Doppler physics

Human hearing can detect frequencies ranging from 20 Hz to 20 KHz. “Ultra”sound operates at frequencies higher (>20 KHz) than the audible range. Systems typically used in medicine have ranges far exceeding this, usually in the 1 to 20 MHz range. In the most basic state, ultrasound detection uses short pulses of high-frequency sound emitted into the body. Combinations of reflection, absorption, and scatter of sound waves to varying tissues are received to form the signal. With the addition of the Doppler effect to ultrasonography, the frequency of the reflected sound can be used to determine the direction of blood flow. Changes in signal intensity provide valuable information with regard to blood flow. Typically, the unidirectional Doppler ultrasound works by these principles.

Doppler: transitioning from preoperative to intraoperative tool

The handheld Doppler unit ( Fig. 1 ) has proven to be one of the most versatile and widely used devices in autologous and especially microvascular breast reconstruction. Commonly used as a postoperative tool for free flap monitoring, preoperative and intraoperative efficacy have also been demonstrated. Preoperatively, it is sometimes used to localize perforator location on the cutaneous surface of the flap. Intraoperatively, it can be used to assist with perforator selection by discriminating between the various signal intensities. In the setting of abdominal or gluteal flap breast reconstruction, knowledge of perforator density and caliber can aid in faster and more effective flap elevation. Although some surgeons prefer the use of CTA, confirmation of the CTA data is almost always performed using a Doppler device. In a prior study using CTA for gluteal flap planning, it was demonstrated that there were an average of 11 perforators in the superior gluteal artery territory with a mean diameter of 0.6 mm. This was very similar to the comparative findings of Doppler ultrasound, as there were 9 perforators with a mean diameter of 0.4 mm. The location of the CTA-documented perforators was confirmed using Doppler.

A standard handheld Doppler unit found throughout nursing units and operating rooms.

The Doppler systems have played a prominent role in the planning and harvest of abdominal flaps. At the skin level, the handheld Doppler can predict the deeper location of abdominal perforators before any dissection. The periumbilical region consistently provides most usable abdominal perforators. Giunta and colleagues evaluated using the handheld Doppler in 46 patients before undergoing breast reconstruction. They found an average of 3.6 perforators in one-sided deep inferior epigastric perforator (DIEP) flap and 3.6 perforators in the superior gluteal artery perforator (SGAP) flap with preoperative Doppler assessment. Interestingly, when the preoperative Doppler skin signals were compared with intraoperative findings, a high false-positive rate of 47.6% and a false-negative rate of 11.0% were demonstrated. In addition, it was found that there was only minor discrepancy between the locations of the skin signal in relation to the deep location of each perforator (average of 0.8 cm vertically and 0.8 cm horizontally). This emphasizes the importance of possibly predicting the location of each perforator to approximately 1 square centimeter. The perforator compression test can potentially decrease a high false-positive rate. By applying external pressure with the Doppler probe, true perforator signals will vanish, leaving deeper source vessels as the remaining signal. This test can help to determine the origin of the Doppler signal.

Clinical studies using Doppler ultrasound have been previously reported. Blondeel and colleagues incorporated Doppler ultrasound in preoperative evaluation for patients who were to undergo DIEP and SGAP reconstruction. Approximately 4.5 perforators were indentified for each side of the abdominal flap and an average of 2.6 perforators for each SGAP flap. As related to the intraoperative findings, Blondeel and colleagues reported a true positive rate of 80.6% and a positive predictive value of 91.9%. These studies highlight the sensitivity, sometimes oversensitivity, of the handheld Doppler at identifying cutaneous vessels. This is an important observation because many perforators are visualized during the elevation of an abdominal flap, many of which are small and will be sacrificed because of suboptimal location or caliber.

The success and efficiency of perforator-based breast reconstruction relies on the surgeon’s experience with choosing the correct perforator(s) and with the technical aspect of perforator dissection. The handheld Doppler can aid the surgeon in selecting the best perforator(s) to optimize flow and perfusion within a flap and minimize the incidence of fat or partial flap necrosis. The following sections will describe the technique of using the handheld Doppler in identifying and selecting reliable perforators.

Doppler: transitioning from preoperative to intraoperative tool

The handheld Doppler unit ( Fig. 1 ) has proven to be one of the most versatile and widely used devices in autologous and especially microvascular breast reconstruction. Commonly used as a postoperative tool for free flap monitoring, preoperative and intraoperative efficacy have also been demonstrated. Preoperatively, it is sometimes used to localize perforator location on the cutaneous surface of the flap. Intraoperatively, it can be used to assist with perforator selection by discriminating between the various signal intensities. In the setting of abdominal or gluteal flap breast reconstruction, knowledge of perforator density and caliber can aid in faster and more effective flap elevation. Although some surgeons prefer the use of CTA, confirmation of the CTA data is almost always performed using a Doppler device. In a prior study using CTA for gluteal flap planning, it was demonstrated that there were an average of 11 perforators in the superior gluteal artery territory with a mean diameter of 0.6 mm. This was very similar to the comparative findings of Doppler ultrasound, as there were 9 perforators with a mean diameter of 0.4 mm. The location of the CTA-documented perforators was confirmed using Doppler.

A standard handheld Doppler unit found throughout nursing units and operating rooms.

The Doppler systems have played a prominent role in the planning and harvest of abdominal flaps. At the skin level, the handheld Doppler can predict the deeper location of abdominal perforators before any dissection. The periumbilical region consistently provides most usable abdominal perforators. Giunta and colleagues evaluated using the handheld Doppler in 46 patients before undergoing breast reconstruction. They found an average of 3.6 perforators in one-sided deep inferior epigastric perforator (DIEP) flap and 3.6 perforators in the superior gluteal artery perforator (SGAP) flap with preoperative Doppler assessment. Interestingly, when the preoperative Doppler skin signals were compared with intraoperative findings, a high false-positive rate of 47.6% and a false-negative rate of 11.0% were demonstrated. In addition, it was found that there was only minor discrepancy between the locations of the skin signal in relation to the deep location of each perforator (average of 0.8 cm vertically and 0.8 cm horizontally). This emphasizes the importance of possibly predicting the location of each perforator to approximately 1 square centimeter. The perforator compression test can potentially decrease a high false-positive rate. By applying external pressure with the Doppler probe, true perforator signals will vanish, leaving deeper source vessels as the remaining signal. This test can help to determine the origin of the Doppler signal.

Clinical studies using Doppler ultrasound have been previously reported. Blondeel and colleagues incorporated Doppler ultrasound in preoperative evaluation for patients who were to undergo DIEP and SGAP reconstruction. Approximately 4.5 perforators were indentified for each side of the abdominal flap and an average of 2.6 perforators for each SGAP flap. As related to the intraoperative findings, Blondeel and colleagues reported a true positive rate of 80.6% and a positive predictive value of 91.9%. These studies highlight the sensitivity, sometimes oversensitivity, of the handheld Doppler at identifying cutaneous vessels. This is an important observation because many perforators are visualized during the elevation of an abdominal flap, many of which are small and will be sacrificed because of suboptimal location or caliber.

The success and efficiency of perforator-based breast reconstruction relies on the surgeon’s experience with choosing the correct perforator(s) and with the technical aspect of perforator dissection. The handheld Doppler can aid the surgeon in selecting the best perforator(s) to optimize flow and perfusion within a flap and minimize the incidence of fat or partial flap necrosis. The following sections will describe the technique of using the handheld Doppler in identifying and selecting reliable perforators.