Computed Tomography Angiography, Perforator Flaps, the Surgeon, and Osirix

Through their experience with 144 patients, the authors evaluate the use of the free open-source DICOM viewer OsiriX for Mac in the preoperative planning of perforator flaps with three objectives: 1) increase the present knowledge related with the preoperative planning of perforator flaps with computed tomography angiography (CTA), 2) evaluate the application in the image post-processing of perforator flaps, and 3) evaluate the performance of the image post-processing when performed by a surgeon. The experience demonstrated that the use of OsiriX allowed an adequate evaluation of different structures and parameters of great preoperative interest in perforator flap surgery: 1) source artery, 2) diameter of artery and vein/s at the hypothetical site of microsurgical anastomoses, 3) course and branching pattern of the flap pedicle, 4) perforator course in the subcutaneous fat (theoretical flap axis), 5) measurement of the skin and fat where the perforator pierced the deep fascia (theoretical flap thickness), 6) measurement of the distance between the point of entrance of the perforator in the subcutaneous fat to the source artery (theoretical maximal pedicle length), and 7) measurement of the perforator diameter where it pierced the deep fascia. For the home user, OsiriX is an efficient alternative, comparable to the more professional applications only available in Radiology services and dedicated professional PACS workstations.


Introduction
Whether or not CTA is justified in the preoperative planning of perforator flaps is still under debate but there is a growing evidence of the associated benefits [1] and, despite the lack of randomized multicentric studies, a substantial and growing amount of bibliography supports the use and security of CTA [1][2]. CTA has demonstrated to improve the surgical results, aid in the selection of the best perforators, minimize surgical trauma and donor site sequelae, reduce economic costs and minimize the surgeon's surgical stress [3][4][5][6][7]. Its potential role in the evaluation of recipient vessels has also proven to be high [8]. Very popular in breast reconstruction with abdominal flaps, its use has rapidly expanded to other body regions because the technique is readily available, extremely fast and shows a low interobserver variability [9][10][11]. However, computed tomography has drawn experts' attention because of its relatively high radiation dose per study [12][13][14]. In this scenario, different measures are imperative: 1) a correct indication of CT studies, 2) the improvement of technology and 3) the optimization of the acquisition parameters to minimize the radiation dose following the ALARA (As Low As Reasonably Available) principle [4,[15][16]. At present, only magnetic resonance angiography can rival CTA [17][18], although some reports have found that the technique is still far from it because of a lesser resolution, higher cost and limited availability [19][20][21]. OsiriX is a free opensource imaging software that transforms an Apple Macintosh into a PACS DICOM workstation to process and visualize medical images [22]. Free in its 32-bit version, the paid 64-bit version gives an extended usage of the processor's RAM memory. The recently released MD version has obtained the Class II Medical Advice certification by the FDA for clinical use. IPhone and IPad HD versions open up a new portable dimension to radiological studies.

Materials And Methods
A retrospective review was made of the authors' experience in the preoperative evaluation of perforators with CTA with three objectives: 1) widen the present knowledge of the use of CTA in preoperative planning of perforator flaps, 2) evaluate the OsiriX application in the DICOM image post-processing of perforator flaps and 3) analyze the performance of the post-processing when done by a surgeon (non-specialist in Radiology). After the appropriate Institutional Ethics Committee approval from the Hospital Fremap Majadahonda and the corresponding written patient informed consent, the research included the studies of 144 consecutive patients who undertook a CTA as one of the routine studies in the preoperative planning of a perforator-based free or pedicled flap reconstruction ( Table 1).   [4][5][6][7][8][9][10][11][12][13][14][15][16], adjusted for an optimal relationship between radiation dose and image quality following the ALARA principle [23], are shown in Table 2.

2: Acquisition parameters and contrast media
The images were processed and analysed by the first author with a 24-inch screen iMac and the 64-bit version of OsiriX. The different reconstruction 2D and 3D modes were used as shown in Table 3.

