Computerized Tomographic and Magnetic Resonance Angiography for Perforator-Based Free Flaps: Technical Considerations

Perforator-based free flaps rely on the appropriate dominant vessel supplying the vascular territory of the flap. Preoperative knowledge of the vascular anatomy can improve outcome and diminish surgical time. Several preoperative imaging techniques exist for surgical planning. Computed tomographic and magnetic resonance angiography are two emerging modalities that provide exceptional anatomic detail. Despite the growing utilization of cross-sectional imaging for preoperative planning, each modality has specific technical considerations that are necessary to consider in order to produce a quality study.

The advent of multidetector computed tomography (MDCT), with an ever-increasing number of detectors and faster gantry rotations, has revolutionized diagnostic radiology, allowing for rapid imaging at an increased resolution. Coupling MDCT technology with improvements to intravenous contrast has made it possible to perform precise imaging during the arterial phase of contrast infusion increasing resolution of smaller vessels. Computed tomographic angiography (CTA) is now routinely used in vascular, abdominal, and transplant surgery for its ability to provide accurate vascular anatomic detail. In most settings it has supplanted conventional invasive catheter angiography as the diagnostic imaging modality of choice for imaging blood vessels. CTA is a noninvasive method for preoperative planning, such as determining tumor resectability, arterial anatomy before organ donation, and extent of peripheral vascular disease. As advanced CTA has become mainstream, it is not surprising that new and novel applications have been developed in other specialties, including reconstructive surgery.

Over the last several decades, the use of perforator-based free flaps has gained appeal because of the reduction in donor-site morbidity common with conventional musculocutaneous flaps. Successful perforator-based free flaps rely on selection of the appropriate dominant vessel supplying the vascular territory of the flap. Generally, anatomic variability increases in distal branches beyond the parent vessel. In addition, anatomic variability tends to increase as vessel size decreases. Improvements to surgical technique, allowing for the harvest of smaller, distal vascular segments has made knowledge of the native vascular anatomy critical during surgical dissection. Anatomic course and perforator diameter are important determining factors for flap perfusion. Furthermore, understanding the perforator distribution and blood supply of a flap can impact the operative time and perfusion of the flap, ultimately affecting the outcome.

Doppler ultrasound is used extensively for free-flap operative planning. Duplex sonography adds the ability to assess the rate of flow in addition to the acoustic characteristics of a vessel. Unfortunately, ultrasound of any form is limited by its subjective nature, time required to perform a quality evaluation, and ultimately its reproducibility. Digital subtraction angiography was at one time considered the gold standard for the identification and mapping of vessels. However, it is an invasive procedure, which always has the risk of complications, such as vessel dissection and access-site hemorrhage. Evaluation of small, perforator-sized vessels requires selective angiography, which increases the risk of vessel injury and still fails to accurately depict the course of the vessel of interest in the surrounding soft tissue.

Magnetic resonance angiography (MRA) and CTA have the advantages of being noninvasive methods of imaging vascular anatomy with high spatial resolution and soft-tissue detail that is easily reproducible. MRA has the advantage of not using ionizing radiation, which is a consideration, particularly when dealing with younger patient populations. These two imaging modalities are discussed further as tools for preoperative evaluation in perforator-based flap reconstruction.

Alonso-Burgos and colleagues published one of the first reports using CTA for reconstructive surgery in which 6 patients were evaluated using CT for deep inferior epigastric perforator (DIEP) tissue flap planning. A 4-detector row MDCT scanner was used with 150 mL of iodixanol contrast medium. The investigators obtained a slice thickness of 1.25 mm and reformatted images into multiplanar reformats, maximum intensity projections (MIP), and 3-dimensional (3-D) volume-rendered images. Arterial perforators were identified and evaluated for vessel diameter, fascial penetration pattern, intramuscular course, origin from the deep inferior epigastric artery, and other anatomic variations. In all 6 patients, accurate main perforators were identified on CTA with no additional vessels found at the time of the surgery. In addition, CTA provided important adjunctive preoperative information, such as muscular diastasis, abdominal wall hernia, and fatty infiltration of potential flaps. This early experience with an early 4-detector row scanner showed promising results for CTA as a noninvasive means for presurgical planning.

The same year Masia and colleagues published a retrospective review of 66 DIEP flap reconstructions in which CTA was used for preoperative planning. The investigators found an average time saved of 1 hour and 40 minutes in cases with preoperative CTA. There were 2 cases with partial necrosis and 1 total failure in the group without prior CTA and only 1 partial necrosis case in the CTA group. The investigators thought that CTA offered a high sensitivity, specificity, and 100% positive predictive value. Furthermore, they were able to highlight the value of CTA as a tool to reduce operative time by identifying the most suitable perforator allowing safe ligation of other smaller vessels.

In 2008, Rozen and colleagues published a series of articles using CTA for DIEP and superficial inferior epigastric artery (SIEA) flaps ( Table 1 ). The first report included 75 patients using a 64-detector row scanner the images reconstructed as 1-mm slices. Of the 75 patients, they specifically describe seven cases in which CTA actually changed the operative plan due to the anatomy, particularly patients with prior abdominal wall surgical history. Later, a second study performed by the same group evaluated 104 reconstructions to determine if there were outcome differences after preoperative CTA. The investigators concluded that preoperative CTA was associated with a statistically significant decrease in flap complications, donor-site morbidity and operative stress for the surgeon. In a similar study, Smit and colleagues also demonstrated a trend toward the reduction in surgical time. Their study compared 70 patients who were evaluated preoperatively with CTA and 68 by preoperative Doppler ultrasound. There was a statistically significant decrease in surgical time and no flap complications in the CTA group.

