*Article* **Is Hyperspectral Imaging Suitable for Assessing Collateral Circulation Prior Radial Forearm Free Flap Harvesting? Comparison of Hyperspectral Imaging and Conventional Allen's Test**

**Diana Heimes 1,\* ,† , Philipp Becker 2,†, Daniel G. E. Thiem <sup>1</sup> , Robert Kuchen <sup>3</sup> , Solomiya Kyyak <sup>1</sup> and Peer W. Kämmerer <sup>1</sup>**


**Abstract:** (1) Background: This cross-sectional study aims to compare a new and non-invasive approach using hyperspectral imaging (HSI) with the conventional modified Allen's test (MAT) for the assessment of collateral perfusion prior to radial forearm free flap harvest in healthy adults. (2) HSI of the right hand of 114 patients was recorded. Here, three recordings were carried out: (I) basic status (perfusion), (II) after occlusion of ulnar and radial artery (occlusion) and (III) after releasing the ulnar artery (reperfusion). At all recordings, tissue oxygenation/superficial perfusion (StO<sup>2</sup> (0–100%); 0–1 mm depth), tissue hemoglobin index (THI (0–100)) and near infrared perfusion index/deep perfusion (NIR (0–100); 0–4 mm depth) were assessed. A modified Allen's test (control) was conducted and compared with the HSI-results. (3) Results: Statistically significant differences between perfusion (I) and artery occlusion (II) and between artery occlusion (II) and reperfusion (III) could be observed within the population with a non-pathological MAT (each <0.001). Significant correlations were observed for the difference between perfusion and reperfusion in THI and the height of the MAT (*p* < 0.05). Within the population with a MAT >8 s, an impairment in reperfusion was shown (each *p* < 0.05) and the difference between perfusion and reperfusion exhibited a strong correlation to the height of the MAT (each *p* < 0.01). (4) Conclusions: The results indicate a reliable differentiation between perfusion and occlusion by HSI. Therefore, HSI could be a useful tool for verification of the correct performance of the MAT as well as to confirm the final diagnosis, as it provides an objective, reproducible method whose results strongly correlate with those obtained by MAT. What is more, it can be easily applied by non-medical personnel.

**Keywords:** hyperspectral imaging; Allen's test; radial forearm free flap; microvascular surgery; microsurgery; reconstructive surgery; perfusion monitoring; flap imaging

## **1. Introduction**

The "Chinese flap", the fascio-cutaneous radial forearm free flap (RFFF) was first described in 1981 by Yang et al. [1,2]. It is used for diverse reconstruction purposes with a survival rate of 97% [3,4]; the popularity of its use has increased due to its pliability and thinness, the ease of flap raising using a two-team approach, a consistent anatomy, and the long and high-caliber vascular pedicle [1–3]. As the flap is vascularized by a segment of the radial artery that needs to be removed during the surgery, it is essential to

**Citation:** Heimes, D.; Becker, P.; Thiem, D.G.E.; Kuchen, R.; Kyyak, S.; Kämmerer, P.W. Is Hyperspectral Imaging Suitable for Assessing Collateral Circulation Prior Radial Forearm Free Flap Harvesting? Comparison of Hyperspectral Imaging and Conventional Allen's Test. *J. Pers. Med.* **2021**, *11*, 531. https://doi.org/10.3390/jpm11060531

Academic Editor: Andreas Arkudas

Received: 16 May 2021 Accepted: 8 June 2021 Published: 9 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

ensure an adequate perfusion of the whole hand by the ulnar artery alone. Though, radial artery occlusion is a common complication with frequencies ranging from 1–33% [5]. Here, the Allen's test, a simple bedside test is traditionally used as a preoperative assessment ahead of RFFF harvest. Its validity depends on the degree of arterial compression applied by the examiner, as well as the subjective evaluation of the reperfusion and a potential error by the hyperextension of the hand [3,6]. Furthermore, there are technical differences in the test procedure as well as the cut-off point for reperfusion (the shorter the cut-off time, the more sensitive and less specific Allen's test is [7]). In addition, Allen's test requires the cooperation of the patient for its correct performance and does not give information about the vascular anatomy [8]. As acute ischemia of the hand after flap raising has been reported, even after satisfactory Allen's test results, further measurement methods have been evaluated to minimize the risk of such ischemic complications [3,4,6]. Here, arteriography is a very precise but also highly invasive method for assessing the vascular anatomy and perfusion of the donor hand. Color flow duplex scanning and doppler ultrasound are noninvasive options that require experience and rely on a subjective interpretation [3]. Besides, transcutaneous pulse oximetry has been shown to correlate with arterial doppler waveforms and therefore provides an objective means of monitoring of potential ischemic complications [9]. an adequate perfusion of the whole hand by the ulnar artery alone. Though, radial artery occlusion is a common complication with frequencies ranging from 1–33% [5]. Here, the Allen's test, a simple bedside test is traditionally used as a preoperative assessment ahead of RFFF harvest. Its validity depends on the degree of arterial compression applied by the examiner, as well as the subjective evaluation of the reperfusion and a potential error by the hyperextension of the hand [3,6]. Furthermore, there are technical differences in the test procedure as well as the cut-off point for reperfusion (the shorter the cut-off time, the more sensitive and less specific Allen's test is [7]). In addition, Allen's test requires the cooperation of the patient for its correct performance and does not give information about the vascular anatomy [8]. As acute ischemia of the hand after flap raising has been reported, even after satisfactory Allen's test results, further measurement methods have been evaluated to minimize the risk of such ischemic complications [3,4,6]. Here, arteriography is a very precise but also highly invasive method for assessing the vascular anatomy and perfusion of the donor hand. Color flow duplex scanning and doppler ultrasound are noninvasive options that require experience and rely on a subjective interpretation [3]. Besides, transcutaneous pulse oximetry has been shown to correlate with arterial doppler waveforms and therefore provides an objective means of monitoring of potential ischemic complications [9].

