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Vertical hot topics_broc

  1. 1. MRI Hot TopicsVertical and Horizontal Fields for MRI s medical
  2. 2. Vertical and Horizontal Fields for MRI: Does Direction Matter?Mathias Blasche, MA, and Brian M. Dale, PhDVertical-Field Experience: can be engineered to site with little modification, asSuccesses and Challenges the field strength moves beyond 0.5T, issues maySiemens has a long history of exploring low-field and often arise.vertical-magnet technology. The MAGNETOM P8 dates The fact that many low-field users desire to migrate toback to 1991. In 1993, Siemens introduced the first high-field must be considered. In fact, more than 75%0.2T C-shaped magnet, the MAGNETOM Open. In 2000, of the new MR systems purchased today, worldwideSiemens continued to explore vertical-field magnets from all vendors, are horizontal-field magnets withand this led to the development of the MAGNETOM 1.5T or higher field strength. These customers wantRhapsody, the first 1.0T vertical-field system. It consists of to increase throughput and productivity, while stilla two-pole magnetic cryostat supported by off-set pillars. addressing the need to image claustrophobic andSiemens has always maintained a leadership role in obese patients. But when the need arises for highestvertical-field magnet design. Other manufacturers have image quality, highest throughput and the full rangeresponded over the years by introducing systems from of advanced applications, there is no alternative to0.2T all the way to 1.0T, continually promising to horizontal-field systems. Recent developments haveintroduce and deliver an equivalent to the MAGNETOM shown the benefits of high-field magnets combinedproducts. Today, the MAGNETOM Concerto offers the with multiple channel RF systems and multiplebest performance available at 0.2T and the MAGNETOM element coils. All major MR vendors are investingC! is a leading competitor at 0.35T. The various open huge development efforts into this field, on theirMAGNETOM vertical-field systems have enabled horizontal-field systems.outpatient imaging centers to offer services to obese The highlight of this development was the introductionand claustrophobic patients. of Tim (Total imaging matrix) technology at the RSNALow- and mid-field systems with vertical field orientation 2003. Tim offers a boost in image quality and allowsare a perfect solution for the cost-sensitive market due integrated Parallel Acquisition Techniques (iPAT) in allto the lower system price and the lower operational directions with high PAT factors, resulting in thecosts. Especially C-shaped open magnets offer high highest productivity. Tim has increased the potentialpatient comfort and improvements of gradient for advanced applications and enables whole-bodyperformance. RF systems and coils make it possible to imaging with highest SNR and spatial resolution.achieve diagnostic image quality for a wide range of This was only possible in local examinations before.applications with reasonable examination times. A high-field open MRI system should therefore combineHowever, when trying to combine the openness of a 1.5T and the previously mentioned advantages ofvertical field magnet with higher-field strength, this Tim technology with the possibility to offer servicestechnology reaches a limit: to obese and claustrophobic patients. The system–Vertical field systems with field strengths beyond 0.5T requirements of these customers can only be met are prohibitively expensive. Their price is in the range with a horizontal-field system. of a horizontal-field system with twice their field The Physics of Field Orientation: Why Doesn’t strength, without realizing the benefits of higher-field Everyone Own a Vertical-Field System? strength’s higher SNR, higher resolution and faster As an experienced vertical-field developer, Siemens examination times. understands the advantages and disadvantages of–Siting requirements can also become an important vertical- versus horizontal-field magnets. If you listen obstacle. As the field strength increases, the weight to the other manufacturers, you might think that and sensitivity to vibration of the MR system will also vertical-field systems are truly optimal for every increase. This puts tremendous constraints on the situation. But as the field strength pushes higher and physical placement of the magnet, possibly requiring higher, you have to stop and ask yourself why those special foundations and extra support, which equals same manufacturers have produced, and continue to higher siting costs. So while 0.2T and 0.35T systems produce, 1.5T systems with horizontal fields. If vertical were really better, shouldn’t they build and provide 1
