Electricity: measuring and testing – Magnetic – Calibration
Reexamination Certificate
2003-02-25
2003-11-18
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Calibration
C324S248000
Reexamination Certificate
active
06650107
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates generally to the field of measuring magnetic fields. More specifically, the present invention is related to calibrating superconducting quantum interference device (SQUID) channels that are used in measuring magnetic fields.
2. Discussion of Prior Art
A SQUID magnetic sensor is at the heart of a sensitive magnetometer aimed at measuring magnetic fields below approximately 10
−10
Tesla (T). This is the range of magnetic fields produced by living organisms (also called biomagnetic fields). For example, the human heart produces fields between 10
−12
T and 10
−10
T just outside of a chest surface. The magnetic fields emanated from the human brain, just outside of the head, are of the order of 10
−14
T-10
−12
T. These numbers can be compared with the earth's magnetic field of about 10
−4
T and the typical urban magnetic noise of 10
−8
T-10
−6
T.
SQUIDs react to a magnetic flux rather than a field. Magnetic flux &PHgr;
B
is defined as the projection of the average magnetic field threading a given area along the area's normal z, times that area A, or mathematically:
&PHgr;
B
=B
Z
A
A low-Tc dc SQUID is an ultra-sensitive, low-noise transducer of magnetic flux &PHgr;
B
- to voltage, consisting of two nominally identical superconducting elements called Josephson junctions serially connected in a superconducting, electrically continuous loop. The SQUID loop is quite small in dimensions, typically 10
−4
-10
−2
mm
2
. Today SQUIDs are typically produced on a chip, using Nb—Al junction technology, with junctions and the SQUID loop made of thin films. The micron-scale dimensions of the layout are defined using photolithographic techniques. The SQUID chip is typically enclosed in a superconducting shield screening the device from ambient magnetic flux. The magnetic flux to be measured is typically intercepted by considerably larger, 10-20 mm diameter loops or coils (called pick-up or detection coils) inductively coupled to a SQUID via an input coil. These coils are usually made of thin insulated superconducting (Niobium) wire wound over some non-conducting cylindrical support, although in some instances they are integrated on a chip with a SQUID. A single coil or a single loop intercepting magnetic field is called a magnetometer. More complex combinations of coils or loops, described in more detail below, form a gradiometer.
Since the SQUID and the coils must be kept in a superconducting state, they are immersed in liquid helium at temperatures only a few degrees above absolute zero (about −460° F., or −269° C., or 4° K.). The double-wall vessel (space between walls being evacuated) intended for keeping and thermally isolating liquid helium is called a dewar. Dewars, in biomagnetic applications, are made largely of fiberglass in order to minimize magnetic interference with SQUIDs. Indeed, even non-magnetic metals are sources of secondary magnetic fields resulting from induced eddy currents.
Magnetocardiography (MCG) systems usually employ an array of sensors, for example 7 to 40. Measuring channel usually refers to one member of such an array, which comprises a single SQUID sensor inductively coupled to an arrangement of detection coils (magnetometer or gradiometer). Both SQUID and detection coils are typically mounted on a fiberglass support rod or on a fiberglass narrow, hollow cylinder. Shielded SQUID with its gradiometer together is usually called a sensor. The electrical leads or interconnects connect the sensor to associated electronic units stationed outside of a dewar at room temperature. Part of a channel that is physically attached to a fiberglass rod or a cylinder is called a probe. Additionally, the probes are essentially modular, so that each probe can be removed and inserted back into the dewar as necessary. Alternatively, all SQUID channels may be connected together in a common (non-modular) structure.
A response of a SQUID channel to a given input can be defined as a ratio of the output voltage to a combination of magnetic fields B=&PHgr;
B
/A found at the detector coils. The form of this combination depends on a gradiometer type. For example,
FIG. 1
illustrates a 2
nd
order symmetrical axial gradiometer consisting of three flat, axial, nominally identical coils or loops wound together. The loops contain 1-2-1 turns in the simplest implementation.
Because of the way the coils are wound, the supercurrents induced in the central loops flow in the direction opposite to the supercurrents in the two outer loops, so that the two outer coils produce signals of opposite polarity to the inner two-turn coil. Thus, this gradiometer produces a signal proportional to:
S
2
=B
Z
(
z
0
)−2
B
z
(
z
0
+l
)+
B
z
(
z
0
+2
l
)
where B
z
(z) is the z-component of magnetic field at a coordinate z, z
0
is the coordinate of a lower detection coil, and l is the distance between neighboring coils called gradiometer's base line, or base. In order to optimize signal-to-noise ratio (SNR), the base is chosen to be approximately equal to half of the distance from the lower detection coil to the magnetic field source (e.g., the heart). In gradiometers designed for heart measurements l is typically chosen to be about 5 cm, because the distance between the lower coil, placed about 2-3 cm above patient's chest, and the heart is approximately 10 cm in a typical adult.
Similarly, one can wind a 3
rd
order gradiometer, which would consist in the simplest implementation of 1-2-2-1 loops, and so on, for even higher orders (see for example A. I. Braginski, H. J. Krause, and J. Vrba, in Handbook of Thin Film Devices, edited by M. H. Francombe, v. 3: Superconducting Film Devices, Chapter 6, p. 149, Academic Press (2000), incorporated here as a reference).
A gradiometer of k's order acts almost as a magnetometer for nearby sources, while it subtracts spatially-constant magnetic field B
z
and spatial derivatives up to order (k−1): dB
z
/dz, dB
2
z
/dz
2
, etc. for distant sources. For example, a 2
nd
order gradiometer subtracts B and dB
z
/dz for distant sources. Thus, in this case the output voltage V divided by S
2
can be considered to be the absolute channel response, in units of Volts/Tesla.
The output voltage V is the result of all electromagnetic processes taking place in the numerous electrical components of a given channel, including induced currents in the detector coils, induction coupling between the input coil and the SQUID, voltage response of a SQUID, filtering and electronic amplification of a signal, etc. Thus, the total channel response mixes SQUID's transfer function with properties of the detection coils as well as with characteristics and settings of the associated SQUID electronics.
In order to determine absolute channel response V/S, one would need to know absolute values of the magnetic field at the positions of gradiometer detection coils. While this can be done, either by an actual measurement or by a calculation for a known field source, for the purposes of the specification, it is sufficient to find channel voltage response V alone, without dividing it by the magnetic signal S, as long as the field source is the same every time V is measured, as will be explained in more detail below. In what follows this voltage V is called the channel response.
As is clear, despite all possible precautions, nominally identical but physically different channels will inevitably present a certain spread of parameters. A number of factors contribute to channel-to-channel differences in the output response. Among these are geometrical and electronic factors.
Geometric factors are easily controlled. In the modular configuration, each probe is inserted into a specially designed space (notch) inside a dewar, so that its position and all the distances with respect to dewar's overall geometrical shape (shell) and to the other probes are fixed and reproduced as well as
Cariomag Imaging, Inc.
Katten Muchin Zavis & Rosenman
Patidar Jay
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