Magnetic Tracer (MT)
More recently, measurements with magnetic tracers have resumed mainly
to study intragastric movements and gastric emptying. Since 1992, Miranda,
Baffa et al., have used an AC (10kHz) bio-susceptometer for this purpose
[Miranda 1992]. In addition to the gastric emptying rate, the rhythmic
gastric contractions are also visible thanks to the strong dependence
of the signals on the distance (proportionally to the distance to the
power -6). One paper has also compared the orocecal transit time (OCTT)
measured by the magnetic method and by the breath H2 output, noting a
close similarity between the two methods [Baffa 1995a]. Recently, they
have also studied the feasibility of obtaining an image of the distribution
of the tracer by scanning the area over the sample with the susceptometer
Another option is to magnetize a ferromagnetic powder, once it has been
ingested (e.g. within a meal), with an external homogeneous field, and
then to measure the remanent magnetic field. This was already mentioned
as a possible solution by Frei et al. . Baffa [1995b], Caneiro
 and Forsman [1998b, 2000] use fluxgate magnetometers to estimate
the strength of the remanent field. The exposed particles are aligned
and exhibit a permanent magnetization. After switching off the external
field, the mixing movements in the GIT will result in a misalignment
of the magnetic moments, and, hence, in a decay of the measured magnetic
field. Once again, most of the publications deal with gastric emptying,
but lately the gastrocolic reflex was also examined [Ferreira 2002].
Magnetic Marker (MM)
We will now discuss the cases where a permanent magnet is used as a
Spelman, Prakash, et al. [Spelman 1994, Prakash 1996, 1999b, Heitkemper 2002],
at the University of Washington, resumed the first study from Wenger with an
ingested magnet and a single magnetometer. They measured contractions in stomach,
small intestine and large intestine, and correlated the magnetic signal with
pressure sensors in the stomach. The marker is a cylindrical Teflon coated
stir-bar magnet, with a diameter of 7.8 mm, a length of 25.0 mm, and a magnetic
moment of 0.18 Am2. The sensor is an electronic compass, a fluxgate sensor
with two orthogonal sensing coils. The sampling rate is set to 1 Hz. With this
method, only the intrinsic frequencies of the GI motility are measured, but
neither the intensity of the contraction nor the position of the marker can
Of course, much more information would be obtained by retrieving the
position of the marker. For this purpose the magnetic field generated
by the marker has to be measured at different locations.
The first approach is to scan different places with one sensor, with
the main disadvantage of time consumption and errors due to movements
of the marker during the scan. Basile et al.  used a single-channel
SQUID 2nd order gradiometer, and a magnetized steel sphere (Ø2
mm, in a 3x6 mm tube, relative density of 1.9 g/cm3), to measure the
oro-anal segmental TT with one scan taken every hour. Forsman et al.
[1995b] implemented the same setup with a fluxgate gradiometer. The time
needed for a scan is 25 s and the volunteer is lying on a pneumatically
driven bed, the small ingested magnet has a magnetization of 1 mAm2 (Ø1.5
mm, 4 mm with the coating, 1.4 g/cm3). Finally, Andrä et al. [1997,
2000, 2001], propose a more complicated system not (yet) tested in vivo.
Their system includes external coils to repeatedly align a spherical
permanent magnet, which is free to rotate in a "bearing liquid" inside
the capsule. The magnet has a magnetization of 20 mAm2 (3.5 mm in diameter,
in a capsule with a diameter of 7 mm, a length of 20 mm, and a final
density of 1.2 g/cm3). In order to eliminate slowly changing background
fields, the measuring procedure comprises in reversing the orientation
of the marker and subsequently taking the difference of the measured
values. The 3 components of the field at one point are measured with
a commercial integrated magnetoresistive sensor (Honeywell). Since the
orientation of the magnet is known, the 3 measured values are enough
to recalculate the position. Note that the external coils need to be
motorized to follow the position of the capsule to insure a constant
orientation of the marker. The temporal resolution is at best 1 Hz, and
the spatial resolution is less than 10 mm within a test volume of 40x40x15
To solve the problem of temporal resolution and inaccuracy due to the
rotation of the marker during the scanning procedure, the next step is
to use an array of sensors.
This has been done for the first time, by Weitschies et al., with multi-channels
SQUIDs (up to 63-channels), similar to the established tool used for magnetoencephalography
and magnetocardiography, leading to a very accurate but expensive tool operating
in an environment shielded against external magnetic fields [Weitschies 1990,
1994, 1997, 1999, Osmanoglou 2002]. A typical temporal resolution of 250 Hz
and a spatial resolution within a range of a few millimeters is achieved with
a magnetic moment of 100 nAm2. The main interest is, for the pharmaceutical
sciences, to investigate the behavior (e.g. transit time) of a solid oral dosage.
