Imaging in Propeller Flap Surgery

Abstract

Since propeller flaps are elevated as island flaps and most often nourished by a single perforator nearby the defect, it is challenging to change the flap design intraoperatively when a reliable perforator cannot be found where expected to exist. Thus, accurate preoperative mapping of perforators is essential in the safe planning of propeller flaps. Various methods have been reported so far: (1) handheld acoustic Doppler sonography (ADS), (2) color duplex sonography (CDS), (3) perforator computed tomographic angiography (P-CTA), and (4) magnetic resonance angiography (MRA). To facilitate the preoperative perforator assessment, P-CTA is currently considered as the gold standard imaging tool in revealing the three-dimensional anatomical details of perforators precisely. Nevertheless, ADS remains the most widely used tool due to its low cost, faster learning, and ease of use despite an undesirable number of false-positive results. CDS can provide hemodynamic characteristics of the perforator and is a valid and safer alternative particularly in patients in whom ionizing radiation and/or contrast exposure should be limited. Although MRA is less accurate in detecting smaller perforators of caliber less than 1.0 mm and the intramuscular course of perforators at the present time, MRA is expected to improve in the future due to the recent developments in technology, making it as accurate as P-CTA. Moreover, it provides the advantage of being radiation-free with fewer contrast reactions.

As described in the definition and classification of propeller flaps presentation in this issue, ¹ propeller flaps can be defined as being (1) island-shaped, (2) sharing a common axis with the perforators, and (3) having the ability to be rotated around that axis ( Fig. 1 ). ² Because propeller flaps allow the transposition of nearby similar tissues to a defect, they carry the advantages of local flaps offering similar skin color and texture as that of the defect area, resulting in good aesthetic outcomes. Additionally, propeller flaps allow shorter operating times since they usually do not require a microsurgical anastomosis. Thus, propeller flaps seem to be more feasible and a better alternative to free flaps, yet they still require accurate patient selection and adequate preoperative planning. Free flaps allow large skin transfers from a distant site, whereas propeller flaps require prior planning due to the use of a limited amount of local skin. Therefore, free flaps are still the gold standard for large defects, and propeller flaps should be considered to be best suited for small-to-medium defects. A propeller flap most commonly is nourished by a single perforator, and when the perforator cannot be found intraoperatively, it is always challenging to change the flap design during the operation or even impossible. Thus, accurate preoperative assessment of reliable perforators is very important in the safe planning of propeller flaps. The ideal perforator assessment tools must be accurate, fast, reproducible, noninvasive, and cost-effective.

Fig. 1.

Schematic illustration of a perforator-based propeller flap.

Preoperative Perforator Assessment Tools

Various methods have been reported to assess the anatomical characteristics of the perforator: (1) handheld acoustic Doppler sonography (ADS), (2) color Duplex sonography (CDS), (3) perforator computed tomographic angiography (P-CTA), and (4) magnetic resonance angiography (MRA). The characteristics of these four tools are summarized in Table 1 . ³

Table 1. Comparison of basic characteristics of perforator assessment tools.

Preoperative planning tools	ADS	CDS	P-CTA	MRA
Portability	Excellent	Good	Not portable	Not portable
Cost/examination (US$)	None or low	Moderate (200)	Relatively high (400)	High (600)
Invasiveness	None	None	Injection of IV contrast	Injection of IV contrast
Operator dependence	Yes	Yes	No	No
Reproducibility	+	+	+++	++
Learning curve	Little	Significant	No	No
Accuracy of perforator detection	High false-positive a	Relatively high	High	High
Hemodynamic information of perforator	No	Yes	No	No
Time to image acquisition	Depends on the operator (∼ 10 min)	Depends on the operator (∼ 30 min)	Short (< 30 s)	Long (20 min)
Resolution (minimal detectable perforator caliber)	No image	0.5 mm	0.3–0.5 mm	1.0 mm
3D view	No image	No	Yes	Yes
Contrast material	–	–	+	+ (safer risk profiles, e.g., gadolinium)
Ionizing radiation exposure	–	–	+	–
Contraindications	None	None	Metal implants b Allergy to the contrast agent Renal insufficiency

Abbreviations: 3D, three-dimensional; ADS, handheld acoustic Doppler sonography; CDS, color duplex sonography; IV, intravenous; MRA, magnetic resonance angiography; P-CTA, perforator computed tomographic angiography.

