Hyperspectral imaging (HSI) is by no means new. Its first use was as a remote sensor to map earth from space, back in the 1970’s. It is, however, an emerging non-invasive clinical screening technology. There are published reports of it being employed to identify and delineate tumor margins, guide surgical planning and assess tissue perfusion of diabetic foot ulcers. We were asked to comment on its utility and suitability in wound care and hyperbaric medicine. Our analysis follows.
HSI is an optical tool that captures sought after information in the form of color pixels in a two-dimensional format. It does so very quickly and there is little to no inter-operator variability while obtaining data…so “operator independent”. HSI is able to measure oxy-hemoglobin through its visual light wavelength. Being an oxy-hemoglobin surrogate, it does not directly measure tissue oxygen per se (which tcpO2 does), and importantly it does not measure plasma-bound oxygen.
When evaluating diabetic foot ulcers, its use has been to determine predictability of healing based upon the amount of oxy-hemoglobin (oxygen delivery) and, interestingly, amount of deoxy-hemoglobin (oxygen consumption). To our knowledge, it has not been used to assess the impact of any related therapeutic intervention for a hypoxic foot wound.
Plasma-bound oxygen is relevant and key to hyperbaric referral assessment and case management. First, we want to know the impact of increased plasma-borne oxygen delivery while breathing 100% oxygen at 1.0 ATA. As a patient breaths oxygen, oxy-hemoglobin concentration increases by a fraction (and this change is measured by HSI) while plasma oxygen concentration increases enormously (and not able to be measured by HSI). This concentration increase, as you will appreciate, is key to determining the extent of local hypoxia reversibility/HBO candidacy. Likewise, HSI cannot be used for in-chamber assessments to determine appropriateness of hyperbaric dose delivery, as it does not have the physical interconnect capability, it would not be intrinsically safe anyway, and again it is not able to measure the impact of HBO’s increased plasma-borne oxygenation levels.
HSI is potentially useful in the wound-healing center setting, as a quick (albeit expensive), assessment of DFU healing likelihood. This is precisely what tcpO2 assessments do, of course. As noted above, HSI is only measuring the state of microvascular perfusion. It does not identify whether any abnormal state is large vessel mediated, something that tcpO2 does very effectively, via 100% oxygen challenge. If an HSI finding suggests low DFU healing likelihood, then HBO referral is an option, given its ability to overcome microvascular impairment via induction of angiogenesis. Of course, for HBO candidacy, a 100% oxygen challenge is required, to confirm the physiologic capacity to respond at the ulcer site to centrally delivered oxygen (adequacy of arterial inflow).
What is clear from all of this is that if HSI is employed, it should always be backed up by a tcpO2 capability. Even in the absence of an HBO referral, tcpO2 measurements will identify whether concurrent large vessel disease is in play. If it is, the patient undergoes further arterial workup to determine any flow augmentation need/possibility. While HSI has applications in several diagnostic settings, we would not recommend investment in this technology for the wound care and hyperbaric medicine setting.
One final aspect is that the reimbursement process for HSI is presently unclear. Some have recommended using the tcpO2 code; others have warned that this would be inappropriate.