Patent Application
20080054901
a is a basic representation of a patient tunnel with a birdcage antenna structure arranged thereon, with inventive wall parts in the region of the ferrules of the birdcage structure.
b is an enlarged representation of the section Z from
An antenna structure 3, for example in the form of conductor foils, plates or the like, is mounted on the outside the tube wall. Generally copper or another metal is used as a conductive material. The shown exemplary embodiment is a high-pass birdcage structure which comprises in total sixteen antenna rods 4 running parallel to the tube axis. The ends of the antenna rods 4 each have end segments 5 widening in a T-shape. The end segments 5 of two adjacent antenna rods 4 are connected via capacitors 6 (which are only schematically shown in
Suitable radio-frequency pulses are fed into the antenna structure 3 via matched feed lines so that the desired radio-frequency field forms inside the patient space B. These feed lines are not shown here for better clarity. The precise design of the patient tunnel, the patient bed, the antenna structure and the manner of the activation of the antenna structure and the further components necessary for this in a magnetic resonance tomography apparatus are sufficiently known to those skilled in the art so that no further explanation is necessary herein.
Schematically shown directly above this representation in Figure is the voltage U applied to the antenna structure 3 upon emission of an RF pulse across the circumference S along the tube-shaped dividing wall 1. It can be seen that particularly high voltages occur directly to the right and left of the capacitors 6, in contrast to which a zero crossing exists between two adjacent capacitors 6 in the middle of the end segments 5. This means that the voltage in the middle region between the capacitors 6 is negligible. If a patient P now directly contacts the inside of the tube wall 1, the electrical field is coupled from the antenna structure 3 into the tissue of the patient P, meaning that displacement currents through the dividing wall 1 into the tissue of the patient occur, with the magnitude of the displacement current resulting from the current density and the area. The magnitude of the current density J(ω) generally is from
J(ω)=jω·D·A=jω·C·U, (2)
wherein j designates the imaginary unit, ω the angular frequency, D=∈0∈·E the electrical flux density and A the area to be considered. According to equation (2), the current density J(ω) is consequently proportional to the dielectric constant ∈ of the wall material and—since E=U/D applies—is simultaneously also proportional to the voltage U at the appertaining location. This leads to the situation that particularly high displacement currents occur at the locations at which a high voltage is applied to the antenna structure. This is schematically represented in
As shown there, the displacement currents JV that propagate through the tube wall 1 into the tissue of the patient P close in the tissue and there lead to a tissue warming. The power introduced into the tissue is thereby proportional to the square of the flowing current. The SAR (Specific Absorption Rate) is defined as follows:
SAR=(κ/2ρ)·|E|2, (3)
wherein κ is the electrical conductivity of the tissue and ρ is the tissue density.
The material GFK that is typically used for the conventional tube walls and dividing walls exhibits a dielectric constant of approximately ∈=5. If a wall thickness of 8 mm is assumed in the dividing wall 1 shown in
How the power introduced into the tissue of the patient P can be reduced simply in the inventive manner is shown in
As can be seen in
The effect of these recesses 10 filled with the wall parts 9 can be seen particularly well using
As explained above by means of the equations (1) and (2), the electrical flux density and thus the magnitude of the displacement current depends on the capacitance of the plate capacitor formed by the dividing wall and thus in particular on its dielectric constant ∈.
In the regions in which no wall parts 9 are located, i.e. in which the wall material of the dividing wall 1 exists in full thickness, as before the wall 1 forms a plate capacitor with the capacitance C=0.55 pF/cm2 (with the values already described above, i.e. 8 mm wall thickness and GFK as a wall material).
However, in the regions in which the wall parts 9 are used the material layers situated atop one another can be considered as series circuits of two capacitances. The total capacitance can then be calculated in a typical manner according to the formula
1/C1=1/C2+1/C3, (4)
wherein C1 is the total capacitance of the dividing wall in this region, C2 is the capacitance over the wall part 9 and C3 is the capacitance over the remaining wall thickness of the initial material of the dividing wall 1 which remains between the recess 10 and the surface of the dividing wall 1 on the side of the patient space.
For the subsequent calculational example it is assumed that a 5 mm-thick hard foam plate with a dielectric constant ∈=1.1 is used as a wall part 9. The remaining wall thickness of the GFK material is then 3 mm. With these values, according to equation (1) a capacitance C3 of 0.195 pF/cm2 results for the hard foam plate and a capacitance C2 of 1.4 pF/cm2 results for the remaining residual wall of the dividing wall 1. The total capacitance C1 is then 0.17 pF/cm2.
Using equation (1), an effective dielectric constant for the dividing wall 1 can be calculated from this in the region of the recesses filled with the wall parts 9. This effective dielectric constant results in ∈=1.56 with the values cited above. This means that the capacitance or the dielectric constant was decreased by a factor of 3.2 in the regions of the wall parts 9 via the inventive measure. Since according to equation (2) the size of the displacement current is proportional to the capacitance, the displacement current is correspondingly decreased by a factor of 3.2 in this exemplary embodiment. Since, as explained above, the introduced power or the SAR is proportional to the square of the flowing current, this entails a SAR reduction by approximately a factor of 10.
It is not a problem that webs remain between the recesses with the wall parts 9, since these webs are located in a region of the zero crossing of the voltage U anyway and here only very low voltages are applied (as shown in the representation of the voltage U over the circumference S in the upper region of
The inventive arrangement ensures that, in critical regions with high E-fields, the dividing wall 1 is replaced by a material with smaller dielectric constant, whereby no impairment whatsoever of the mechanical stability of the dividing wall occurs due to the special design embodiment. The mass of the dividing wall likewise remains largely identically large. No only the bearing function but also the noise-damping properties are thus retained. Nevertheless, the danger of unacceptably high local SAR values is distinctly reduced via the invention. The entire design can additionally be produced in an extraordinarily cost-effective manner. Only one further work step is required in which corresponding recesses are initially introduced into the dividing wall after the manufacture and these are filled with the suitable wall parts (made, for example, from hard foam). Depending on the manufacturing process it is also possible to mold the recesses at the same time in the manufacturing of the dividing wall. The further assembly of the antenna structure on the dividing wall or at the dividing wall can be implemented in a conventional manner. The dividing wall itself can likewise also be treated further, for example surface-treated and mounted.
In conclusion it is noted again that the design of a patient tunnel described in detail in the preceding is only one exemplary embodiment which can be modified in the most varied manner by the average person skilled in the art without leaving the scope of the invention. The invention has been explained in the context of a magnetic resonance tomography apparatus used for medical applications, but it is not limited to such applications, and can also be utilized in scientific applications.
Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 040 574.9 | Aug 2006 | DE | national |