^a A false-positive rate is exaggerated in thin patients and reversed in obese patients. ¹²

^bPerforators may not be clearly determined because of the artifacts of metal implants.

Handheld Acoustic Doppler Sonography

Handheld ADS was the first method described for the assessment of perforators. It is still the most commonly used because it is handy, portable, and relatively inexpensive. ⁴ Handheld ADS is available in 8 and 10 MHz that can detect vessels located up to 20 mm from the skin surface. It offers the simplicity of use and a lesser learning curve. Handheld ADS can be used to identify the location of perforators preoperatively, confirm the pulsation of a pedicle perforator with a sterile probe intraoperatively, and monitor flap perfusion postoperatively. Although handheld ADS is a convenient tool to detect the location of perforators, the disadvantage of this technique is that it is too sensitive, demonstrating a high false-positive rate, up to 47%. ⁵

Khan and Miller revealed that perforators with a smaller diameter (< 0.4 mm) and a tortuous course show unacceptably high false-positive results. ⁶ It is also suggested that the false-positive rate is exaggerated in thin patients and just the reverse in obese patients. ⁷

Because there is no visualization of the audible signals, it is sometimes difficult to distinguish the acoustic signal of perforators from the main source vessels themselves. Moreover, handheld ADS may find perforators that are too small to sustain a perforator flap. Therefore, the obtained information has a wide margin of interpretation possibilities. When searching for perforators with ADS signals, it has been advised to vary the amount of pressure and angle applied with the probe to the skin surface. The sound made by the main source vessel will still be heard when the probe goes proximal or distal, whereas the perforator is heard only at one location. Although handheld ADS is recognized as being less accurate in detecting perforators and no studies have shown improved outcomes so far, surgeons will prefer to use it in clinical practice due to its widespread availability. For some, the handheld ADS is useful as a screening tool for mapping perforators in free-style flaps planning ⁸ and as a complementary tool to P-CTA or MRA, as will be described later as part of the discussion of those options.

Color Duplex Sonography

To overcome the disadvantages of handheld ADS, CDS was developed as a noninvasive perforator assessment tool. A literature search has shown that CDS provides much more reliable information than ADS. ^{6 9 10} The use of CDS provides a color-coded visual image about the main source vessels and their perforating branches ( Fig. 2 ). Not only does CDS reveal the position of a perforator, but it also detects hemodynamic characteristics including the diameter, course, and flow rate and velocity. The advantages of CDS can be summarized as being noninvasive, requiring neither intravenous contrast nor radiation, and being of a lower cost when compared with other imaging tools. However, its use is limited by the fact that it is relatively time-consuming, approximately 30 to 60 minutes for the usual study. CDS also requires adequate experience to handle the tool appropriately in order to obtain optimal data, as well as knowledge of perforator anatomy. Additionally, its real-life dynamics makes it less reproducible and does not allow sharing of three-dimensional vascular image among surgeons. Therefore, the authors recommend that the use of CDS should be limited to selected cases such as patients with a metal implant, allergy to the contrast agent, or renal insufficiency.

Fig. 2.

Color duplex sonography provides color-coded visual image about the main source vessels (asterisk) and their perforating branches (triangle).

