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Quantitative assessment of melanoma metastases at 7T: Susceptibility and T1 mapping
Sina Straub1, Frederik B. Laun1,2, Martin T. Freitag3, Heinz-Peter Schlemmer3, Mark E. Ladd1, and Till M. Schneider3,4

1Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany, 2Radiology, University Hospital Erlangen, Erlangen, Germany, 3Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany, 4Neuroradiology, University of Heidelberg, Heidelberg, Germany

Synopsis

Susceptibility values and T1 values of melanoma metastases measured at 7T are shown. There is an ongoing discussion about whether melanin or hemorrhage dominate the contrast mechanisms observed in melanoma metastases. Susceptibility maps as well as T1 maps benefit from the high contrast and high resolution available at 7T, though we observed paramagnetic susceptibility in areas corresponding to vessels or hemorrhagic events, but no general relation between susceptibility values and T1 values of individual metastases

Introduction

According to their appearance in magnetic resonance imaging (MRI), melanoma metastases have been categorized as showing a melanotic pattern, an amelanotic pattern or other patterns; for the melanotic pattern, high signal intensity in T1-weighted (T1w) images is characteristic1, 2. The T1-shortening effect of melanin is thought to be based on binding of paramagnetic metal ions3. Hemorrhagic metastases and hemorrhages can, however, show similar hyperintensities in the T1w signal depending on the time evolution of the hemorrhage. Moreover, melanin content has been histopathologically correlated to hyperintensity in T1w images4. Nevertheless, it is still unclear and scare quantified which MR signal changes are accounted for by blood and which by melanin.

Methods

The study was conducted in accordance with the Declaration of Helsinki. Institutional review board approval was obtained and all subjects provided written informed consent. Four stage IV melanoma patients (mean age 58 ± 15 years) with 37 metastases were evaluated quantitatively.

The patients were scanned on a 7 T whole-body MR system (Magnetom 7 T, Siemens Healthcare, Germany) with a 24-channel head-matrix coil. A 3D gradient-echo (GRE) sequence (pre-contrast) and a MP2RAGE (pre- and post-contrast) were acquired in three patients. The first patient terminated the examination after only the gradient echo sequence has been acquired. Sequence parameters are shown in Table 1. Brain masks were generated from the magnitude images using FSL-BET5. Phase images were unwrapped using Laplacian-based phase unwrapping6-8, and the background field was removed with V-SHARP7, 8 with kernel size up to 25 mm. Susceptibility maps were calculated in Matlab using the iLSQR method6, 9. Susceptibility maps were referenced with respect to cerebrospinal fluid from the atrium of the lateral ventricles. T1-weighted images and T1 maps (pre- and post-contrast) were co-registered to the first echo of the GRE magnitude images for each patient using affine registration in FSL-FLIRT10. Melanoma metastases were manually identified based on post-contrast T1-weighted images and T1 difference images (T1(precontrast)-T1(post-contrast)) in the Medical Imaging Interaction Toolkit (MITK)11, 12 for at most ten metastases per patient. For the patient for whom only GRE data were obtained, metastases were identified on GRE magnitude images.

Results

In Patients 1, 2, and 4, ten metastases were included in the evaluation, and in Patient 3, only seven metastases could be identified. In Patient 1, metastases could be identified on GRE magnitude data as they showed hyperintense contrast (Figure 1). This behavior was not observed for the metastases in the other patients.

In Figure 2, boxplots of the mean susceptibility values (a) and the mean T1 values (b) are shown. For Patient 1, no T1 values were available as only GRE data were measured. Patient 3, who shows a wider range in susceptibility values, also shows a wide range in T1 values.

Figure 3 shows scatter plots of susceptibility values and T1 values (a) and of T1 values before and after contrast agent injection. Susceptibility values and T1 values of the individual metastases do not seem related. The minimum mean susceptibility observed for an individual metastasis was -0.074 ppm (with T1 value of 1935 ms), and the maximum mean susceptibility was 0.160 ppm with a T1 value of 2821 ms, which was also the maximum T1 value observed. The minimum mean T1 observed for an individual metastasis was 1342 ms (with susceptibility of -0.030 ppm). Contrast agent uptake in individual metastases also seemed to vary (b).

In Figure 4, susceptibility maps and T1 maps before and after contrast agent enhancement are shown as well as difference images of the T1 maps before and after contrast agent enhancement. Arrow heads point at the metastases, which can be delineated in all images. In susceptibility maps, metastases appear nearly isointense with fine paramagnetic (bright) structures. Due to the T1 shortening effect of contrast agent, metastases display low T1 values after contrast agent injection.

Discussion and Conclusion

Susceptibility values observed here were in agreement with results shown at lower field strength13. Melanoma metastases are known to show T1w hyperintense contrast1-3 associated with melanin. In this work, we observed this only in one patient, in whom the metastases were in accordance with the melanotic pattern, but showed no strongly paramagnetic susceptibility effects. Furthermore, we observed fine paramagnetic structures especially at the borders of the metastases that could be in agreement with small vessels or hemorrhagic events. Due to the high contrast and high resolution available at 7T, QSM and T1 mapping at 7T could help solve the question of which contrast mechanisms guide the appearance of melanoma metastases in larger studies, ideally with histology information available.

Acknowledgements

No acknowledgement found.

References

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  13. Straub S, Laun FB, Freitag MT, et al. Assessment of melanin content and its influence on susceptibility contrast in melanoma metastases. Proc. Intl. Soc. Mag. Reson. Med. 25 (2017):1965.

Figures

Table 1: Measurement parameters of the multi-echo gradient echo and MR2RAGE sequences.

Figure 1: Echoes 1 and 2 of GRE magnitude images and a susceptibility map show one melanoma metastasis (arrow head). The metastasis is only hardly visible on the susceptibility map. All images are pre contrast agent injection.

Figure 2: Boxplots of mean susceptibility values (a) and mean T1 values (b) of all metastases for each patient.

Scatter plot of mean susceptibility values for each melanoma metastasis and mean T1 values (a). (b) shows a scatter plot of mean T1 values before and after contrast enhancement (CE). Each dot represents mean values for one metastasis; blue dots represent metastases in Patient 2, orange in Patient 3, and yellow in Patient 4.

QSM and T1 parameter maps. The first column shows susceptibility maps, the second and third columns show T1 maps before and after contrast agent injection, respectively, and the fourth column shows difference maps of column 3 and 4. Metastases are indicated by arrow heads.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)
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