An integrated dc squid gradiometer for biomagnetic application

Abstract

SUMMARYA f i r s t -o r d e r off-diagonal gradiometer f o r biomagnetic measurement is fabricated and tested. The gradiometer consisted of two f i e l d pick-up c o i l s , a planar-type DC-SQUID with two superconducting loops connected i n p a r a l l e l , two multiturn input c o i l s , and a modulation-feedback c o i l . The s i z e of t h e pick-up c o i l s was 6 x 6 mm2 with a basel i n e of 8 mm. The o v e r a l l s i z e of t h e gradiometer was 15 x 7.5 mm2. The r e s o l u t i o n of t h e magnetic f i e l d gradient of t h e gradiometer increased from 11 t o 1 . 8 p T / m / m corresponding t o frequencies from 1 t o 600 Hz. The resolution became nearly white i n t h e frequency range above 100 Hz. The i n t r i n s i c balance of t h e gradiometer was b e t t e r than 1000 ppm f o r t h e f i e l d perpendicular t o i t s plane. The magnetocardiogram and t h e alpha rhythmic f i e l d s were obtained e a s i l y using t h i s gradiometer i n a conventional e l e c t r omagnetically shielded room. . Introduction Magnetic f i e l d s caused by a c t i v i t i e s ofHowever, t h e biomagnetic t h e human brain o r h e a r t are very weak (10-10 -10-14 T).f i e l d could n o t be measured without t h e improvement of SQUID (superconducting quantum i n t e r f e r e n c e device) [ 1 1 . Biomagnetic m e asurement using SQUID has t h e following chara c t e r i s t i c s : (1) noninvasive o r noncontact measurement; and ( 2 ) estimation of t h e source l o c a l i z a t i o n from t h e s p a t i a l d i s t r i b u t i o n of t h e magnetic f i e l d measured o u t s i d e t h e body. Therefore, biomagnetic measurement i s a useful method f o r t h e i n v e s t i g a t i o n of b r a i n functions [ 2 ] . Magnetically shielded rooms made of high permeability materials are uncommon because of t h e i r high cost. Thus, i t is necessary t o develop a SQUID with which t h e biomagnetic f i e l d can be measured i n an ambient laboratory o r i n a conventional electromagn e t i c a l l y shielded room. For t h i s purpose, t h e f i r s t -and t h e second-order SQUID gradiometers have been used [6-131. The gradiometer has a s p e c i a l arrangement of pick-up c o i l s t o eliminate magnetic f i e l d s caused by o r i g i n s f a r from t h e gradiometer. 88 Medical research i n t h i s f i e l d a l s o i s being c a r r i e d out. It includes t h e measurement of t h e magnetocardiogram (MCG) of myoc a r d i a l i n f a r c t i o n [ 3 , 41, and source l o c a li z a t i o n of f o c a l epilepsy [ 51. The biomagnetic f i e l d is very weak comTherefore, i t is important t o pared with t h e Earth magnetic f i e l d (%5 x 10-5 T). develop a highly s e n s i t i v e SQUID and a l s o t o eliminate t h e EarthThere are two types of SQUID: t h e RF-SQUID which has one Josephson junction, and the DC-SQUID which has two Josephson junct i o n s . I n t h e e a r l y stage, t h e pick-up c o i l was made by winding superconducting w i r e around a bobbin. The input c o i l connected superconductive1.y t o t h ...

Development of SQUID sensor for nondestructive evaluation: Optimization of the integrated concentric DC‐SQUID gradiometer

“…Since the bias current to the device was fed through the damping resistors, a resistive gradient is seen at the maximum critical current on the IV characteristic [11]. 4(a) and 4(b).…”

“…L spoke is the inductance of the radial spoke-shaped wiring from spot C. Figure 2 shows the equivalent circuit of the concentric multiloop DC-SQUID gradiometer. In order to achieve symmetrical feed of the bias current to the device, damping resistors (R d ) are installed in the bias leads [11].…”

“…The modulated voltage 'V was 40 PV. Since the bias current to the device was fed through the damping resistors, a resistive gradient is seen at the maximum critical current on the IV characteristic [11]. L s was estimated from the critical current modulation of the IV characteristic and M fs was estimated from the )V characteristic.…”