A method for imaging a plurality of particles using a particle analysis system configured for combined viscosity and geometric hydrofocusing, the particles included in a biological fluid sample having a sample fluid viscosity, the method comprising: flowing a sheath fluid along a flowpath of a flowcell , the sheath fluid having a sheath fluid viscosity that differs from the sample fluid viscosity by a viscosity difference in a predetermined viscosity difference range; injecting the biological fluid sample into the flowing sheath fluid within the flowcell so as to provide a sample fluid stream enveloped by the sheath fluid ; flowing the sample fluid stream and the sheath fluid through a reduction in flowpath size toward an imaging site , such that a viscosity hydrofocusing effect induced by an interaction between the sheath fluid and the sample fluid stream associated with the viscosity difference, in combination with a geometric hydrofocusing effect induced by an interaction between the sheath fluid and the sample fluid stream associated with the reduction in flowpath size, is effective to provide a target imaging state in at least some of the plurality of particles at the imaging site while a viscosity agent in the sheath fluid retains viability of cells in the sample fluid stream leaving structure and content of the cells intact when the cells extend from the sample fluid stream into the flowing sheath fluid ; and imaging the plurality of particles at the imaging site , characterized in that : the sheath fluid viscosity of the sheath fluid is greater than the sample fluid viscosity of the biological fluid sample , and the reduction in flowpath size is a reduction in cross-sectional area of the flowpath by a ratio in the range of 10: 1 to 100:1. flowing a sheath fluid along a flowpath of a flowcell , the sheath fluid having a sheath fluid viscosity that differs from the sample fluid viscosity by a viscosity difference in a predetermined viscosity difference range; injecting the biological fluid sample into the flowing sheath fluid within the flowcell so as to provide a sample fluid stream enveloped by the sheath fluid ; flowing the sample fluid stream and the sheath fluid through a reduction in flowpath size toward an imaging site , such that a viscosity hydrofocusing effect induced by an interaction between the sheath fluid and the sample fluid stream associated with the viscosity difference, in combination with a geometric hydrofocusing effect induced by an interaction between the sheath fluid and the sample fluid stream associated with the reduction in flowpath size, is effective to provide a target imaging state in at least some of the plurality of particles at the imaging site while a viscosity agent in the sheath fluid retains viability of cells in the sample fluid stream leaving structure and content of the cells intact when the cells extend from the sample fluid stream into the flowing sheath fluid ; and imaging the plurality of particles at the imaging site , characterized in that : the sheath fluid viscosity of the sheath fluid is greater than the sample fluid viscosity of the biological fluid sample , and the reduction in flowpath size is a reduction in cross-sectional area of the flowpath by a ratio in the range of 10: 1 to 100:1. the sheath fluid viscosity of the sheath fluid is greater than the sample fluid viscosity of the biological fluid sample , and the reduction in flowpath size is a reduction in cross-sectional area of the flowpath by a ratio in the range of 10: 1 to 100:1. <EOS>