Perforator Computed Tomographic Angiography

The increased spatial resolution offered by multidetector-row computed tomography (MDCT) provides highly accurate multiplanar and three-dimensional reconstructed images. Moreover, this has made it possible to reveal small perforators, as small as 0.5 mm in diameter. P-CTA can provide surgeons with detailed three-dimensional images of vessels such as perforator's location, diameter, and course, in relation to other anatomical structures. P-CTA can not only detect information at the superficial layer above the deep fascia (like ADS) but also show the intramuscular course of perforator branches down to the main source vessels. This information allows surgeons to perform a much easier muscle dissection during flap elevation if needed.

Several other authors have reported the clinical benefits of P-CTA for preoperative detection of perforators. Rozen et al ¹¹ revealed a 99.6% sensitivity and 99.6% positive predictive value for identifying and mapping deep inferior epigastric artery perforators (DIEAPs) with P-CTA. A 100% predictive value of P-CTA was well shown in a series of DIEAP flaps for breast reconstruction by Masia et al. ¹² In three studies comparing CDS with P-CTA, all the authors concluded that P-CTA was superior to CDS with regard to its accuracy in identifying perforators. ^{13 14 15} Moreover, systematic reviews and meta-analyses found a significant clinical advantage of P-CTA over CDS, which reached statistical significance. ^{16 17} Thus, it is recognized that P-CTA is the current gold standard for perforator mapping. ¹⁸ By using this technique, surgeons can see and share images preoperatively, which can shorten the operation time, ^{12 19 20 21 22 23 24} reduce complications such as flap necrosis, ^{12 23 24} reduce operator's stress levels by 41%, ¹⁹ and improve the total treatment outcomes. P-CTA has been also reported as a useful tool in propeller flap planning based on 13 operative cases. ² All perforators identified by P-CTA preoperatively were located accurately during the operation without any errors. The distance between the preoperative supposed positions and the intraoperative actual positions were within 1 cm, and there were no false-positives or false-negatives. Consequently, it was not necessary to change the flap design intraoperatively in any of the cases. Furthermore, the actual operation time was 23% less than the scheduled time. From a cost-effectiveness point of view, Rozen et al showed that the P-CTA could reduce the operation time and shorten hospital stay compared with operations without any preoperative P-CTA, resulting in reduced overall costs. ²⁵

The major disadvantages of P-CTA are the exposure to ionizing radiation and the necessity to use a contrast medium, which may cause allergic reactions and/or contrast-induced nephropathy in patients with renal failure. Limiting the scanning field to only the area of interest can reduce radiation exposure. ^{26 27} Only one comparative study by Cina et al has shown an equivalent accuracy of CDS versus P-CTA and suggested that P-CTA should not be used in certain cases to avoid radiation exposure. ²⁸

Computed Tomographic Scanning Technique

The current main CT used by the authors is a 64-row MDCT (LightSpeed VCT, GE Healthcare) ( Fig. 3 ). P-CTA studies are performed jointly by a plastic surgeon, a radiologist, and a radiology technician. Scan parameters in our facility, Nippon Medical School, are listed in Table 2 . ² Patients are scanned in a position matching the operative positioning and were instructed to hold their breath during the imaging of the main trunk. The scan range is limited to the tissue that is to be used intraoperatively to decrease the radiation exposure to the patient and improve the image quality. The entire CT scan is performed with a rotation speed of 0.4 second/rotation, a detector coverage of 40 mm, and a detector configuration of 0.625 mm and 64 rows. This acquisition protocol allows for a table speed of 137.5 mm/second and a scan time of approximately 10 seconds. For P-CTA, axial images of 0.625 mm in thickness are reconstructed with an interval of 0.1- to 0.3-mm overlapping technique and transferred to a workstation (Advantage Workstation 4.4, GE Healthcare) ( Fig. 4 ). The obtained images are reconstructed using the maximum intensity projection technique and the volume-rendering technique. The detected perforator positions can then be marked on the patient's skin in direct correspondence to the coordinate data from the MDCT ( Fig. 5 ). A propeller flap can then be designed including that perforator as the flap pedicle, and raised until the previously marked perforator is visualized. Usually, the perforator identified by P-CTA preoperatively ( Fig. 6 ) is accurately located intraoperatively without any errors ( Fig. 7 ). After identifying the reliable perforator visually, an additional incision is made circumferentially to harvest the flap as an island flap following the usual protocols.