  3. 3. a vertical-field 1.5T scanner? Below we examine the –Therefore, chemical-shift artifacts are inherently smallclaims in support of vertical fields and differentiate in any application involving high-speed imaging.the hype from the truth. So, lower bandwidths are in theory better for imagingA. Solenoid Coils since they maximize SNR. But, this is only one of several concerns when considering clinical imaging.The Competition Claims:Vertical-field magnet orientation allows the use of Modern MR scanners have strong gradient systems thatsolenoid coils rather than the saddle-coil design of provide for high-speed imaging. Many applications, i.e.,horizontal-field systems. Solenoid coils have inherently gradient echo, MR angiography, Turbo Spin Echo, EPI,higher SNR than saddle coils. TrueFISP, etc., require fast time of acquisition which inherently results in using high-receiver bandwidthsThe Truth about Solenoid Coils: and lower chemical-shift artifacts even at higher fields.This claim compares and is only true when considering Therefore, for most applications, chemical shift and,surface coils of the same dimensions. The claim is ultimately, bandwidth are not an issue. The real issue inbased on a comparison of a solenoid coil to an old most clinical applications is SNR which is a direct resultfashioned linearly polarized 1-channel coil. However, of the magnetic-field strength. Essentially, the low-fieldtodays high-field systems feature coils with circular advocates are claiming that “slower is better” with thepolarization and offer multiple channel array coils receiver bandwidth argument.with a higher number of coil elements per region of C. Filling Factorinterest. This increases SNR far beyond the capabilitiesof a 1-channel linearly polarized coil and has proven Background:advantageous in image quality and applications; A coil placed close to the region of interest will have ai.e., iPAT in all directions. The “solenoid coil” claim greater signal, and a smaller coil in relation to the regionis not a fair comparison of state-of-the-art technology. of interest will result in less noise. Therefore a coil that is completely “filled” by exactly what is being imagedB. Smaller Receiver Bandwidth has a high “filling factor” and has ideal SNR properties.Background: The Competition Claims:The chemical-shift effect is proportional to the fieldstrength. Therefore, chemical-shift artifacts increase Some vendors claim that their vertical-field coils havewith higher-field strength and decrease with lower-field a better filling factor than the horizontal-field coils ofstrengths. In order to decrease the chemical-shift their competitors.artifact, a higher receiver bandwidth must be selected, The Truth about Filling Factor:resulting in lower SNR. As mentioned above, small coils located close to theThe Competition Claims: region of interest achieve the best SNR. Today, MRILow-field system advocates argue that lower-field manufacturers accomplish this by using phased-arraysystems use a smaller receiver bandwidth minimizing technology and by developing many different-sizedthe chemical shift artifact. They claim that this (partly) coils for scanning particular regions of anatomy.negates the SNR gains of higher-field strength. This is one area in which solenoid coils, as used in vertical-field systems, reach a limit of efficiency.The Truth about Receiver Bandwidth: Solenoid coils must surround the imaged body part.The physics of chemical shift artifacts and field strength This limits the ability to use a combination of small,is very clear. Briefly: efficient coil elements. Compared to surface-coil–Twice the field strength equals twice the SNR. technology commonly applied at high field strengths,–To keep a constant chemical-shift artifact at the solenoid coils have an intrinsically larger field-of-view higher-field strength, one must use twice the receiver which increases patient-induced noise and therefore bandwidth, which, in turn, reduces the SNR by a decreases SNR. factor of the square root of two. D. Pre-Amplifiers–Therefore, even where chemical shift is a concern, twice the field strength still results in a 41% The Competition Claims: increase in SNR! Some vendors claim to have better pre-amplifiers–Additionally, it is important to note that high-speed compared to competitors. imaging requires high-receiver bandwidth. The Truth about Pre-Amplifiers: Low-receiver bandwidth means that the duration At this time in MR development, pre-amplifiers have of the echo is longer. Short TR and TE can only be literally become a commodity. Siemens and most achieved with high-receiver bandwidth. 2
