Photoacoustic detection shows its sensitive side
Lauren I. Rugani
Detection of metastasis often
proves to be difficult, either because investigation comes too soon in the development
of tumor cells or because the cells have begun to disseminate away from the primary
tumor site. Current detection techniques include lymph node or bone marrow assays
and tracking the spread of stained cells through the lymphatic system. Besides the
inaccuracy of such methods, it has been predicted that malignant melanomas can bypass
the lymphatic system altogether and instead spread through the circulatory system.
Because tumor cells are estimated to appear in
the circulatory system on the order of one cell per 106 normal blood cells, detection
techniques must achieve an accurate sensitivity to such low concentrations. To meet
this requirement, a team of researchers from the University of Missouri-Columbia
has proposed an in vitro photoacoustic detection method, in which pulsed laser energy
is absorbed by a chromophore — the region of a cell that is responsible for
its color — and acoustic energy propagates as a result of thermoelastic expansion
of the cells. Because the signal emitted by melanoma cells is unique, they cannot
be mistaken for normal cells.
The detection mechanism employs a frequency-tripled
450-nm Nd:YAG laser from Opotek Inc. of Carlsbad, Calif., that delivers 5-ns pulses
to a customized flow cell used for excitation and subsequent acoustic wave detection
of a solution containing a tissue sample with a human malignant melanoma cell line.
The resulting photoacoustic signals — detected by a polyvinylidene difluoride
film attached to the flow cell — are converted to voltage signals.
Researchers customized a flow cell to enable photoacoustic detection
of melanoma cells in solution, adding a piezoelectric polyvinylidene difluoride
(PVDF) film to record the acoustic signals generated by the cells after they were
irradiated with an Nd:YAG laser. Schematic reprinted with permission of Optics Letters.
The investigators tested the sensitivity
of the technique by introducing latex microspheres as melanoma tissue phantoms,
placing the spheres in saline solution in concentrations ranging from 700 to 7.12
x 106 microspheres per milliliter. Although the detection limit for these trials
was 700 spheres per 20 ml of solution, accounting for the area of the excitation
path may enable as few as 10 cells to be present to maintain a definitive signal.
They found a linear relationship between the concentration of phantom melanoma cells
in the solution and the peak excitation value.
They then conducted trials using a
concentration of 2.3 x 106 live melanoma cells in the same amount of solution. Irradiation
of the sample with 12 mJ of light energy resulted in a peak excitation after 2.5
μs — an expected signal for melanoma. A staining technique applied to
the cells revealed that <2 percent of the cells contained melanin. The researchers
thus likened melanoma detection to a 4-mV signal for ∋80 melanin-containing
cells in the excitation path.
The linear relationship between excitation
and cell concentration illustrates the system’s ability to discern different
acoustic wave magnitudes and to determine unknown melanoma concentrations by analyzing
peak excitation values. The technique can be applied during any stage of metastasis;
early detection may help in disease prevention, whereas application in later stages
may help analyze the effectiveness of chemotherapeutic treatments.
The researchers plan to advance their
studies to clinical trials involving patients with class IV metastasis in an attempt
to detect melanoma cells in the bloodstream. Other trials may include attaching
secondary molecules or antibodies to enhance detection.
Optics Letters, Oct. 15, 2006, pp. 2998-3000.
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