Differentiation of Breast Cancer from Fibroadenoma with Dual-Echo Dynamic Contrast-Enhanced MRI

Dynamic contrast-enhanced magnetic resonance imaging (DCE MRI) of the breast is a routinely used imaging method which is highly sensitive for detecting breast malignancy. Specificity, though, remains suboptimal. Dynamic susceptibility contrast magnetic resonance imaging (DSC MRI), an alternative dynamic contrast imaging technique, evaluates perfusion-related parameters unique from DCE MRI. Previous work has shown that the combination of DSC MRI with DCE MRI can improve diagnostic specificity, though an additional administration of intravenous contrast is required. Dual-echo MRI can measure both T1W DCE MRI and T2*W DSC MRI parameters with a single contrast bolus, but has not been previously implemented in breast imaging. We have developed a dual-echo gradient-echo sequence to perform such simultaneous measurements in the breast, and use it to calculate the semi-quantitative T1W and T2*W related parameters such as peak enhancement ratio, time of maximal enhancement, regional blood flow, and regional blood volume in 20 malignant lesions and 10 benign fibroadenomas in 38 patients. Imaging parameters were compared to surgical or biopsy obtained tissue samples. Receiver operating characteristic (ROC) curves and area under the ROC curves were calculated for each parameter and combination of parameters. The time of maximal enhancement derived from DCE MRI had a 90% sensitivity and 69% specificity for predicting malignancy. When combined with DSC MRI derived regional blood flow and volume parameters, sensitivity remained unchanged at 90% but specificity increased to 80%. In conclusion, we show that dual-echo MRI with a single administration of contrast agent can simultaneously measure both T1W and T2*W related perfusion and kinetic parameters in the breast and the combination of DCE MRI and DSC MRI parameters improves the diagnostic performance of breast MRI to differentiate breast cancer from benign fibroadenomas.


Introduction
Magnetic resonance imaging (MRI) has become an important technique for breast cancer detection, diagnosis, and staging [1]. When lesion morphology is combined with the dynamic analysis of contrast kinetics within breast lesions, the overall sensitivity of MRI is nearly 90%, and specificity varies between 67% and 72% [2,3]. Compared to all other imaging techniques (including ultrasonography and mammography), the negative predictive value of MRI remains the highest of all modalities [4,5].
Conventional dynamic contrast enhanced MRI (DCE MRI) is the most widely used and clinically validated technique for breast cancer MRI. It not only provides morphological information, but also typically uses high spatial resolution to estimate T 1 W-related contrast uptake parameters. Despite high sensitivity, diagnostic specificity remains unsatisfactory [6,7]. An alternative dynamic contrast technique, known as dynamic susceptibility-contrast MRI (DSC MRI) uses high temporal resolution to obtain perfusionrelated parameters based on T 2 *measurements, such as relative regional blood volume (rBV)and relative regional blood flow (rBF) [8][9][10]. Perfusion-related parameters can differentiate malignant from benign lesions [8][9][10]. The combination of conventional DCE MRI and DSC MRI with two administrations of contrast agent (CA) has demonstrated the capability to substantially improve the diagnostic specificity of breast MRI [7,11].
Using dual-echo MRI, and with a single administration of contrast agent, T 1 W and T 2 *W related measurements can be simultaneously acquired, with the first echo acquiring T 1 W (DCE MRI) data and the second echo (6.3 ms later) acquiring T 2 *W data (DSC MRI) [12][13][14]. To date, dual-echo MRI has not been performed in the breast. We have developed a dual gradient echo (GRE) sequence and have used it to evaluate both T 1 W and T 2 *W related parameters in differentiating breast cancer from the most common benign breast mass, fibroadenoma.

Ethics Statement
The study was approved by the ethics committee of the First Affiliated Hospital of Zhejiang Chinese Medical University, Hangzhou, China, and performed in accordance with the ethical guidelines of the Declaration of Helsinki. After a throughout explanation of the study to the patient, written informed consent was obtained from 37 patients and 1 guardian on the behalf of the minor.

Patients
Between May 2011 to March 2012, 46 patients with suspected breast cancer based on mammography and the BIRADS category being 4 or 5 underwent dynamic contrast enhanced MRI and dual-echo dynamic contrast enhanced MRI examination. Of the 46 patients, 38 patients had subsequent surgery or biopsy with pathologic correlation (mean age: 45 years, range: 15-65 years).There were 45 lesions evaluated in 38 patients, with 5 patients having two lesions and one patient having three lesions. In patients with multiple lesions, only the largest lesion was analyzed in this study. All patients had normal renal function (eGFR.60 ml/min/1.73 m 2 ).

