Noncontrast Magnetic Resonance Imaging for Preoperative Perforator Mapping

Identifying the position, course, and caliber of the dominant perforator is extremely valuable in the preoperative study for perforator surgery. Besides reliability, the ideal technique should offer low cost and high availability and reproducibility. It should be fast, easy to interpret, and free of morbidity. Multidetector-row computed tomography (MDTC) and magnetic resonance imaging (MRI) provide images that are easy to interpret, and assess the perforator’s caliber and localization and its intramuscular course and anatomic relationships. Noncontrast MRI avoids radiation to the patient and eliminates the need for intravenous contrast medium. This article discusses this method and presents our experience.

The deep inferior epigastric artery perforator (DIEP) flap has gained immense popularity in breast reconstruction since its introduction in the 1990s. It provides fat and skin with characteristics that are similar to those of the normal breast and spares the rectus abdominis muscle or fascia, thereby minimizing donor site morbidity. One of the key points in breast reconstruction with DIEP flap is choosing the best supplying perforator and several factors should be kept in mind when doing so. The ideal perforator vessel should have a large caliber, a short intramuscular course, the easiest dissection, a suitable location within the flap, and subcutaneous branching with intraflap axiality. In our experience, after performing more than 600 DIEP flaps, we have identified perforator vessels with a totally extramuscular course in 12% of cases. These extramuscular vessels initially follow a retromuscular plane before piercing the muscular fascia in the exact abdominal midline. They are thus paramuscular perforator vessels rather than musculocutaneous perforator vessels. We consider these vessels to be ideal because their course facilitates dissection.

Perforator vessels arising from the deep inferior epigastric system are anatomically highly variable in number, location, caliber, and relationships with surrounding structures In view of this variability, is it valuable to have a reliable method that accurately identifies and locates the dominant perforator before surgery. Precise imaging can help to select the best hemiabdomen to raise, to differentiate between superficial and deep epigastric vessels and to combine 2 or more perforator vessels when there is no dominant vessel. A precise image allows planning of the operative technique, reduction of operating time, and improvement in operative outcomes. Using preoperative imaging techniques to study the epigastric vessels, we have decreased the number of postoperative complications.

Methods most widely used at present for the preoperative study of abdominal perforating vessels

Several techniques are available for the preoperative mapping of abdominal perforating vessels: handheld Doppler ultrasound, color Doppler imaging, computed tomography-angiography (CTA), and, more recently, magnetic resonance imaging (MRI).

The handheld Doppler ultrasound has been in use since the early days of microsurgery. This easy-to-use, inexpensive technique is performed by surgeons to locate the perforating arteries before perforator flap elevation, and it is the method most commonly used to locate an individual vessel before surgery. However, correlation between the audible volume of the signal and the diameter of the perforator vessel is poor and often imprecise. It offers only a limited amount of information and cannot distinguish perforator vessels from main axial vessels. It does not provide any data on the course of the perforator vessel because the information is given as an acoustic signal. The number of false positives is high, reaching 47% in one series. The value of Doppler sonography in this setting is therefore questionable. Doppler sonography may also be too sensitive because even minuscule vessels that are not large enough to support a perforator flap can be selected for abdominal perforator surgery. Despite these drawbacks, handheld Doppler ultrasound remains useful in our daily practice and helps us to assess the situation and the course of the superficial epigastric vessels.

Color Doppler imaging provides more information than Doppler sonography. It is a highly reliable technique to identify and locate the dominant perforator vessel. It provides a good evaluation of the main axial vessels and their perforator vessels. Moreover, the caliber and hemodynamic characteristics of the perforator vessels can be observed directly on color Doppler imaging. It provides information about blood flow direction, pattern, and velocity. The high sensitivity and the 100% predictive value of this technique have made it a good diagnostic tool in the planning of DIEP flaps. However, color Doppler imaging also has some limitations; it is time consuming for the radiologist to perform and patients are often uncomfortable because they must remain in the same position for nearly 1 hour. In addition, it requires the presence of highly skilled sonographers with knowledge of perforator flap surgery, and its results are technician dependent. In addition, color Doppler imaging does not provide anatomic images that show the surgeon the anatomic relationship between the deep inferior epigastric artery and its perforator branches and other structures along its route. These important limitations have contributed to its disuse in microsurgical units.

