We found a significant reduction in the mean ADC value of the dominant fibroid following embolisation. This reduction in ADC suggests that cellular integrity is disrupted, causing cellular dehydration [10
]. Histologically, it is known that fibroids, which are not degenerate, are composed of densely packed smooth muscle cells with varying amounts of intervening collagen [16
]. Prior to embolisation, these clusters of cells with intact cell membranes result in a restricted pattern of diffusion, which decreases following infarction.
We also noted a slight reduction in the mean ADC value of the myometrium following embolisation. This was unexpected, and may reflect a degree of myometrial damage or, given the small ROI for measurement (10–18 mm), a technical problem.
Reduction of myometrial ADC value is significantly less than the dominant fibroid (p
=0.005), suggesting that myometrial reperfusion occurs. Perfusion studies performed immediately after UAE have shown maintained myometrial perfusion [17
A moderately strong and significantly positive correlation was identified (r
=0.007) between the baseline ADC values and subsequent dominant fibroid shrinkage. The exact reason for this is not clear; however, the degree of volume reduction may depend to some extent on the intrinsic histological nature of the fibroid. It has been suggested that this may relate to the degree of hyaline degeneration of fibroids [11
], and this is a possibility, particularly since there is no way of differentiating these from non-degenerate fibroids on standard MRI sequences [18
We did not identify any significant correlation between post-embolisation ADC values and percentage volume reduction (r=0.17, p=0.54). This is probably secondary to loss of the cellular architecture following embolisation, which reduces ADC values.
Our study used SDNR to objectively measure the degree of contrast enhancement. This technique has been advocated for quantitative measurement of focal masses on MRI [12
] and has been used in the evaluation of pituitary lesions [19
], and permits an objective measure of contrast enhancement, rather than a subjective score.
Using the above technique, we did not identify any correlation between the ADC value and SDNR, pre- or post-embolisation. A recently published study that utilised a slightly different measure of contrast enhancement (signal measurement on dynamic contrast imaging) has also failed to show any correlation between the two parameters [11
]. However, one previous study, where a subjective percentage scale of enhancement scoring was used, did show a positive correlation between post-embolisation ADC value and degree of baseline contrast enhancement [10
]. This difference is difficult to explain, but may arise because of different study methodology.
ADC value measures net movement of water molecules within tissue, and is therefore dependent on its histological nature and its cellular/fibrous content. Signal intensity measures degree of contrast enhancement, which in turn reflects extracellular contrast distribution. Both of these measure different values, and this is likely to be a plausible explanation for the lack of correlation between these two variables in our study.
We did not identify any significant correlation between percentage volume reduction and SDNR pre- or post-embolisation. It has also been demonstrated on perfusion imaging that poorly perfused and well-perfused leiomyomas show no difference in mean volume reduction [17
], and lack of correlation between degree of contrast enhancement and volume reduction is not different from previous studies [17
There were several limitations to our study. It was retrospective and only included 15 patients, lacking power to detect true correlations between pairs of variables. We chose to assess only the dominant fibroid rather than the total fibroid burden. Our ADC calculations were based only on b=1000 values as we felt that there may be perfusion-related discrepancy at lower b-values. The signal-to-noise ratio in the b=1000 images were adequate for ADC calculation. Given the small ROI for calculation, the ADC values of the myometrium may have been prone to a degree of sampling error.
In conclusion, we have shown a significant positive correlation between baseline ADC values and subsequent dominant fibroid volume reduction. Larger prospective studies are necessary to confirm whether diffusion-weighted MRI offers any useful additional benefit to standard CEMRI in UAE.