Abstract: Virtual Dissection MRI of Internal Mammary Lymphadenopathy

Synopsis
MRI of Internal Mammary lymph nodes can detect lymphadenopathy in breast cancer patients, which implies a higher cancer stage and worse prognosis for the patient. The MRI scanning technique is described, including an image post-processing method using Photoshop layers and the Eraser tool, dubbed “virtual dissection”, which facilitates the depiction of anatomic structures that cross between MRI slices. MRI of 40 stage II breast cancer patients shows 18% with lymphadenopathy.

Methods
Forty (40) adult female breast cancer patients with stage IIA or higher disease gave informed consent for the study. MRI scans were performed on a 1.5T scanner ( Philips Intera, release 8, Best), with the patient lying prone on a surface coil anterior to the sternum. Coronal turbo spin echo T1 weighted images used the following parameters: TR 400ms, TE 16ms, turbo factor 3, matrix 512×512, FOV 16cm, slice thickness 4.0mm, gap 0.4mm, flip angle 80, NSA 4. Six slices are obtained, and then the pulse sequence is repeated with the slices shifted 2.2mm anteriorly, resulting in a total of 12 slices spaced 2.2mm apart. Total scan time in the magnet is 10 minutes, including patient positioning and scout views. Patients with suspected lymphadenopathy will undergo PET scanning and MRI follow-up after treatment. Internal Mammary MRI results are not currently being used to upstage patients to stage IIIB. All patients receive chemotherapy and radiotherapy ports cover the retrosternal region.

Although the resulting images are suitable for direct intepretation by the radiologist, additional “virtual dissection” post-processing was performed: The twelve MR images are transferred from a PC with PACS viewing software (Radworks, GEMS, Milwaukee) into Photoshop software (Adobe, Mountain View, CA) by Copy & Paste of each image. Each image pasted into Photoshop becomes one Layer, superimposed over other layers. The anterior images become the top layers, which temporarily cover over the posterior images on the lower layers. As the “Eraser” tool erases pixels from one layer, they become transparent, which allows pixels from a lower layer to show through. The result is a “virtual dissection”, by removing (erasing) fat and muscle to reveal deeper structures such as the internal mammary vessels, lymph nodes and lymphatic ducts.

Results
A Virtual Dissection image is shown in Figure 1, illustrating bilateral Internal Mammary lymphadenopathy in a patient with bilateral breast cancer. Figure 2 shows a prominent normal lymph node with a fatty hilum, in a different patient. Interpretation requires a judgement based on node size, asymmetry compared to other nodes, and degree of fatty signal in the node hilum. Of 40 patients scanned, the interpretation was: 7 (18%) had definite lymphadenopathy, 6 (15%) had probable lymphadenopathy, 6 had prominent benign-appearing lymph nodes, 15 had small benign lymph nodes, and 6 had no visible lymph nodes.

Virtual dissection using Photoshop is very useful, particularly during the early phase of gaining experience with interpretation. In most cases, there are veins penetrating inward from the breasts at the 1-2 or 2-3 intercostal levels, which have spiral paths that can resemble lymph nodes on standard coronal images. The virtual dissection technique helps to depict their connections to the internal mammary veins. Intercostal veins are often only appreciated as being blood vessels on the virtual dissection images. Lymphatic ducts are occasionally seen, for example, crossing behind the left 3rd costal cartilage, lateral to the artery on figures 1 and 2. Small benign lymph nodes measuring 2 to 4mm short-axis thickness are commonly seen in many patients. Lymph nodes measuring greater than 5.0 mm short-axis thickness, with minimal fatty hilum, are likely to be lymphadenopathy.

Discussion
The remarkable detail of high resolution MRI in the Internal Mammary region provides the possibility of a non-invasive rapid staging test for breast cancer, that could potentially modify treatment plans. Internal mammary (IM) lymphadenopathy is an important independent risk factor that confers a worse prognosis for the patient [1]. A Japanese study of Internal Mammary MRI ( at 256×256 matrix size), found that IM lymph nodes having  5mm thickness had an 82% positive predictive value, based on biopsy proof[2]. Published surgical reviews of internal mammary biopsies generally find an incidence of about 18% internal mammary lymphadenopathy [3], but incidence can be 20% to 30% depending on patient selection criteria. The current technique of 512×512 matrix size, overlapping slices and virtual dissection in Photoshop, should provide even greater accuracy and will probably surpass the historical sensitivity of surgical vision-guided dissection. Further evaluation of this technique seems worthwhile.

Acknowledgements : Alberta Cancer Board – Research Initiative Program.
References
[1] Veronesi et al, Ann Surg 1983 Dec;198(6):681-4. [2] Kinoshita et al, Radiat Med 1999; 17:189-93. [3] Kikuchi A. Rinsho Hoshasen 1989; 34:471-9.

 
 

Figure 1: Bilateral Internal Mammary Lymphadenopathy

Figure 2: A normal lymph node at the left 3-4 intercostal space level.