Dual-Echo based DCE MRI and DSC MRI
A dual-echo based gradient echo sequence has been implemented to acquire both DCE MR and DSC MR images simultaneously with the 1st echo as T 1 W and the 2nd echo as T 2 *W. The signal intensity of a GRE can be given by Where, r 0 denotes the spin density, h denotes the flip angle, TR denotes the repetition time, TE denotes echo time, T 1 (t) denotes the longitude relaxation time at time t. The equation clearly shows that the signal of DCE MR and DSC MR images are determined   by not only T 1 component, but also the T 2 * effect. That is to say, there are errors associated with the commonly used CK model for perfusion which assumes a constant T 2 *, and the commonly used CK model for permeability which assume a constant T 1 .
Simultaneously acquired T 1 W and T 2 W* data make it possible to correct the errors.
To take into account the T 2 * effect in permeability analysis, we first calculate T 2 * map by taking the advantage of the dual echo pulse sequence using Equation (2), Where S echo1 (t) and S echo2 (t) are signal intensities from the 1st echo (TE 1 ) and 2nd echo (TE 2 ), respectively. To exclude T 2 * dependency in each pixel, the signal intensity S(t) of Equation (1) is divided by the corresponding T 2 * exponential component thereby obtaining only T 1 -dependent signal intensity S T1 (t) for T 1 W DCE MR imaging, To take into account the T 1 effect on perfusion, we take the advantage of the dual-echo sequence to calculate only T 2 *dependent signal intensity S T2* (t) using Equation (4),

MR Imaging Technique
All imaging was performed on a 3T whole-body MRI scanner (Verio, Siemens, Germany) with a sixteen-channel phased-array breast coil. Data were obtained for routine clinical diagnosis, which included routine clinical semi-quantitative DCE MRI in addition to the experimental dual-echo DCE/DSC MRI sequence.

MR Image Analysis
Image data was saved and transferred to an offline workstation. Custom software written in house with Matlab 7.6 (MathWorks, Natick, Mass, USA) was used for subsequent analysis.
Peak enhancement ratio (PER) and time from contrast agent arrival to peak enhancement (T max ) are semi-quantitative T 1 W related parameters and they are often used in DCE MRI. Relative regional blood flow and regional flood volume are T 2 *W related parameters (perfusion parameters) and the most relevant parameters obtained in dynamic susceptibility-contrast MRI (DSC MRI). The semi-quantitative T 1 W related parameters, including PER and T max of conventional DCE MRI (c-PER, c-T max ) and dual-echo MRI (d-PER, d-T max ), and the T 2 *W related parameters such as rBV and rBFof dual-echo MRI (d-rBV, d-rBF) were calculated [15][16][17][18][19]. In the calculating of dual-echo MRI parameters, T 2 * effects were removed in T 1 W related parameters analysis and T 1 effect would be taken into account in perfusion analysis.

Statistical Analysis
The parametric variables were compared using one-way ANOVA. Pathology results from tissue sampling were considered the gold standard. The mean and variance were used in this setting. Receiver operating curves (ROC) and the area under the ROC curve (AUROC) were calculated as a descriptive tool to assess the overall discrimination of individual parameters and combined parameters. Sensitivity, specificity and Kappa statistic were used with respect to the diagnostic performance. Analysis was performed with SPSS 19 software (SPSS Inc., Chicago, IL). A P value of less than 0.05 was considered to indicate a statistically significant difference.

