You might be thinking of circular array tomography, a computer
assisted imaging techinque.
http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11303935&dopt=Abstract
J Acoust Soc Am. 2001 Mar;109(3):1219-25.
Acoustic nonlinearity parameter tomography for biological tissues via
parametric array from a circular piston source--theoretical analysis
and computer simulations.
Zhang D, Chen X, Gong X.
State Key Lab of Modern Acoustics, Institute of Acoustics, Nanjing
University, China.
The acoustic nonlinearity parameter B/A describes the nonlinear
features of a medium and may become a novel parameter for ultrasonic
tissue characterization. This paper presents a theoretical analysis
for acoustic nonlinear parameter tomography via a parametric array. As
two primary waves of different frequencies are radiated simultaneously
from a circular piston source, a secondary wave at the difference
frequency is generated due to the nonlinear interaction of the primary
waves. The axial and radial distributions of sound pressure amplitude
for the generated difference frequency wave in the near field are
calculated by a superposition of Gaussian beams. The calculated
results indicated that the difference frequency component of the
parametric array grows linearly with distance from the piston source.
It therefore provides a better source to do the acoustic nonlinearity
parameter tomography because the fundamental and second harmonic
signals both have a near field that goes through many oscillations due
to diffraction. By using a finite-amplitude insert substitution method
and a filtered convolution algorithm, a computer simulation for B/A
tomography from the calculated sound pressure of the difference
frequency wave is studied. For biological tissues, the sound
attenuation is considered and compensated in the image reconstruction.
Nonlinear parameter computed tomography (CT) images for several
biological sample models are obtained with quite good quality in this
study.
PMID: 11303935 [PubMed]
http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=6606811&dopt=Abstract
Phys Med Biol. 1983 Nov;28(11):1219-34.
High-resolution circular ring
positron tomograph with dichotomic sampling: Dichotom-I.
Cho ZH, Hilal SK, Ra JB, Hong KS, Bigler RE, Yoshizumi T, Wolf AP,
Fowler JS.
The circular ring transaxial positron camera developed earlier was
refitted with a new dichotomic sampling scheme and aperture
collimators on the detector array to improve the sampling and the
overall system resolution. The z-axis slice thickness collimators were
also limited to 1 cm, which corresponds to a slice thickness of 0.5 cm
FWHM. Two different types of aperture collimators were adopted for
high resolution (HR) and very high resolution (VHR) imaging,
respectively. In HR mode a resolution of 6.5 mm FWHM was obtained
without appreciable degradation of overall sensitivity, which
represents a threefold improvement in resolution over the original
system. In phantom studies with HR mode a sensitivity of 4500 counts
s-1 muCi-1 cm-3 was obtained for a 20 cm diameter uniform phantom
filled with water. A VHR mode experiment was also conducted to observe
the ultimate resolution capability of the Dichotom-I system, and a
resolution of 4.2 mm FWHM was obtained at the expense of sensitivity
which was reduced by a factor of four from the HR mode experiment. The
experience gained with Dichotom-I suggests a relatively simple and
inexpensive modification of the existing NaI(T1) ring positron
cameras, most of which suffer from low resolution due to poor sampling
and poor intrinsic detector resolution.
PMID: 6606811 [PubMed - indexed for MEDLINE]
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