2. Vertical and Horizontal Fields for MRI: Does Direction Matter?
Mathias Blasche, MA, and Brian M. Dale, PhD
Vertical-Field Experience: can be engineered to site with little modification, as
Successes and Challenges the field strength moves beyond 0.5T, issues may
Siemens 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 to
back 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, worldwide
Siemens continued to explore vertical-field magnets
from all vendors, are horizontal-field magnets with
and this led to the development of the MAGNETOM
1.5T or higher field strength. These customers want
Rhapsody, the first 1.0T vertical-field system. It consists of
to increase throughput and productivity, while still
a two-pole magnetic cryostat supported by off-set pillars.
addressing the need to image claustrophobic and
Siemens has always maintained a leadership role in obese patients. But when the need arises for highest
vertical-field magnet design. Other manufacturers have image quality, highest throughput and the full range
responded over the years by introducing systems from of advanced applications, there is no alternative to
0.2T all the way to 1.0T, continually promising to horizontal-field systems. Recent developments have
introduce and deliver an equivalent to the MAGNETOM shown the benefits of high-field magnets combined
products. Today, the MAGNETOM Concerto offers the with multiple channel RF systems and multiple
best performance available at 0.2T and the MAGNETOM element coils. All major MR vendors are investing
C! is a leading competitor at 0.35T. The various open huge development efforts into this field, on their
MAGNETOM vertical-field systems have enabled horizontal-field systems.
outpatient imaging centers to offer services to obese
The highlight of this development was the introduction
and claustrophobic patients.
of Tim (Total imaging matrix) technology at the RSNA
Low- and mid-field systems with vertical field orientation 2003. Tim offers a boost in image quality and allows
are a perfect solution for the cost-sensitive market due integrated Parallel Acquisition Techniques (iPAT) in all
to the lower system price and the lower operational directions with high PAT factors, resulting in the
costs. Especially C-shaped open magnets offer high highest productivity. Tim has increased the potential
patient comfort and improvements of gradient for advanced applications and enables whole-body
performance. 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 combine
However, when trying to combine the openness of a 1.5T and the previously mentioned advantages of
vertical field magnet with higher-field strength, this Tim technology with the possibility to offer services
technology 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. a vertical-field 1.5T scanner? Below we examine the –Therefore, chemical-shift artifacts are inherently small
claims 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 imaging
A. 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 that
solenoid 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 bandwidths
The 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 in
based on a comparison of a solenoid coil to an old most clinical applications is SNR which is a direct result
fashioned linearly polarized 1-channel coil. However, of the magnetic-field strength. Essentially, the low-field
today's high-field systems feature coils with circular advocates are claiming that “slower is better” with the
polarization and offer multiple channel array coils receiver bandwidth argument.
with a higher number of coil elements per region of
C. Filling Factor
interest. This increases SNR far beyond the capabilities
of 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 a
i.e., iPAT in all directions. The “solenoid coil” claim greater signal, and a smaller coil in relation to the region
is 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 imaged
B. 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 field
strength. Therefore, chemical-shift artifacts increase Some vendors claim that their vertical-field coils have
with higher-field strength and decrease with lower-field a better filling factor than the horizontal-field coils of
strengths. 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 the
The Competition Claims: region of interest achieve the best SNR. Today, MRI
Low-field system advocates argue that lower-field manufacturers accomplish this by using phased-array
systems use a smaller receiver bandwidth minimizing technology and by developing many different-sized
the 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. 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 be
the 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 the
pre-amplifiers with 0.5 dB noise specification, so simultaneous use of multiple small coils and coil
there is actually no differentiation. elements. MAGNETOM Espree can be configured
with Tim [76x18], which has 18 RF channels with
However, there is a differentiation in the location
up to 76 seamlessly integrated coil elements. No other
of 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 productivity
transmission. Siemens places the pre-amp closer to
with PAT factors up to 12. It also allows a virtual
the coil compared to most competitors and this does
205cm field-of-view along the z-axis. This means
help the MR signal. Therefore, the Siemens approach
a patient can be imaged from head to toe without
of including the pre-amplifier as part of the coil
the need to reposition the patient or the coils.
maximizes the benefits overall.
The Physics
Three Key Issues Vertical-Field Supporters
Don’t Want You to Know RF coil loops must be placed perpendicular to the
orientation of the main magnetic field in order to
We have seen that claims about “Receiver Bandwidth”
receive the MR signal. This is the limiting factor for
and “Pre-amplifiers” are actually not differentiators.
the use of surface coils on any specific anatomy relative
Additionally, we now know that “Solenoid Coils”
to the main-field orientation. For example, in a
and “Filling Factor” are actually arguments in favor
horizontal field the coil or multiple coil elements
of high-field horizontal systems over mid-field vertical
(loops) must be placed anterior, posterior, right and/or
systems. But there is more. There are three important
left of the patient. If a coil were placed at the top of the
advantages inherent in a horizontal-field orientation,
patients head like a “halo” for example, the coil would
three key points the vertical-field supporters
not pick up any signal since the main field runs through
don’t want you to know.
the coil. The same physics applies to vertical-field
1. 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 posterior
With a vertical-field orientation, the human anatomy will not receive signal since the main field now runs
does 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 that
solenoid coils have to go around the whole body is exactly how other coils must be positioned for other
circumference 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 circle
coil 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 Better
RF system with 8 or more coil elements is therefore not
Use of Parallel Acquisition Techniques
beneficial with a vertical-field orientation. At present,
Parallel Imaging is applied along the phase-encoding
there is no coil available for a vertical-field system that
direction and this requires that multiple coil elements
allows the use of more than 4 RF channels in one FoV.
exist in that direction. The axial orientation is one of the
This limits the capabilities in advanced applications, as
most commonly used orientations. Axial images require
expected from a high-field system, especially the use of
Parallel Acquisition Techniques.
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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 be
direction, the latter being the most frequent, e.g., for made or supported by physics that vertical-field systems
abdominal imaging. perform like horizontal, higher-field strength magnets.
Anterior-posterior phase encoding—and therefore Summary
most 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 elements
design, because coil elements are placed in the and many receiver channels is necessary. These
anterior-posterior direction. systems alone can offer the image quality and workflow
Vertical field does not allow an alignment of coil elements advantages that are expected from a high-performance
in 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 inherently
the 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 argue
advantages for Parallel Imaging. These advantages otherwise, but their claims are either incorrect or simply
are important when it comes to high speed and not relevant to the modern clinical practice. Certain
throughput, 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 trend
3. Only Horizontal-Field Allows toward multiple-channel phased array coil technology.
Optimal Spine Coil Design
The optimal coil design for spine examinations is a flat, At the same time, the market remains stable for systems
multi-channel coil positioned posterior to the patient. below 0.5T. Here, the MAGNETOM C! and MAGNETOM
Such a coil is optimized for spine examinations since it Concerto offer cost-efficient solutions, addressing the
allows you to place many small elements very close to needs of the diagnostic market focusing on patient
the target anatomy. It also minimizes artifacts from other comfort. Indeed they can match the image quality
anterior 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 any
A vertical field does not allow such a design. As described advantages that may be gained. In conclusion, the
above, physics dictates that the RF field of the coil (B1) market clearly shows that 0.6T to 1.0T vertical-field
must 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 Espree
an 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-field
magnets, therefore, require spine exams to be performed For additional specific information on Siemens MR
with suboptimal (solenoid) coils around the body (see products, please visit our Web page at
Figure 1A). These coils are larger and therefore they http://www.siemens.com/medical.
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