness (d(w)=0.4 mm). All measurements confirmed the theoretical results. Consequences for the construction of vascular stents are discussed, as well as problems with image artifacts due to rf enhancement near solid conductive implants.

RF artifacts caused by metallic implants or instruments which get more prominent at 3 T: an in vitro study.

Metallic devices with high electrical conductivity inside or adjacent to the body might lead to marked alterations of the RF amplitude B1 in the tissue under investigation, especially at increased RF frequency, and if specific conditions for electromagnetic resonance are fulfilled. RF-metal interaction effects were investigated systematically at B0=0.2, 1.5 and 3 T analyzing correlated image artifacts for copper wires (d=1 mm, L=53 and 27 cm), and for following instruments and implants made of titanium or nitinol: biopsy needles, hip prostheses, vascular stents and aneurysm clips. The samples were examined in Gd-DTPA-doped 140 mM NaCl solution using spin-echo (SE) sequences with high readout bandwidth. Automatic transmitter adjustment V(T,auto) and manually reduced transmitter voltage VT were applied in order to detect B1 enhancement. At 0.2 T, beyond the shielding of the luminal region of the stents, no RF effects were observed. At 1.5 T, the copper wires caused distinct RF artifacts. At 3 T, RF artifacts also appeared for the hip prostheses and the biopsy needles. Stents with pronounced luminal shielding at lower field strength revealed marked B1 enhancement close to their outer surface.

Radio frequency versus susceptibility effects of small conductive implants--a systematic MRI study on aneurysm clips at 1.5 and 3 T.

PURPOSE: Metallic implants cause enlarged artifacts in magnetic resonance (MR) images at higher magnetic fields, B0, due to their magnetic susceptibility. Interactions of conductive material with radio frequency (RF) pulses also change for higher field strengths, B0, due to the frequency dependence of resonance conditions. Systematic measurements on commercial aneurysm clips and simplified copper models were performed in order to investigate both phenomena at 1.5 and 3 T. MATERIALS AND METHODS: Six different commercial aneurysm clips made of titanium, straight copper wires and bent copper models were examined in Gd-DTPA-doped water. RF-related effects were measured by adapted 2D and 3D spin-echo sequences. For reliable differentiation from susceptibility-related effects, variable transmitter voltages were applied. In addition, RF-induced heating was controlled by an infrared (IR) camera. RESULTS: At 3 T, a significant RF-induced electric response could be demonstrated for the copper samples and more moderate for one of the commercial clips, dependent on the geometrical structure determining possible resonant RF coupling. Related RF effects could be distinguished from susceptibility artifacts: a signal enhancement at reduced transmitter voltages indicated locally amplified B1-field amplitudes. No significant heating effect could be measured by IR measurements. CONCLUSION: MR imaging was used to analyze possible RF-induced effects. At 3 T, resonant RF coupling even of small metallic implants has to be considered carefully. Despite a local enhancement of the RF amplitude, no significant RF-induced heating inside the surrounding fluid was found. A direct thermal endangering of patients seems to be unlikely, but extremely high B1-field amplitudes might occur adjacent to the metallic surface with potential nonthermal affection of tissue.

Bifunctional chimeric SuperCD suicide gene -YCD: YUPRT fusion is highly effective in a rat hepatoma model.

AIM: To investigate the effects of catalytically superior gene-directed enzyme prodrug therapy systems on a rat hepatoma model. METHODS: To increase hepatoma cell chemosensitivity for the prodrug 5-fluorocytosine (5-FC), we generated a chimeric bifunctional SuperCD suicide gene, a fusion of the yeast cytosine deaminase (YCD) and the yeast uracil phosphoribosyltransferase (YUPRT) gene. RESULTS: In vitro stably transduced Morris rat hepatoma cells (MH) expressing the bifunctional SuperCD suicide gene (MH SuperCD) showed a clearly marked enhancement in cell killing when incubated with 5-FC as compared with MH cells stably expressing YCD solely (MH YCD) or the cytosine deaminase gene of bacterial origin (MH BCD), respectively. In vivo, MH SuperCD tumors implanted both subcutaneously as well as orthotopically into the livers of syngeneic ACI rats demonstrated significant tumor regressions (P<0.01) under both high dose as well as low dose systemic 5-FC application, whereas MH tumors without transgene expression (MH naive) showed rapid progression. For the first time, an order of in vivo suicide gene effectiveness (SuperCD>> YCD>>BCD>>>negative control) was defined as a result of a direct in vivo comparison of all three suicide genes. CONCLUSION: Bifunctional SuperCD suicide gene expression is highly effective in a rat hepatoma model, thereby significantly improving both the therapeutic index and the efficacy of hepatocellular carcinoma killing by fluorocytosine.

