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Variation of Internal Mammary Vessels: Vein Lateral to Artery: Two Cases

The knowledge on the normal anatomy and variation of the internal mammary vessels (IM) is essential to the safety of parasternal percutaneous procedure. A 1995 study by Arnez et al. (1) identified four different patterns of the IMs, with the internal mammary vein (IMV) lying medial to the internal mammary artery (IMA) in 95% of the 34 cadavers (Types 1 and 2) (2). In the 3%, the main trunk of IMV lies lateral to the IMA above the upper border of the second rib, below which it divides into a lateral and medial branch (Type 3); in the remianing 2% (single cadaver), a single-trunk IMV lies lateral to the IMA throughout the parasternal areas (Type 4). To our knowledge, however, the last two patterns have never been demonstrated for cross-sectional imaging to this date. The crossing of the IMA over the IMV has never been reported at the first intercostal space level. We present two cases of unusual IM course incidentally discovered on the multi-detector CT (MDCT), with the IMV running entirely or laterally to the IMA in the first intercostal space. CASE REPORT


INTRODUCTION
The knowledge on the normal anatomy and variation of the internal mammary vessels (IM) is essential to the safety of parasternal percutaneous procedure. A 1995 study by Arnez et al. (1) identified four different patterns of the IMs, with the internal mammary vein (IMV) lying medial to the internal mammary artery (IMA) in 95% of the 34 cadavers (Types 1 and 2) (2). In the 3%, the main trunk of IMV lies lateral to the IMA above the upper border of the second rib, below which it divides into a lateral and medial branch (Type 3); in the remianing 2% (single cadaver), a single-trunk IMV lies lateral to the IMA throughout the parasternal areas (Type 4). To our knowledge, however, the last two patterns have never been demonstrated for cross-sectional imaging to this date. The crossing of the IMA over the IMV has never been reported at the first intercostal space level.
We present two cases of unusual IM course incidentally discovered on the multi-detector CT (MDCT), with the IMV running entirely or laterally to the IMA in the first intercostal space.

Case 1
A 25-year-old female is presented with a palpable supraclavicular mass. Neck and chest CT were being obtained for evaluation, which shows a large mass in the anterior mediastinum.
The patient is being referred to the radiology department for transthoracic biopsy of the mediastinal mass, when it was noticed that the main trunk of the IMV was running laterally to the IMA on both sides of the mass, at the level of the first intercostal space (Fig. 1A). In the second intercostal space, the left and right IMAs were accompanied by two branches of IMVs lying lateral and medial to each of them. Above the first intercostals space, the IMVs were medial to the IMAs as in most patients, crossing IMAs were near the inferior margin of the first rib.
Although biopsy could be done either medially or laterally to the IMs, the lateral path was chosen to avoid injuries to the Congenital variations of the internal mammary vascular anatomy are rare and may be encountered during various percutaneous transthoracic procedures. Hereby, we present two cases of congenital variations of the internal mammary vessel incidentally found on CT, with the vein entirely or partially lying lateral to the artery in the first intercostals space. The imaging findings and the clinical implications of these rare anatomical variations are being presented. trunk of the right IMV was running laterally to the IMA in its entire course. The left IMV was running medially to the concomitant IMA in the upper half of the first intercostal space, but was laterally crossed by the IMA in the middle of the intercostal space to the artery at the more caudal levels (Fig. 2). The biopsy was successfully done via a paravertebral approach, yielding the diagnosis of squamous cell carcinoma.

DISCUSSION
Percutaneous transthoracic procedure is actively being performed for evaluation of pulmonary or mediastinal lesions (2).
When performing percutaneous procedure via an anterior approach, familiarity to the anatomy of the IMs is essential for the safety of the procedures (3). However, there is a disparity between the results of cadaveric and imaging studies, and whether the IMV runs medially to its concomitant artery in the first or second intercostal space.
In the 1995 cadaveric study, Arnez et al. (1) identified four different types of IM anatomy (Fig. 3): the IMV was found medial to the IMA throughout its course in 95% of cases (Types 1 and 2); In 5% of cases the IMV was lateral to the IMA (Types 3 and 4). According to Arnez et al., the most common arrangement is Type 1: a single IMV lying medial to the IMA divides

Case 2
A 73-year-old male is being presented with cough and sputum that he had suffered for several months. A chest radiograph revealed a cavitary mass in the lower right lobe. The patient is being referred to the radiology department for biopsy of the mass. Diagnostic chest CT was being obtained, which showed an incidental variation of internal mammary vessels: the main  When planning a biopsy of a smaller mass in a similar location, the distance between the planned needle path and the internal mammary vessels can be shorter, and the operator may have to weigh the risk of the parasternal approach against that of the lateral approach. It has been emphasized to avoid both the IMA and IMV during a percutaneous procedure (3, 4), and there have been many reports of fatal hemorrhages from the IMA during both percutaneous and surgical procedures (1,(5)(6)(7)(8)   be carefully reviewed to see whether the IMV is medial or lateral to the IMA. Usually, the IMV runs medially to the IMA in the first or second intercostals space, and it would be wise to plan a parasternal biopsy in such a patient. Nevertheless, when the IMV lies laterally to the IMA as in our first case, it may be safer to set the biopsy route laterally to the IMs unless there is enough distance between the sternal margin and the IMA.
In our second case, the IMV was found to cross over the IMA in the middle of the first intercostals space. To our knowledge, this particular pattern of IM course has never been reported in a cadaveric or imaging study. Although the parasternal route was not used in the biopsy of the second case, this case underscores the importance of careful review for IM course when preparing for a parasternal biopsy, which can be facilitated by the use of MDCT.
In summary, we present two cases in which the lateral location of the IMV and the medial location of the IMA were welldemonstrated for MDCT at the level of the first intercostals space. It is helpful to consider these variations in the IM course when planning and performing percutaneous biopsy with an anterior approach.