TABLE 4: Studied parameters and structures
In each study, different points of interest were marked to evaluate and measure the regions of study: 1) reference points with a surface translation (umbilicus, anterior superior iliac spine, greater trochanter, malleoli), 2) point of piercing of the deep fascia by the perforator and 3) site of pedicle emergence from the source artery. The study included the following measurements: 1) distance between previously defined points of interest and 2) external diameter of small-calibre vessels (smaller than 3-4 mm) at the hypothetical suture site and at the piercing point of the deep fascia.

Results
Overall, CTA demonstrated an excellent competence in the visualization of the vascular and musculoskeletal anatomy. As expected, the orthogonal 2D MPR and 3D MPR were the most valuable and easy to learn modes of visualization (Figure 1).  The curved 3D MPR demonstrated to be the best mode for a global vessel evaluation because it can "stretch and straighten" vessels. Of the two possible views (straightened and stretched), the latter preserves the isometry and allowed an approximate measurement of the vessel length despite its tortuous course ( Figure  3).

Discussion
Despite the growing body of literature regarding the benefits of CTA in perforator flap surgery, no publication specifically addresses which specific data can be obtained from the technique. Based on the study, the authors have found that CTA can provide valuable information regarding perforator flap surgery: 1. Detection of possible vessel malformations or obstructions that might even contraindicate the surgical flap (typical examples would be the distal obstruction of the superficial femoral artery when planning an anterolateral thigh flap or the absence of the peroneal artery in fibula transfers).
2. Selection of the most beneficial (not necessarily the largest) according to its size, situation and pedicle length ( Figure 5).

FIGURE 5: 2D MPR mode (coronal)
The 2D MPR mode (coronal) is also helpful in the measurement of the pedicle length, approximate external diameter of the perforator and flap thickness.
3. Evaluation of the flap pedicle from the source artery to the deep fascia. The technique allows the recognition of its length, course, approximate diameter, and relationships with neighboring structures ( Figure 6).

FIGURE 8: Neighboring slices
The evaluation of neighboring slices translates the connection between perforasomes into images, thus guiding flap design with safety.
7. Estimation of the perforator course in the subcutaneous fat. A prevalent suprafascial course will advise against aggressive intraoperative flap defatting. On the contrary, a predominant subdermal course will advise a careful flap deepithelization (Figure 9).

FIGURE 9: Perforator course in the subcutaneous fat
The perforator course in the subcutaneous fat provides the key for an immediate, safe deepithelization and defatting of the flap. 8. Evaluation of the quality and distribution of recipient vessels and identification of possible degenerative or traumatic vessel lesions.
Radiologists are, of course, the specialists in image post-processing but only the surgeon is able to translate the image findings into surgical decisions. Consequently, more and more surgeons are showing interest in image post-processing but, in most hospitals, the availability of professional workstations is limited and their use is mostly restricted to radiologists under a busy clinical activity. Built-in DICOM viewers included on the CD/DVD are reduced versions of the original applications with limited processing capabilities that do not allow a competent image post-processing. OsiriX fills the gap with efficiency and, at present, it is probably the best alternative for the home user. Two factors have a direct influence in image postprocessing of DICOM files: software and informant. There is data enough to sustain the excellent performance of OsiriX in the management of DICOM files. OsiriX, except for the FDA/CE-approved MD version, is not certified as a commercial medical device for primary diagnostic imaging and its use in clinical practice might have ethical and legal implications. Consequently, although the benefit of using the application to improve the preoperative knowledge of the particular patient's anatomy is unquestionable, an adequate learning period is fundamental because a deficient use of the tool can cause erroneous information that might be potentially harmful for the patients.

Conclusions
CTA has demonstrated to be of great aid in the preoperative planning of perforator flaps. Until a noninvasive non-radiating imaging technology can replace it, great efforts must be done to minimize the risk of radiation following the ALARA (As Low As Reasonably Acceptable) principle. From a surgeon's perspective, getting involved in image post-processing is beneficial to optimize the translation of image findings into surgical decisions. Until the professional imaging applications included in manufactured workstations are available to the individual user, OsiriX fills the gap with extreme efficiency.