Technical Note: MR-visualization of interventional devices using transient field alterations and balanced steady-state free precession imaging.

PURPOSE: In interventional magnetic resonance imaging, instruments can be equipped with conducting wires for visualization by current application. The potential of sequence triggered application of transient direct currents in balanced steady-state free precession (bSSFP) imaging is demonstrated. METHODS: A conductor and a modified catheter were examined in water phantoms and in an ex vivo porcine liver. The current was switched by a trigger pulse in the bSSFP sequence in an interval between radiofrequency pulse and signal acquisition. Magnitude and phase images were recorded. Regions with transient field alterations were evaluated by a postprocessing algorithm. A phase mask was computed and overlaid with the magnitude image. RESULTS: Transient field alterations caused continuous phase shifts, which were separated by the postprocessing algorithm from phase jumps due to persistent field alterations. The overlaid images revealed the position of the conductor. The modified catheter generated visible phase offset in all orientations toward the static magnetic field and could be unambiguously localized in the ex vivo porcine liver. CONCLUSIONS: The application of a sequence triggered, direct current in combination with phase imaging allows conspicuous localization of interventional devices with a bSSFP sequence.

Numerical modeling of needle tip artifacts in MR gradient echo imaging.

Exact determination of needle tip position is obsolete for interventional procedures under control of magnetic resonance imaging (MRI). Exact needle tip navigation is complicated by the paramagnetism of microsurgical instruments: Local magnetic field inhomogeneities are induced resulting in position encoding artifacts and in signal voids in the surrounding of instruments and especially near their tips. The artifacts generated by the susceptibility of the material are not only dependent on the material properties themselves and on the applied MRI sequences and parameters, but also on the geometric shape of the instruments and on the orientation to the static magnetic field in the MR unit. A numerical model based on superposition of induced elementary dipole fields was developed for studying the field distortions near paramagnetic needle tips. The model was validated by comparison with experimental data using field mapping MRI techniques. Comparison between experimental data and numerical simulations revealed good correspondence for the induced field inhomogeneities. Further systematic numerical studies of the field distribution were performed for variable types of concentric and asymmetric tip shapes, for different ratios between tip length and needle diameter, and for different orientations of the needle axis in the external static magnetic field. Based on the computed local inhomogeneities of the magnetic field in the surroundings of the needle tips, signal voids in usual gradient echo images were simulated for a prediction of the artifacts. The practically relevant spatial relation between those artifacts and the hidden tip of the needle was calculated for the different tip shapes and orientations in the external field. As needle tip determination is crucial in interventional procedures, e.g., in taking biopsies, the present model can help to instruct the physician prior to surgical interventions in better estimating the needle tip position for different orientations and needle tip shapes as they appear in interventional procedures. As manufacturing prototypes with subsequent measurements of artifacts in MRI are a costly procedure the presented model may also help to optimize shapes of needle tips and of other parts of MR-compatible instruments and implants with low expense prior to production if some shape parameters can be chosen freely.

Inductively coupled rf coils for examinations of small animals and objects in standard whole-body MR scanners.

Inductively coupled solenoid coils fitting to objects in the size of mice or rats were developed to adapt modem whole-body MR scanners featuring sufficient gradient strength for animal examinations with high spatial resolution. Homogenous receiver characteristics is achievable over almost the whole inner region of the solenoid coils. The SNR can be increased by a factor 2 to 6 with the adapting coils for examinations using the head coil as connected receiver. Standard sequences on clinical 1.5 T scanners can be applied with adapted transmitter voltages. For example, a SNR value of about 30 is achievable in a mouse liver after 10 minutes measuring time using a 2-D spin echo imaging sequence and a size of 0.3 x 0.3 x 0.8 mm3 for the picture elements.

Sodium 3-D MRI of the human torso using a volume coil.

Sodium MR imaging is considered to provide clinically important information about the human body that is not achievable by hydrogen-based approaches. However, due to the low natural abundance in biological tissues, sodium signals usually lead to low spatial resolution, low SNR, and long acquisition times compared to conventional 1H imaging, even using well-adapted surface coils. For our study, a volume coil was designed with nearly homogeneous excitation/receive characteristics and a suitable geometry fitting the human torso. A sufficient penetration throughout the entire thorax, abdomen, or pelvis is provided allowing for sodium imaging of the kidneys, the liver with gall bladder, or the myocardium. All measurements were performed on a 1.5 T whole body scanner using a spoiled 3-D gradient echo sequence. Imaging parameters TE, TR, and readout bandwidth were optimized for sensitive recording of the sodium component with slow transverse relaxation. Nonselective RF excitation pulses with a duration of 2.5 ms and rectangular shape were applied to avoid SAR problems. Narrow receiver bandwidth and excitation near the Ernst angle provided clinically practicable examinations with measuring times of less than 15 min at a spatial resolution of 8 x 8 x 8 mm3. Under these conditions, SNR of 11 for the kidneys and vertebral disks, 9 for the spinal canal, and 6 for the liver was achieved. A special 3-D spin echo sequence was used to determine T2, times which resulted to 15.3 +/- 1.1 ms for liver, 27.7 +/- 7.2 ms for kidneys, and 24.0 +/- 4.7 ms for the content of the spinal canal.