  4. 4. Figure 1A: Rigid coils—Large rigid coils away from Figure 1B: Tim Matrix coils—Flexible, light-weight Figure 2: In order to receive signal, surface RF coils must bethe patient reduce filling factor and SNR. Tim Matrix coils assume the patient’s curvature oriented with their field perpendicular to the main magnetic and optimize filling factor and effectively SNR. field. This results in different potential surface-coil locations for horizontal and vertical-field magnets.other coil manufacturers offer the industry-leading In contrast, a horizontal-field orientation allows thepre-amplifiers with 0.5 dB noise specification, so simultaneous use of multiple small coils and coilthere is actually no differentiation. elements. MAGNETOM Espree can be configured with Tim [76x18], which has 18 RF channels withHowever, there is a differentiation in the location up to 76 seamlessly integrated coil elements. No otherof the pre-amplifiers. The further the signal travels manufacturer of open MRI systems even comes close.prior to the pre-amplifier, the greater the loss due to Clinically, this provides highest speed and productivitytransmission. Siemens places the pre-amp closer to with PAT factors up to 12. It also allows a virtualthe coil compared to most competitors and this does 205cm field-of-view along the z-axis. This meanshelp the MR signal. Therefore, the Siemens approach a patient can be imaged from head to toe withoutof including the pre-amplifier as part of the coil the need to reposition the patient or the coils.maximizes the benefits overall. The PhysicsThree Key Issues Vertical-Field SupportersDon’t Want You to Know RF coil loops must be placed perpendicular to the orientation of the main magnetic field in order toWe have seen that claims about “Receiver Bandwidth” receive the MR signal. This is the limiting factor forand “Pre-amplifiers” are actually not differentiators. the use of surface coils on any specific anatomy relativeAdditionally, we now know that “Solenoid Coils” to the main-field orientation. For example, in aand “Filling Factor” are actually arguments in favor horizontal field the coil or multiple coil elementsof high-field horizontal systems over mid-field vertical (loops) must be placed anterior, posterior, right and/orsystems. But there is more. There are three important left of the patient. If a coil were placed at the top of theadvantages inherent in a horizontal-field orientation, patients head like a “halo” for example, the coil wouldthree key points the vertical-field supporters not pick up any signal since the main field runs throughdon’t want you to know. the coil. The same physics applies to vertical-field1. Horizontal-Field Allows Higher Number magnets. The coil must be perpendicular to the field, of Coil Elements/RF Channels so now a coil element/loop placed anterior or posteriorWith a vertical-field orientation, the human anatomy will not receive signal since the main field now runsdoes not allow a sensible use of many small surface through the coil in this orientation. The “halo” coil,coils as shown in Figure 2. Instead, the available though, would now work in the vertical field and thatsolenoid coils have to go around the whole body is exactly how other coils must be positioned for othercircumference as shown in Figure 1A. These large coils anatomical structures. A coil for the lumbar spine,have an inherent SNR disadvantage compared to small therefore, must be large enough to completely circlecoil elements, but due to the human anatomy, smaller the patient’s abdomen and pelvis.coils cannot be designed for vertical fields. The use of an 2. Horizontal-Field Allows BetterRF system with 8 or more coil elements is therefore not Use of Parallel Acquisition Techniquesbeneficial with a vertical-field orientation. At present, Parallel Imaging is applied along the phase-encodingthere is no coil available for a vertical-field system that direction and this requires that multiple coil elementsallows the use of more than 4 RF channels in one FoV. exist in that direction. The axial orientation is one of theThis limits the capabilities in advanced applications, as most commonly used orientations. Axial images requireexpected from a high-field system, especially the use ofParallel Acquisition Techniques. 3
  5. 5. receive more noise and breathing artifacts. In the end, vertical-field “spine” coils have lower SNR and create more artifacts than horizontal-field spine coils. How to Address the High Field Open Market So we are left with the question of how to address the need to provide an open experience with sufficient field strength to perform advanced applications. MAGNETOM Espree is the first Open Bore system in the world providing the openness of a 70cm bore and only a 125cm system length, an openness similar to a CT system. This is coupled with a 1.5T field strength and Tim technology.Figure 3. It is absolutely clear that there is no vertical-field system on the market that can provide these advantages.either a left-right or anterior-posterior phase encoding It should also be made clear that no argument can bedirection, the latter being the most frequent, e.g., for made or supported by physics that vertical-field systemsabdominal imaging. perform