Fig. 3.

The gold standard 64-row multidetector computed tomography device.

Table 2. Multidetector-row computed tomography scan parameters.

Parameter
Scanner	64-slice MDCT scanner (LightSpeed VCT, GE Healthcare)
Detector configuration	64-row × 0.625-mm slice thickness
Detector coverage	40 mm
Helical detector pitch	0.516–0.984
Gantry rotation speed	0.4 s/rot
Tube potential	120 kVp
Tube current	600 mA (dose modulation)
Contrast	Iopamidol 370 mg iodine/mL (Iopamiron 370, Bayer Yakuhin Ltd.)
Volume	BW × (0.8–0.1)mL + Phy. Saline 20mL
Injection rate	BW × (0.08–0.1) mL/s (upper limit: 5mL/s)
Bolus tracking method	Smart prep
Initiation of CT scanning	Increase of >150 HU at aorta or the parent artery from which perforators emerge
Image reconstruction	0.3-mm overlapping axial images

Abbreviations: BW, body weight; CT, computed tomography; HU, Hounsfield units.

Fig. 4.

Reconstructed images are transferred to a workstation, which generates the reformatted images in multiple planes (coronal, axial, and sagittal) and three-dimensional volume-rendered images.

Fig. 5.

Detected perforator positions are marked on the patient's skin ( cross ) in direct correspondence to the coordinate data from the multidetector computed tomography.

Fig. 6.

The point where a perforator penetrates the deep fascia overlying the muscle is within the dotted circle.

Fig. 7.

The perforator as identified by P-CTA preoperatively is accurately located intraoperatively without any errors.

Magnetic Resonance Angiography

Contrast (gadolinium) enhanced magnetic resonance imaging (MRI), which is termed magnetic resonance angiography (MRA), has progressed recently and shown to be accurate for identifying the three-dimensional anatomy of perforators as well as P-CTA. ²⁹ MRA has notable advantages over P-CTA as it is based on magnetism instead of radiation and can be used with a noniodine contrast medium, making it a safer option for the patient. Although perforating vessels of more than 1.0 mm diameter can reliably be visualized by MRA, the depiction of smaller perforators, less than 1.0 mm, is less accurate with MRA. ^{30 31} Cina et al ³² conducted a comparative study of P-CTA and MRA targeting DIEAPs and found that the accuracy rate in identifying dominant perforators was the same (91.3%) for both techniques, whereas the accuracy in detecting the intramuscular course of the perforators was 97.1% for P-CTA and 88.4% for MRA. Other disadvantages of MRA include long study time, high cost, and a contraindication for use in cases with implanted metal devices, cardiac pacemakers, or very claustrophobic patients. In conclusion, P-CTA is superior to MRA regarding accuracy in detecting smaller perforators at the present time; however, technological developments in MRA predict an improved accuracy equivalent to P-CTA and more reliability, as it does not require radiation or a contrast medium.

Conclusion

A propeller flap is an island-shaped flap nourished most commonly by a single perforator. When that perforator cannot be found intraoperatively, it is often difficult to change the flap design during the operation. Thus, accurate preoperative perforator assessment of perforators in the planning of propeller flaps is more important than for free flaps to avoid intraoperative surprises or embarrassment. At the present time, P-CTA remains the gold standard for perforator mapping as it reduces the operating time and complications, which will then result in improved comprehensive clinical outcomes. CDS is a valid alternative particularly for cases where radiation and/or contrast exposure should be limited. The future of MRA appears promising due to continued technological developments that may make it as accurate as P-CTA, in addition to its advantages including avoidance of exposure to ionizing radiation and less adverse reactions to contrast medium.