Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2003-05-02
2004-11-30
Arana, Louis (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06825665
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed in general to magnetic resonance tomography as employed in medicine for examining patients. The present invention is specifically directed to a method for manufacturing a carrier tube for the body coil of an MRI apparatus.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully utilized as an imaging method in medicine and in biophysics for more than 15 years. In this examination modality, the subject is disposed in a strong, constant magnetic field. The nuclear spins of the atoms in the subject that were previously irregularly oriented are aligned as a result. Radio-frequency energy can then excite these “ordered” nuclear spins to a specific oscillation. This oscillation generates the actual measured signal in MRT that is picked up with suitable reception coils. The examination subject can be spatially encoded in all three spatial directions by utilizing non-uniform magnetic fields generated by gradient coils. The method allows a free selection of the slice to be images, allowing tomograms of the human body to be acquired in all directions. As a tomographic imaging method in medical diagnostics, MRT is distinguished first and foremost by a versatile contrast capability as a “non-invasive” examination modality. MRT currently employs applications with high gradient power that enable an excellent image quality with measuring times on the order of magnitude of seconds and minutes.
The constant technological improvement of the components of MRT devices and the introduction of fast imaging sequences have created an increasing number of medical application in medicine. Real-time imaging for supporting minimally invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are a few examples.
FIG. 9
shows a schematic section through a conventional MRT apparatus. The section shows further components of the interior that is surrounded by the basic field magnet
1
. The basic field magnet
1
contains superconducting magnet coils that are situated in liquid helium and is surrounded by a magnet envelope
12
in the form of a two-shell vessel. The cryo-head
15
that is attached to the magnet envelope
12
at the outside is responsible for keeping the temperature constant. The gradient coil
2
is concentrically suspended via carrying elements
7
in the interior surrounded by the magnet envelope
12
(also called magnet vessel). A carrying tube with the radio-frequency antenna applied thereon is likewise concentrically introduced in the interior of the gradient coil
2
. The carrying tube and RF antenna are referred to below as an RF resonator or as a “body coil”
13
. The gradient coil
2
and the body coil
13
thus represent two cylinders inserted into one another with a radial spacing therebetween—in the form of an air gap—amounting only to about 3 cm. The RF antenna converts RF pulses emitted by a power transmitter into a magnetic alternating field for exciting the atomic nuclei of the patient
18
, and subsequently converts the alternating field emanating from the precessing nuclear moment into a voltage supplied to the reception branch. The upper part of the body coil
13
is mechanically connected to the magnet envelope
12
via a cladding
29
that is funnel-shaped. Tongues
30
(see
FIG. 10
) are mounted at the lower part of the body coil
13
, the body coil
13
being mechanically connected via these tongues
30
to the lower part of the magnet envelope
12
via a cladding
29
as well as with carrying elements
7
. The tongues
30
as well as the body coil
13
are mechanically connected to bed rails
33
. Under certain circumstances, the tongues
30
are considered as belonging to the body coil
13
. The patient
18
on a patient bed
19
is moved into the opening in the interior of the system via glide rails
17
. The patient bed is disposed on a vertically adjustable carrying frame
16
.
The gradient coil
2
is likewise composed of a carrying tube
6
having an exterior on which three windings (coils) are disposed that each generate a gradient that is proportional to the current supplied to the coil. The three gradients are perpendicular to one another. A radio-frequency shield (RF shield)
20
that shields the coils from the radio-frequency field of the RF antenna is applied on the inside of the carrying tube
6
. As shown in
FIG. 11
, the gradient coil
2
has an x-coil
3
, a y-coil
4
and a z-coil
5
that are respectively wound around the carrying tube
6
and thus respectively generate gradient fields in the directions of the Cartesian coordinates x, y and z. Each of these coils is equipped with its own power supply in order to generate independent current pulses with the correct amplitude and at the correct time in conformity with the sequence programmed in the pulse sequence controller. The required currents lie at approximately 250 A. Since the gradient switching times should be as short as possible, current rise rates on the order of magnitude of 250 kA/s are required. In an extremely strong magnetic field as is generated by the basic field magnet
1
(typically between 0.22 and 1.5 Tesla), such switching events involve strong mechanical oscillations due to the Lorentz forces that thereby occur, these mechanical oscillations leading to considerable noise.
The following demands are made of the body coil
13
of an MRT apparatus:
For space reasons, a tube wall thickness of only up to 10 mm can be accepted. The material of the body coil should comprise an optimally low power absorption of RF power, i.e. must be electrically non-conductive. The body coil must be MR-compatible, i.e. non-imaging in the sense of magnetic resonance (for example, it dare not contain any water). Since the body coil is supposed to carry the patient bed with patient, the body coil must comprise high mechanical shape stability. In order to shield the noise generated mainly by the gradient coil as well as possible, to body coil should be optimally long without comprising interruptions. For design-oriented reasons, however, the funnel-shaped widened portion (cladding
29
) of the patient tunnel should also begin as far inside as possible, which leads to a very short body coil and does not meet the noise-related demands.
In conventional solutions, short cylindrical Gfk tubes of epoxy resin are employed for the body coil, the functional elements of the RF antenna being applied thereon in the form of planar copper conductors. For manufacturing such tubes, a rotating arbor is wrapped with resin-saturated fiberglass rovings and is cured (possibly upon application of heat). This solution involves compromises that have a significant disadvantage with respect to one of the two aspects of noise or design: Although the body coil is short, the funnel-shaped widened portion is not a part of the body coil but is composed of a separate plastic part (cladding
29
). This is inadequate for meeting the noise-reducing demands since it lacks the necessary mass and rigidity. Second, the interface between the body coil and the funnel-shaped cladding represents an acoustic weak point.
SUMMARY OF THE INVENTION
It is an object of the present invention to optimize the noise and design properties as well as the electromechanical stability of a magnetic resonance tomography apparatus.
This object is inventively achieved in a magnetic resonance tomography apparatus having a basic field magnet surrounded by a magnet envelope that surrounds and limits an interior space, with a gradient coil system disposed in this interior space, and a body coil having an RF antenna and a carrying tube disposed in the gradient coil system as an inner encapsulation cylinder, and wherein the magnet envelope and the gradient coil system are optically as well as acoustically closed by the body coil and a diaphragm at the end faces and in the interior. The body coil is inventively manufactured by a vacuum casting process or a vacuum die-casting process.
A
Eberler Ludwig
Eberler Michael
Kolbeck Thomas
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