*J. Pers. Med.* **2021**, *11*, x FOR PEER REVIEW 2 of 17

of the radial artery that needs to be removed during the surgery, it is essential to ensure

Hyperspectral imaging (HSI) is an imaging modality for medical applications and has been tested for the determination of perfusion parameters for diabetic foot and skin ulcer [10–13], tissue perfusion measurement and wound analysis [14,15] as well as flap monitoring [16]. TIVITA™ (Diaspective Vision, Pepelow, Germany) is a new, contact-free HSI system for the assessment of tissue oxygenation and perfusion. It is an internal pushbroom imaging spectrograph (CMOS camera) and acquires a three-dimensional HyperCube with spatial (x, y; resolution 0.1 mm/pixel at 50 cm distance) and spectral (λ; resolution 5 nm) dimensions [17]. Every point in a row (x-axis) is analyzed in parallel; the row is moved along the y-axis and the spectral dimension (λ) is generated [18,19]. The system detects hemoglobin and its derivates oxyhemoglobin, deoxyhemoglobin and water [20]. Optical remission spectroscopy in the visible and near infrared range (400–1000 nm) allows contact-free acquisition of information about tissue properties, such as tissue oxygenation/superficial perfusion (StO<sup>2</sup> (0–100%); 0–1 mm depth), tissue hemoglobin index (THI (0–100)), near infrared perfusion index/deep perfusion (NIR (0–100); 0–4 mm depth) and tissue water index (TWI (0–100)) without influencing the tissue (Figure 1). Hyperspectral imaging (HSI) is an imaging modality for medical applications and has been tested for the determination of perfusion parameters for diabetic foot and skin ulcer [10–13], tissue perfusion measurement and wound analysis [14,15] as well as flap monitoring [16]. TIVITA™ (Diaspective Vision, Pepelow, Germany) is a new, contact-free HSI system for the assessment of tissue oxygenation and perfusion. It is an internal pushbroom imaging spectrograph (CMOS camera) and acquires a three-dimensional HyperCube with spatial (x, y; resolution 0.1 mm/pixel at 50 cm distance) and spectral (λ; resolution 5 nm) dimensions [17]*.* Every point in a row (x-axis) is analyzed in parallel; the row is moved along the y-axis and the spectral dimension (λ) is generated [18,19]. The system detects hemoglobin and its derivates oxyhemoglobin, deoxyhemoglobin and water [20]. Optical remission spectroscopy in the visible and near infrared range (400–1000 nm) allows contact-free acquisition of information about tissue properties, such as tissue oxygenation/superficial perfusion (StO2 (0–100%); 0–1 mm depth), tissue hemoglobin index (THI (0–100)), near infrared perfusion index/deep perfusion (NIR (0–100); 0–4 mm depth) and tissue water index (TWI (0–100)) without influencing the tissue (Figure 1).

**Figure 1.** Wavelengths of different types of light. The hyperspectral camera processes visual light with a wavelength from 380 to 740 nm and light in the near infrared range from 750 to 1000 nm. **Figure 1.** Wavelengths of different types of light. The hyperspectral camera processes visual light with a wavelength from 380 to 740 nm and light in the near infrared range from 750 to 1000 nm.

StO2 reflects the percentage of hemoglobin oxygen saturation in the capillary area of the tissue microcirculation and shows changes in tissue oxygenation. Thus, StO2 represents the tissue oxygen saturation, which is mainly due to the blood volume in the venous part (75%) of the microcirculation after the oxygen has been released to the surrounding tissue. The parameter NIR perfusion (near infrared) describes the quality of the blood flow, which is determined by the relative oxygen saturation of the hemoglobin and the relative hemoglobin content in the microcirculatory system in deeper tissue layers of 4 to 6 mm. This parameter can be used to identify undersupplied tissue areas in deeper layers. The color scale ranges from red (high perfusion) to blue (low perfusion). THI describes StO<sup>2</sup> reflects the percentage of hemoglobin oxygen saturation in the capillary area of the tissue microcirculation and shows changes in tissue oxygenation. Thus, StO<sup>2</sup> represents the tissue oxygen saturation, which is mainly due to the blood volume in the venous part (75%) of the microcirculation after the oxygen has been released to the surrounding tissue. The parameter NIR perfusion (near infrared) describes the quality of the blood flow, which is determined by the relative oxygen saturation of the hemoglobin and the relative hemoglobin content in the microcirculatory system in deeper tissue layers of 4 to 6 mm. This parameter can be used to identify undersupplied tissue areas in deeper layers. The color scale ranges from red (high perfusion) to blue (low perfusion). THI describes the existing hemoglobin distribution in the superficial microcirculatory system.

By this means, index parameter problems with the arterial supply or the venous drainage can be recognized. The color scale ranges from red (high hemoglobin content) to blue (low hemoglobin content). As an index value, TWI describes the relative water content in the tissue area under consideration [21]. The tissue is irradiated with white light and the remitted light is detected; scattering and absorption by tissue structures depends on the wavelength [13,19] and light penetration depth is approximately 0.8 mm (500 nm) to 2.6 mm (1000 nm). The method has previously been described by Klucke et al. [17].

In the need of an objective, reliable and investigator-independent method to evaluate the vascular perfusion of the donor hand, this cross-sectional study aims to compare a new and non-invasive approach using HSI with the conventional Allen's test for the assessment of collateral perfusion prior to RFFF harvest in healthy adults. The findings obtained in this study will be used to set limits for the evaluation of hyperspectral data, in order to facilitate interpretation and perioperative assessment.