Comparison of the Semi-quantitative Conventional DCE MRI and Dual-echo MRI Parameters between Breast Cancer and Fibroadenoma
Breast cancers displayed a lower value of T max with conventional DCE MRI (c-T max ) than fibroadenomas (P = 0.014). Breast cancers also had lower values of T max , rBF and rBV with dualecho MRI (d-T max , d-rBF and d-rBV) than fibroadenomas (P = 0.017, P = 0.029, P = 0.046, respectively). No significant difference regarding peak enhancement ratio (PER) of conventional DCE MRI and dual-echo MRI (P = 0.423, 0.252, respectively) was found between breast cancers and fibroadenoma (Table 1).
ROC analysis of both semi-quantitative conventional DCE MRI and dual-echo MRI parameters are given in Table 2, with ROC curves shown in Figure 1. The most relevant factors for discriminating breast cancer from fibroadenoma, based on the areas under the ROC curve (AUROC) were c-T max , d-T max , d-rBF and d-rBV, (0.800, 0.794, 0.613,0.619, respectively). Using ROC analysis, a c-T max ,249 s had a sensitivity of 90% and a specificity of 68.8% for predicting malignancy, a d-T max ,183(s) had a sensitivity of 80% and a specificity of 75% for predicting malignancy, a d-rBF,0.024 had a sensitivity of 50% and a specificity of 80% for predicting malignancy, a d-rBV,0.005 had a sensitivity of 60% and a specificity of 80% for predicting malignancy.
Combining the d-T max of dual-echo MRI with rBV, sensitivity is 90% and specificity becomes 70%. When combining the three factors of T max , rBF and rBV, sensitivity remains unchanged at 90% but specificity increases to 80% (Table3).

Discussion
Conventional T 1 W DCE MRI typically uses high spatial resolution to estimate T 1 W related semi-quantitative parameters such as c-PER and c-T max . Despite high sensitivity, diagnostic specificity remains unsatisfactory [7,11].The addition of perfusionrelated parameters obtained from T 2 *W DSC MRI, such as rBF and rBV [8][9][10], has been shown to increase examination sensitivity and specificity. Improved specificity likely is due in part to an increased number of capillaries and a greater mean capillary diameter in malignant tissues compared to those in benign tissues [7].
It has been shown that simultaneous measurement of T 1 W and T 2 *W parameters can improve the accuracy of both perfusion parameters and permeability kinetics using dynamic contrast enhancement [14]. In particular, T 2 * effects of gadolinium contrast agents, which are more pronounced with increased concentrations, can be measured to reduce underestimation of peak enhancement and overestimation of permeability [14]. A dual-echo gradient echo sequence is a requirement to perform simultaneous T 1 W and T 2 *W measurements using a single contrast dose, and eliminates motion artifact between measurements, since echoes are spaced less than 7 ms apart. Despite the attractiveness of such a technique, dual-echo MRI has not been performed in the breast until now due to various technical challenges [20][21][22].Furthermore, the effects of T 1 -corrected perfusion and T 2 *-corrected T 1 W semi-quantitative parameters have never been evaluated within breast tissue. In fact, there are no comparison studies between normal, benign, or malignant breast with regard to T 2 * effects on routinely-acquired T 1 W dynamic perfusion parameters, and systemic errors can be considerable (particularly within tumors) when this effect is not considered. The dual-echo technique magnetic resonance sequence and analysis software we have implemented leverages multiple recent techniques developed by our group for imaging the breast [23][24][25] to explore the clinical potential of reducing the false positive rate in clinical breast MRI.
The combination of conventional DCE MRI and DSC MRI with two administrations of contrast agent has demonstrated substantially improvement in the diagnostic specificity of breast MRI [7,11,12]. Our data show that dual-echo MRI can simultaneously measure the T 1 W and T 2 *W related parameters and improves the accuracy of differentiating breast cancer from fibroadenomas. Dual-echo MRI with a single administration of contrast agent has the advantage of eliminating the expense of a second contrast dose administration, the elimination of a second imaging sequence. Furthermore, the dual-contrast dose technique necessitates a delay between contrast dose administrations to allow washout of residual contrast from the first administration; the dual-echo MRI technique obviates this delay, which typically is 15 minutes. Finally, reduction of total contrast agent administered improves the safety profile of the examination.
This study has several limitations. Only a small number of patients were included, as the intent was an initial investigation into the feasibility and applicability of dual-echo MRI in the human breast. A larger number of patients would permit improved sensitivity, specificity, and accuracy estimations in various subtypes of malignant and benign lesions. Furthermore, our current implementation of dual-echo MRI only allows a single slice to be acquired through a lesion with sufficient temporal and spatial resolution. However, we anticipate that with further iterations in sequence design and improvements in MRI technology a substantially larger imaging volume is achievable.

Conclusions
In conclusion, we show that dual-echo MRI with a single administration of contrast agent can simultaneously measure both T 1 W and T 2 *W related kinetic and perfusion parameters in the breast. We further show that combining T 1 W DCE MRI measurement of contrast kinetics with T 2 *W DSC MRI perfusion measurements improves the diagnostic performance of breast MRI to differentiate breast cancer from benign fibroadenomas.