Since 2003, the multidetector-row computed tomography (MDCT) scan has proved to be highly reliable in preoperative planning of abdominal free flap breast reconstruction and has shown excellent results, significantly reducing operative time and complications. Unlike the handheld Doppler and colored duplex-Doppler ultrasound, it provides anatomic images that are easy to interpret and offers information on the caliber, location, and course of any perforating vessel. With the recent development of MDCT, a considerable number of thin-sliced computed tomography (CT) images are obtained in a short time. Intravenous contrast medium can be injected at high velocities, and excellent images of the vasculature are obtained. The increased spatial resolution offered by MDCT allows highly accurate multiplanar and 3D reconstructed images to be obtained. Moreover, this technique is easily reproducible and fast to perform, thereby minimizing patient discomfort and health care costs compared with the color Doppler technique. In addition, it provides unique and valuable information for surgical planning. The main drawbacks of CT are unnecessary radiation to the patient (effective dose is 5.6 mSv, similar to conventional abdominal CT scan ) and potential systemic allergic reactions to the intravenous contrast medium.

Noncontrast MRI

To overcome the limitation of radiation with the MDCT technique, in 2005 we began to investigate the possibility of using MRI for abdominal perforator mapping. In the following 3 years we worked with different kinds of MRI technologies, but all of them needed a contrast injection to obtain a good quality image of the perforator vessels. These MRI techniques allowed us to visualize perforator vessels with the same reliability as MDCT. A comparative study with 30 patients showed that there were no false positives. However, we found that the technique had several drawbacks. First, intravenous contrast was still necessary to obtain adequate quality images. Second, the image resolution did not allow precise analysis of the perforators’ intramuscular course and its anatomic relationships. In addition, the 3D image reconstruction was not as accurate as with MDCT. Although this is not essential for surgery, such pictures can be useful for teaching purposes. Other disadvantages were the possible claustrophobic feeling for the patients and that they had to lie face down to avoid respiratory movement.

However, we decided continue, and 1 year later we began to study the usefulness of 3-Tesla (T) MRI. This technique provided high-resolution images that allowed us to study the intramuscular and subfascial course of the perforating vessels. We acquired the images in less time, and contrast visibility with the same dose of gadolinium was better. Other investigators have since stressed the value of this technique. However, this technology is expensive and not available at all centers. In addition, intravenous contrast administration is still required.

During our investigation at this time our attention was drawn to methods being used for the study of renal tumors with noncontrast MRI. This technique allowed good visualization of the vessels and we began to consider the possibility of using the same sequence for preoperative perforator mapping of the abdominal wall in female patients undergoing breast reconstruction with DIEP flap after mastectomy. In 2007, we found a new 1.5-T MRI acquisition sequence that provided specific vascular imaging without using contrast material.

At first, we used a 1.5-T magnetic resonance (MR) system (Excelart Vantage; Toshiba Medical Systems, Tokyo, Japan) equipped with a pair of 4-by-4 phased array coils. We used a respiratory-triggered, 3D true, steady-state, free-precession (SSFP) imaging sequence with time spatial inversion pulse (T-SLIP). We performed this in anterior coronal and axial planes using the following parameters: repetition time (TR)/echo time (TE)/facet angle (FA), 5.2 milliseconds/2.6 milliseconds/120°; slice thickness, 1.5 mm for coronal and 5 mm for axial planes; slice number, 40 to 50, no gap; field of view, 400×350 mm, matrix 256×256; number of acquisitions, 1. The acquisition time ranged from 20 to 30 minutes.

Despite the correct localization of the dominant perforator in all patients, in some cases we noted a low definition of the perforator course inside the muscle, which meant that some critical information for an effective perforator mapping was lost compared with MDCT.

