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Computed tomographic (CT) and high-resolution ultrasound (HRUS) imaging of experimental brain abscess were correlated with neuropathologic findings in nine mongrel dogs. The HRUS scan was more sensitive to different histologic features than the CT scan but both accurately delineated the evolution of the experimental brain abscess. All stages of abscess evolution were characterized by an appearance of an echogenic rim with a hypoechoic center. In the early stages the echogenicity of the abscess was related primarily to marked cellular infiltration, while in the late stages extensive collagen deposition correlated closely with the echo pattern. The size of the abscess in the cerebritis stages appeared smaller on the HRUS scan than on the CT scan because the latter modality detected the extensive cerebritis around the developing necrotic center whereas the HRUS scan did not. This discrepancy disappeared in the capsule stages. The HRUS scan provided a more accurate depiction of the neuropathologic characteristics of the necrotic than did the CT scan. Healing of the abscess, indicated by a decrease in size of the hypoechoic center, was accurately detected by the HRUS scan.

A high-sensitivity superconducting magnetometer has been used to measure geomagnetic activity at Stanford, California, in the 0.1-to 14-Hz frequency range. The measurements, which covered the 2-month interval January 26 to March 26, 1974, consistently show a minimum of activity in the interval 3-7 Hz, where conventional measurement systems are likely to be limited by their internal noise. At frequencies, below the minimum, in the range 0.1-3 Hz, there is typically a monotonic decrease of background activity with frequencyf. The variation is approximately of the form f-' 25 for the Pc 1 frequency range (0.2-5 Hz). At frequencies above the minimum, in the range 7-14 Hz, the activity is dominated by the first Schumann resonance. The peak amplitude of this activity is usually low in comparison with the amplitude of the background activity at 1 Hz. No splitting of the first Schumann resonance peak into a triplet could be detected, but a doublet structure was observed on 7 days in the 2-month measurement interval. This paper reports high-sensitivity measurements of geomagnetic activity in the frequency range 0.1-14 Hz made with a superconducting magnetometer system [Buxton andFraser-Smith, 1974] at a mid-latitude location (Stanford, California; geomagnetic latitude 43.5øN). The data are new in two respects. First, unlike past measurements, which emphasize the properties of particular large-amplitude events, our measurements are concerned primarily with the properties of the underlying component of geomagnetic activity in the Pc I pulsation frequency range (0.2-5 Hz) which forms a comparatively stable background to the better studied largeamplitude events. We believe that the data presented here are the first fully calibrated measurements of this background activity. Second, our measurements consistently show a minimum of activity in the interval 3-7 Hz (i.e., at the junction of the ultralow frequency band (ULF: frequencies less than •5 Hz) with the extremely low frequency band (ELF: frequencies in the range •5 Hz to 3kHz)), where conventional ULF/ELF measurement systems are likely to be limited by their internal noise. The internal noise level of the Stanford superconducting magnetometer is significantly lower than the noise level in these conventional systems, and it has been shown to be lower than the level of natural background activity in the frequency range 3-7 Hz [Buxton and Fraser-Smith, 1974]. Thus our data are probably the first largely noise-free measurements of geomagnetic activity at frequencies near 5 Hz. In addition to these two features of our data the superconducting magnetometer measurements fill a gap between background measurements made by other investigators at frequencies below 1 Hz and above 7 Hz (both frequencies are approximate; superconducting magnetometer measurements at lower frequencies have recently been reported by Zimmerman and Campbell [1975]). It is possible as a result to relate the natural background in the frequency range 0.1-14 Hz to the background outside the frequency range, i.e.,...

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We present the first measurements of spectra that characterize noise in the frequency range 5×10−4 to 20 Hz of a superconducting magnetic gradiometer operating in a magnetically quiet environment. Spectra are effectively white at frequencies above about 0.1 Hz with a spectral power density of 0.03 (pT/m)2 Hz−1(1 pT/m=10−12 T/m=10−10 G/cm =10−3 γ/m) and are of the form S (f) =S (fo)(fo/f)γ at frequencies below 0.1 Hz. A value of 1.3 for γ with S (fo) =0.002 (pT/m)2 Hz−1 at fo=1Hz is representative of values observed during the quietest observational periods. Intrinsic noise of the superconducting quantum interference device (SQUID) then dominates observed spectra at frequencies below 0.1 Hz.