[Artifacts in MRT caused by instruments and implants]. / Artefakte in der MRT durch Instrumente und Implantate.

Metallic instruments and implants can cause severe image artifacts in magnetic resonance imaging (MRI). Besides the properties of the materials and the geometrical arrangement of the devices, the applied MRI sequence type and its parameters (echo time, voxel size, read-out bandwidth, orientations of encoding directions, etc.) play also an important role. These interactions are presented in a systematic survey. A detailed description of the basic physical mechanisms underlying the generation of artifacts is also provided.

Magnetic resonance visualization of conductive structures by sequence-triggered direct currents and spin-echo phase imaging.

PURPOSE: Instrument visualization in interventional magnetic resonance imaging (MRI) is commonly performed via susceptibility artifacts. Unfortunately, this approach suffers from limited conspicuity in inhomogeneous tissue and disturbed spatial encoding. Also, susceptibility artifacts are controllable only by sequence parameters. This work presents the basics of a new visualization method overcoming such problems by applying sequence-triggered direct current (DC) pulses in spin-echo (SE) imaging. SE phase images allow for background free current path localization. METHODS: Application of a sequence-triggered DC pulse in SE imaging, e.g., during a time period between radiofrequency excitation and refocusing, results in transient field inhomogeneities. Dependent on the additional z-magnetic field from the DC, a phase offset results despite the refocusing pulse. False spatial encoding is avoided by DC application during periods when read-out or slice-encoding gradients are inactive. A water phantom containing a brass conductor (water equivalent susceptibility) and a titanium needle (serving as susceptibility source) was used to demonstrate the feasibility. Artifact dependence on current strength and orientation was examined. RESULTS: Without DC, the brass conductor was only visible due to its water displacement. The titanium needle showed typical susceptibility artifacts. Applying triggered DC pulses, the phase offset of spins near the conductor appeared. Because SE phase images are homogenous also in regions of persistent field inhomogeneities, the position of the conductor could be determined with high reliability. Artifact characteristic could be easily controlled by amperage leaving sequence parameters unchanged. For an angle of 30° between current and static field visualization was still possible. CONCLUSIONS: SE phase images display the position of a conductor carrying pulsed DC free from artifacts caused by persistent field inhomogeneities. Magnitude and phase images are acquired simultaneously under the same conditions without the use of extra measurement time. The presented technique offers many advantages for precise instrument localization in interventional MRI.

Image-guided radiofrequency ablation of hepatocellular carcinoma (HCC): is MR guidance more effective than CT guidance?

OBJECTIVES: The purpose of the study was to retrospectively compare technique effectiveness of computed tomography (CT)-guided versus magnetic resonance (MR)-guided radiofrequency (RF) ablation of hepatocellular carcinoma (HCC). MATERIALS AND METHODS: In 35 consecutive patients 53 CT-guided (n=29) or MR-guided (n=24) ablation procedures were performed in the treatment of 56 (CT: 29; MR: 27) HCC. The entire ablation procedure was performed at a multislice CT-scanner or an interventional 0.2-Tesla MR-scanner. Assessment of treatment response was based on dynamic MR imaging at 1.5Tesla. The mean follow-up was 22.9 months. Primary technique effectiveness was assessed 4 months after ablation therapy. Secondary technique effectiveness was assessed 4 months after a facultative second ablation procedure. Primary and secondary technique effectiveness of CT-guided and MR-guided RF ablation was compared by using Chi-Square (likelihood ratio) test. RESULTS: Primary technique effectiveness after a single session was achieved in 26/27 (96.3%) HCC after MR-guided RF ablation and 23/29 (79.3%) HCC after CT-guided RF ablation (Chi-Square: p=0.04). Secondary technique effectiveness was achieved in 26/27 (96.3%) HCC after MR-guided RF ablation and in 26/29 (89.7%) HCC after CT-guided RF ablation (Chi-Square: p=0.32). A local tumor progression was detected in 8/52 (15.4%) tumors after initial technique effectiveness. Major complications were detected after 3/53 (5.7%) ablation procedures. CONCLUSIONS: CT-guided and MR-guided RF ablations are locally effective therapies in the treatment of HCC. Due to a higher rate of primary technique effectiveness MR-guided RF ablation may reduce the number of required sessions for complete tumor treatment.