This is the reason why the latest studies deal with disintegrating capsules
[Weitschies 2001, Hartmann 2002]. The complexity and the cost of the infrastructure
prevent the use of this system for a lot of applications, especially if long
term measurements are necessary. Having said this, some interesting recordings
have been published concerning the whole GIT: esophagal, gastric and duodenal
transit of a non-disintegrating capsule [Weitschies 1999], and even a mass
movement of the marker through the transverse colon [Weitschies 2000].
One of their latest papers (Weitschies et al., in collaboration with
Universität Jena, Germany) [Romanus 2002], brings together the works
described above. Although the application does not concern the GIT, the
experiment is interesting: a SQUID sensor, operating in an unshielded
environment, detects the magnetic relaxation of magnetic nanoparticles
in mice after intravenous injection.
Prakash et al.  (see in vivo measurements with a single sensor,
at the beginning of this section), have tested, but only in vitro, an
array of 8 fluxgate magnetometers, in a shielded booth (see above for
magnet specification). In parallel, they published a nice solution for
solving the equations to localize a single dipole, with some constraints
on the positioning of the sensors [Prakash 1999a].
For this review of minimally-invasive solid markers for the GIT to be
exhaustive, we must mention the work of Ewe et al. [1989, 1991, 1995]
who used a portable metal detector to track a hollow metallic sphere
(Ø6 mm, 1.4 g/cm3). The localization accuracy is 0.5 to 1 cm for
a detection distance between 2 to 12 cm. An interesting result is the
good correlation between the transit time of the metallic marker and
the Hinton radio-opaque markers method.
Magnetic markers are also used to track the position of catheters, especially
in cardiology. The advantage is that clinicians may locate and navigate
their device without fluoroscopy (or at least with less x-rays), and
without cumbersome systems, in order to move from an expensive operating
room to less costly procedure rooms. Moreover, fluoroscopy is only a
two-dimensional imaging system, whereas the three dimensions can be retrieved
with a magnetic marker.
In the previous section, an ingested maker and external sensors were
used, whereas here, different solutions for the tracking of catheters
have been developed. Indeed, an electrical connection to the outside,
through the catheter, is possible. In this case, the magnetic marker,
a magnetic dipole in fact, may be produced by a coil driven by an alternative
current. Furthermore, the sensor(s) may be located in the catheter, while
the magnetic source remains outside. Almost all the imaginable strategies
have been realized, and different products exist on the market. The systems
with emitting coils have two obvious advantages: Firstly, the information
is modulated at high frequencies, thus the large quasi-DC noise due to
the inhomogeneous magnetic field of the earth is no longer a problem.
Secondly, many markers may be tracked simultaneously by multiplexing
the signals, in time or in frequency. On the other hand, the ease with
which one can adapt a permanent magnet to a standard device and the absence
of power supply for the magnetic marker, leading to less expensive devices,
are good assets for solutions based on permanent magnets.
The next paragraphs list the existing devices based either on a permanent
or on a modulated magnetic dipole, to find the position of an indwelling
device. Note that other non-invasive techniques, on which this review
will not focus, are also used, such as ultrasound (measuring distances
by measuring the time between two crystals) [Meyer 1997, Vilkomerson
1992, 1997, Vesely 1998], electrical impedance (measuring distances by
measuring the impedance between two electrodes) [Wittkampf 1997, 1999,
Wrublewski 1999], or radiofrequency (measuring distances by measuring
the power of the signal) [Iddan 1997]; when at least three distances
have been measured, the three-dimensional position can be recalculated
by triangulation. These later solutions are not very accurate, due largely
to the inhomogeneities of the medium, the most accurate systems being
based on coils. This emphasizes another advantage of the magnetic dipole
solution: the human body can be considered as a homogeneous medium for
the magnetic field (same permeability as the air), at least up to a certain
frequency (100 kHz, according to our own measurement).
Permanent magnetic dipole
Lucent Medical System Inc. (ZortranTM), are the only ones who opted
for a permanent magnet [Golden 1995, Somogyi 1999, Tobin 2000, Beck 2001].
Their work has been developed together with the University of Washington
(see above). Three cylindrical magnets are needed (total length 16.8
mm, Ø1.8 mm). The hand-held detector utilizes four integrated
magnetoresistive magnetic field sensors (Honeywell). The position of
the tube tip is displayed on a transparent screen 6x4 cm. Very little
detailed information about the accuracy of the positioning system are
given. A new prototype with 16 sensors has also been tested in nasogastric
Another firm, Teslev Scientific Inc. (project Cathfinder, funded by
SIMS Deltec), tried to achieve the same goal with a permanent magnet.
