A. Duncan et al., MEASUREMENT OF CRANIAL OPTICAL-PATH LENGTH AS A FUNCTION OF AGE USINGPHASE-RESOLVED NEAR-INFRARED SPECTROSCOPY, Pediatric research, 39(5), 1996, pp. 889-894
Near infrared spectroscopy (NIRS) has been used to measure concentrati
on changes of cerebral hemoglobin and cytochrome in neonates, children
, and adults, to study cerebral oxygenation and hemodynamics. To deriv
e quantitative concentration changes from measurements of light attenu
ation, the optical path length must be known. This is obtained by mult
iplying the source/ detector separation by a laboratory measured diffe
rential path length factor (DPF) which accounts for the increased dist
ance traveled by light due to scattering. DPF has been measured by tim
e of flight techniques on small populations of adults and postmortem i
nfants. The values for adults are greater than those for newborns, and
it is not clear how to interpolate the present data for studies on ch
ildren. Recent developments in instrumentation using phase resolved sp
ectroscopy techniques have produced a bedside unit which can measure o
ptical path length on any subject. We have developed an intensity modu
lated optical spectrometer which measures path length at four waveleng
ths. Two hundred and eighty three subjects from 1 d of age to 50 y wer
e studied. Measurements were made at a fixed frequency of 200 MHz and
a source detector separation of 4.5 cm. Results suggest a slowly varyi
ng age dependence of DPF, following the relation DPF690 = 5.38 + 0.049
A(0.877), DPF744 = 5.11 + 0.106A(0.723), DPF807 = 4.99 + 0.067A(0.814)
, and DPF832 = 4.67 + 0.062A(0.819) where DPF690 is the DPF measured a
t 690 nm and A is age is expressed in years from full term. There was
a wide scatter of values, however, implying that ideally DPF should be
measured at the time of each study.