like horizontal, higher-field strength magnets.Anterior-posterior phase encoding—and therefore Summarymost Parallel Imaging applications in axial orientation In order to achieve the performance of a state-of-the-art—can only be achieved with a horizontal-field 1.5T scanner, an RF system with many coil elementsdesign, because coil elements are placed in the and many receiver channels is necessary. Theseanterior-posterior direction. systems alone can offer the image quality and workflowVertical field does not allow an alignment of coil elements advantages that are expected from a high-performancein anterior-posterior direction (See Figure 2), so Parallel system. Tim technology perfectly addresses these needs.Imaging cannot be applied in a-p direction. This limits The argument that vertical-field systems are inherentlythe use of Parallel Imaging in axial slice orientation. better than horizontal-field systems is simply not true.In summary, horizontal-field orientation offers distinct Vertical-field advocates may go to great lengths to argueadvantages for Parallel Imaging. These advantages otherwise, but their claims are either incorrect or simplyare important when it comes to high speed and not relevant to the modern clinical practice. Certainthroughput, as it is expected from a high-field system: vendors use these arguments to justify the existence of their extremely expensive vertical-field open products–A higher number of RF channels/coil elements can in the 0.6T to 1T range and attempt to position them be used, resulting in higher PAT factors. against superior technology.–Parallel Imaging can be done in all three directions (with the Tim coils), while vertical field does not However, the MR market clearly shows the need for allow iPAT in the anterior-posterior direction, which true high-field strength. The buying trend is toward is important for axial slice orientation. 1.5T and beyond, which represents approximately 75% of all MR systems bought today. There is also a trend3. Only Horizontal-Field Allows toward multiple-channel phased array coil technology. Optimal Spine Coil DesignThe optimal coil design for spine examinations is a flat, At the same time, the market remains stable for systemsmulti-channel coil positioned posterior to the patient. below 0.5T. Here, the MAGNETOM C! and MAGNETOMSuch a coil is optimized for spine examinations since it Concerto offer cost-efficient solutions, addressing theallows you to place many small elements very close to needs of the diagnostic market focusing on patientthe target anatomy. It also minimizes artifacts from other comfort. Indeed they can match the image qualityanterior organs, such as cardiac motion or respiration. of many 0.6T to 0.7T systems, where the limits of vertical-field technology start to diminish anyA vertical field does not allow such a design. As described advantages that may be gained. In conclusion, theabove, physics dictates that the RF field of the coil (B1) market clearly shows that 0.6T to 1.0T vertical-fieldmust not be parallel to the main magnetic field (B0). But scanners are a bad compromise between “high-field”a ring coil underneath the patient (or multiple rings in and patient comfort, while the MAGNETOM Espreean array coil) has a vertical B1 field, which is parallel to the provides the best answer to this complex problem.B0 field of a vertical magnet (see Figure 3). Vertical-fieldmagnets, therefore, require spine exams to be performed For additional specific information on Siemens MRwith suboptimal (solenoid) coils around the body (see products, please visit our Web page atFigure 1A). These coils are larger and therefore they 4
  6. 6. On account of certain regional limitations of sales rightsand service availability, we cannot guarantee that allproducts included in this brochure are available throughthe Siemens sales organization worldwide. Availabilityand packaging may vary by country and is subject tochange without prior notice. Some/All of the featuresand products described herein may not be available inthe United States.The information in this document contains generaltechnical descriptions of specifications and options aswell as standard and optional features which do notalways have to be present in individual cases.Siemens reserves the right to modify the design,packaging, specifications and options described hereinwithout prior notice. Please contact your local Siemenssales representative for the most current information.Note: Any technical data contained in this documentmay vary within defined tolerances. Original imagesalways lose a certain amount of detail when reproduced.Siemens Medical Solutions USA, Inc. Siemens MedicalMagnetic Resonance Division Solutions that help51 Valley Stream ParkwayMalvern, PA 19355-1406 USATelephone: © 2005 Siemens Medical Solutions Order No. A91004-M2220-M100-10-4A00 Printed in USA