They stopped their project in 1994. SIMS Deltec has now developed another
system based on coils (see below). The idea was to use a cylindrical
magnet, 0.64 mm in diameter and 7.6 mm in length, together with eight
induction type sensors (inductance changes of a ferrite core coil in
a resonant circuit) not all in the same plane. Standard fluxgate sensors
were not suitable due to their high cost.
High frequency signal emitted by a coil
When a modulated signal is used, the background magnetic field of the
earth does not interfere with the measurement, therefore a small handheld
detector can be moved over the patient's skin to track the marker. Cath-finderTM
from Deltec [Strohl 1990, Starkhammar 1990, 1996] and CathTrackTM [Strohl
1995] are two inexpensive handheld tracking systems for catheters. They
use a small coil in the catheter as a sensor, and external emitting coils.
To keep the system simple, the position of the marker can be calculated
only if the locator is centered above the marker and perpendicular to
it, leaving only two-degrees-of-freedom (depth and orientation in the
horizontal plane). With a detection coil of 1 mm in diameter, or even
less, the detection depth is 18 cm and the accuracy approximately 1 cm.
Working on the same principle, more expensive systems are more accurate
and determine all six-degrees-of-freedom of the tip of the catheter (the
three translations and the three rotations). Actually, many markers are
used, and thus not only the tip but all the catheter can be visualized.
CARTOTM and NOGATM are the reference systems [Ben-Haim 1996, Acker 1998].
Developed by Biosense Webster, they are used in cardiology, and have
a resolution better than 1 mm, at a distance of 30 cm. Another device,
developed at St Mark's Hospital in London [Bladen 1993, Saunders 1995,
Kitchen 1999], is used mainly for colonoscopy. The catheter contains
12 to 15 sensor coils 2 mm wide and spaced at 12 cm intervals.
With external emitting coils, yet other sensors have been used. ARTMA,
for endonasal surgery, uses integrated Hall sensors attached to the head,
as external landmarks, and to surgical instruments [Gunkel 1995]. A navigation
system for endovascular interventions, still in prototype form, at the
Delft University of Technology [Tanase 2003], is based on integrated
Hall or magnetoresistive sensors. This imaging system, attached to a
fluoroscope, aims to reduce the x ray dose needed for an intervention.
The advantage of the use of these sensors over coils, can certainly be
proven at low frequency and small size.
Finally, systems with emitting coils in the catheter can also be found.
A handheld "Cathlocator" at the Royal Adelaide Hospital [Williams
1996] has been tested (conflicting data about the accuracy was given).
The idea was also patented in 1991, for a more complex system to be used
together with x-rays, by a leader in medical imaging [Darrow 1993]. Note
that when the emitting coil is external, the field may be stronger and
the field gradient in space can be kept more constant. Otherwise, with
a small emitting coil, the field decreases very rapidly with distance,
which requires a larger dynamic range.
A few other systems, developed for other applications, work on the same
principles as the ones described in the previous section. The main difference
is that the size of the marker can be larger. Ascension Technology propose
a versatile product Flock of BirdsTM [www.ascension-tech.com], with an
accuracy of 0.5 mm and 0.1° at 30 cm, based on a pulsed dc magnetic
Magnets together with Hall sensors are widely used in one-degree-of-freedom
applications, mostly angular but also linear positioning systems [Demierre
2003]. However, the detection range is, in these examples, very limited.
Maenaka et al.  describe an application with three-degrees-of-freedom.
We have seen that small magnets and ferromagnetic powders have already
been used to assess the gastrointestinal motility. Yet, these method
are, for the moment, limited. Most of the measurements concern stomach
contractions and emptying. If we consider the rest of the GIT, almost
no study exists. The explanation is that the existing techniques are
cumbersome, very expensive, and most of the time need a shielded environment,
limiting the number and duration of the recordings. The remaining less
expensive systems, are based on a single sensor (eventually two). Consequently,
they allow only the recording of the intrinsic rhythmic activity, which
is not very useful if the position of the marker is not known. The devices
using a single mobile sensor to make a mapping of the magnetic field,
strongly limit the time accuracy, and also the spatial accuracy due to
movements of the marker during the scan; moreover, we end up with a cumbersome
apparatus. For all these methods, the major intrinsic constraint is the
magnetic field of the earth.
An interesting alternative is to use a modulated electromagnetic signal
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such as better spatial accuracy, simultaneous tracking of several markers,
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