Table 1

Comparison of recently available publications evaluating CTA for DIEP flap harvesting

Authors	Year	Number of Patients/Flaps	Free Flap	Reported Success Rate (%)
Alonso-Burgos et al	2006	6	DIEP	100
Masia et al	2006	66	DIEP	100
Rozen et al ^,^a	2008	75	DIEP/SIEA	100
Rozen et al	2008	88	DIEP	100
Schaverien et al	2008	12	DIEP	—
Rozen et al	2008	10	DIEP	—
Phillips et al	2008	65	DIEP/SIEA	—
Clavero et al	2009	126	DIEP	100
Masia et al	2009	162	DIEP	99
Rozen et al	2009	26	DIEP	—
Smit et al	2009	70	DIEP	100
Pacifico et al	2009	60	DIEP	—
Rozen et al ^b	2009	6	DIEP	100
Whitaker et al	2009	325	DIEP	—
Rozen et al	2009	10	DIEP	100
Kim et al	2010	58	ALT	—
Chen et al	2010	32	ALT	83
Gattaura et al	2010	100	DIEP	—
Masia et al	2010	357	DIEP	100
Zhang et al	2010	4	ALT	—
Visscher et al	2010	10	DIEP	—
Katz et al ^,^c	2010	86	DIEP	—
Ting et al ^,^a	2010	1	DCIA	—
Rad et al ^,^c	2010	12	SGAP/LSGAP	—
Gacto-Sanchez et al ^,^b	2010	70	DIEP	10
Cina et al ^,^b	2010	45	DIEP	91
Ribuffo et al	2010	41	OFFF	100

Abbreviations: ALT, anterolateral thigh flap; OFFF, osteocutaneous fibula free-flap; SGAP/LSGAP, superior gluteal artery perforator/lateral supragenicular artery.

a Case reports.

b Connotes a comparison of Doppler ultrasound and CTA.

c Studies using CTA but not evaluating CTA outcome.

Despite CTA strengths in arterial imaging, limitations do exist. A comparison study of Doppler to CTA for DIEP flap planning in 2010 evaluated 45 patients preoperatively examined with both Doppler and CTA. In this series, the dominant perforator used for the flap was found in 44 patients with Doppler and 41 patients with CTA. Additionally, among CTA patients, there was a disagreement in perforator size described on the CTA compared with what was found during surgery. Overestimation on the CTA was attributed to a summation of the perforating artery and adjacent vein during measurement, likely secondary to volume averaging. The investigators did find that CTA provided a better analysis of the intramuscular course of the vessels as well as assessment of superficial venous communication, and that overall CTA provided a global picture to the surgeon.

MRA has continued to improve on its ability to visualize vessels distinctly within adjacent soft tissue and lack of ionizing radiation. This ability is in large part because of faster gradients and improved sequences. A recent study confirmed the potential of MRA for preoperative planning. Continued work in this area especially with newer technological advancements, including the introduction of blood-pool contrast agents, should ultimately give MRA a distinct advantage in this field.

CTA technique

Proper acquisition of the raw data is critical for correct vascular assessment of the donor area. Although CTA acquisition involves the manipulation of a small set of variables, familiarity with them insures correct image acquisition and therefore reduces the risk for repeated contrast boluses and exposures to ionizing radiation.

Patient preparation

Enteric contrast is avoided before any CTA acquisition. Patients are asked to disrobe and only wear a hospital gown without tying it. Because 3-D shaded surface rendering will be used to depict anatomic location of the perforators in relation to the overlying skin, any material distorting the normal contour of the abdomen is avoided. Furthermore, patients are positioned with arms at their sides to again replicate the natural neutral position of the abdominal soft tissue during the future surgery. For similar reasons no retention devices are used.

A large-gauge (18 gauge or larger) peripheral intravenous line is placed in the antecubital region of the arm. The large size is required for the rapid infusion of contrast, which is essential for adequate opacification of small vessels. Such vessels, as in the case of abdominal perforators, which range in size between 1 to 4 mm, require an adequate contrast bolus to be properly imaged. Extensive literature regarding contrast bolus is available in the coronary CTA literature regarding the ideal rate of contrast infusion for vessels of similar size in that region of the body. As a general rule, approximately 1 to 2 g of iodine is injected per second to allow the best visualization of vessels of this caliber. Although iodine concentrations per milliliter of contrast vary for CTA, the highest available concentration is used. This concentration is generally 350 to 370 mg/mL in the United States and 400 mg/mL in Europe. The principle can be simply stated: a lower iodine concentration will require a higher injection rate to obtain equivalent image quality. Furthermore, most contrast agents have a high viscosity and therefore generate high pressures at the suggested high flow rates. It is therefore prudent to place the largest intravenous access feasible and adjust the flow rate accordingly. In most patients flow rates between 4 to 6 mL/s should be attainable keeping in mind that patients with a larger body habitus will require higher flow rates. Finally, when performing examination with high injection rates it is recommended to use a test bolus of at least 10 mL of normal saline to test the intravenous access before infusion of contrast bolus. This practice will minimize the risk of infiltration of the iodinated contrast, which in turn may cause significant morbidity ranging from arm discomfort to a compartment syndrome.

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