We then started working with Toshiba engineers in an attempt to improve the image acquisition sequence. We decided to switch to a new MR angiography technique called fresh blood imaging (FBI) with a Toshiba ZGV Vantage ATLAS 1.5-T, ultrashort-bore body MR system (Toshiba Medical Systems, Tokyo, Japan). This technique provided accurate location of the dominant perforator, good definition of its intramuscular course and excellent evaluation of the superficial inferior epigastric system. We were also able to define the perforator branching within the subcutaneous abdominal tissue and evaluate the vascular connections between the superficial and the deep inferior epigastric vessels ( Figs. 1–3 ).

With the noncontrast SSFP imaging sequence 1.5-T MRI technique, we found 42% insufficiency in definition of the perforator intramuscular course. However, with the MR angiography (MRA) technique FBI, we obtained a better definition of the perforator intramuscular course in all cases, and we were also able to assess the reliability of the superficial inferior epigastric system and its vascular connections with the deep system. As a result, we have since performed the preoperative study of patients having breast reconstruction with non–contrast-enhanced MR angiography technique FBI.

The first step with this technique is to acquire multiplanar images with the patients supine; the same position as they will be placed at surgery. No prior patient preparation is needed. We use high-speed parallel imaging (speeder technology) to achieve accelerated scan times. Initially, sagittal scouts are acquired to locate the inferior abdominal wall and to delimit the study zone. A sequence phase 3D+5_FSfbi is used in the anterior coronal plane with the following parameters: TR, 2694; TE, 80; slice thickness, 1.5 mm; number of slices, 50; number of acquisitions, 1; 512×512 matrix; field of view, 380×380 mm; TI, 160; and resp+ECG gate. A sequence phase 3D+5_FSfbi is then performed in the axial plane with the following parameters: TR, 2900; TE, 78; slice thickness, 3 mm; number of slices, 56; number of acquisitions, 1; 704×704 matrix; field of view, 380×380 mm; TI, 160; and resp+ECG gate. The anterior coronal plane phase only includes the anterior abdominal wall, from a plane immediately below the pubis to the xiphoid process of the sternum. The axial plane phase includes the area from the infrapubic zone to 3 cm above the umbilicus. The acquisition time ranges from 10 to 20 minutes. Multiplanar formatted images and 3D volume rendered images are regenerated on a Vitrea computer workstation (Vitrea version 3.0.1. Vital Images, Plymouth, MN, USA).

The images obtained are interpreted by both the radiologist and the plastic surgeon who is going to harvest the DIEP flap. The team chooses the perforator considered the most suitable according to the following criteria: largest caliber, best location, and shortest intramuscular course. The perforator selected gives a pair of x / y coordinates based on an axial system centered on the umbilicus and the flap can be raised based on the dominant perforator. The surgeon is provided with 3 types of image: axial, sagittal, and coronal.

The axial views and sagittal reconstructions are of great help in the assessment of the perforator vessel to evaluate its dependence on the main trunk or any direct branch of the deep inferior epigastric artery and to delimit its origin on the fascia and its distribution through subcutaneous fat and skin. Rendered reconstructions allow us to mark on the patient’s skin the exact point where the perforator vessel emerges through the fascia of the rectus abdominis muscle. When we choose the best the perforator in the imaging technique, we look for the point where it pierces the fascia in the axial view and we mark an arrow on the skin at this level. From there, the arrow will appear in all views. We draw a coordinate x / y axis on the umbilicus and we make measurements to locate the exit of the perforating vessel in relation to the umbilicus. First, we measure the distance from the midline to the perforator in the axial view, and this is given the value x . The second measure is done in the sagittal view, measuring from the umbilicus level to the exit of the perforator. This is the y value. If we transfer these values from the computer to paper, we locate the exact point where we will find the perforator when we raise the flap. Before we complete the study with the radiologist, we like to look at the coronal cuts to assess the connections between the superficial and deep systems and visualize the subcutaneous and branching pattern of the deep inferior epigastric arterial system ( Fig. 4 ).