Advances in breast MRI: diffusion-weighted imaging of the breast
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- Kuroki, Y. & Nasu, K. Breast Cancer (2008) 15: 212. doi:10.1007/s12282-008-0050-3
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DWI, which is less affected by the state of the background mammary gland, has sufficient capability to diagnose invasive, noninvasive, and invasive lobular carcinoma. DWI is different from conventional methods based on blood flow data and has the potential to provide useful information for the evaluation of NAC, which is considered to be insufficient at present. Having the ability to provide steady, high-resolution tissue images, DWI is expected to play an important role in future breast cancer diagnosis. DWI, however, is not a complete method of diagnosis. Limits exist in spatial resolution and problems with ADC. In consideration of these shortcomings, we need to apply DWI to clinical practice while taking advantage of its high contrast resolution.
KeywordsBreast cancerDiffusion-weighted imaging
DWI is an imaging method that uses in vivo thermal diffusion of water molecules. Diffusion-weighted images regularly used in clinical practice reflect the diffusion of water molecules in the extracellular fluid space and allow estimation of cellularity and tissue structure. This article outlines essentials of parameter setting of DWI for scanning mammary glands and the application of DWI to breast cancer diagnosis.
Characteristics and roles of DWI
DWI reveals cellularity and yields very high contrast resolution of tissues. Because DWI is low in spatial resolution, comparison with other types of imaging data is required to supplement anatomical data and signal interpretation. The main roles of DWI in its application to mammary gland imaging are diagnostic identification of potential lesions and reduction of oversight. More specifically, DWI identifies lesions by non-contrast imaging, identifies lesions when mastopathy exists in the background, and detects multiple ipsilateral breast cancers and contralateral breast cancers.
Areas of high signal intensity do not always represent malignant lesions. The common diagnostic procedure, therefore, involves identification of potential lesions by DWI and acquisition of supporting information from other imaging techniques. DWI data are then reinterpreted in conjunction with the supporting data.
Procedure of DWI
Example of DWI scan parameters
Achieva 1.5-T release 2.5.3 (Philips)
Thick/gap: 5/0 mm
Coil: 7ch breast body
Slices: 24; NEX: 3
SENSE factor: 2
Scan time: 137 s
FOV: 320 mm
Measured voxel size: 2.5 × 3.16 × 5 mm
Matrix: 128 × 91
Recon. voxel size: 1.25 × 1.25 × 5 mm
Application of parallel imaging method
Presently, DWI of the body is based on data acquisition by single-shot echo-planar imaging (SS-EPI). The SS-EPI method, however, is easily affected by susceptibility artifacts. As another advance in MRI, parallel imaging techniques have been developed with the aim of reducing imaging time and improving spatial resolution, as exemplified by the sensitivity encoding (SENSE) technique . Image distortion is determined by the phase-shift-per-line interval in k-space. By applying the parallel imaging technique to SS-EPI-DWI, the number of phase-encoding steps is reduced. As a result, the phase shift is reduced when the entire k-space is filled, thus leading to reduced image distortion.
Image distortion also can be reduced by reducing the field of view or increasing the pixel size, but these approaches may conflict with clinical needs. Accordingly, application of the parallel imaging technique to DWI may be warranted. Diffusion-weighted images with quality sufficient for daily clinical practice were finally achieved through the development and application of the parallel imaging technique. Comprehensive clinical application of DWI to the body has now begun.
Selection of fat suppression technique
Fat suppression is essential to breast DWI. The two main fat suppression techniques presently used for DWI are the short time inversion recovery (STIR) and chemical shift selective suppression (CHESS) methods. The STIR method used for diffusion-weighted whole-body imaging with background body signal suppression (DWIBS) provides steady fat suppression, but its signal-to-noise ratio is lower than that of CHESS. The CHESS method does not always provide uniform fat suppression, but its signal-to-noise ratio is higher.
Selection of b factor
Previous studies have indicated that the spread of cancers that develop mainly through breast ducts is not as restricted as that of invasive cancers [3, 4]. Therefore, if the b factor is set too high, cancers may be detected in a reduced form. In severe cases of mastopathy, the background mammary gland signal may not be suppressed sufficiently even when a b factor of 1,000 s/mm2 is used. If multiple b factor values could be selected, a combination of 750 and 1,500 s/mm2, or 1,000 and 1,500 s/mm2, may be useful.
In diffusion-weighted images, ideally the normal mammary gland signal should be suppressed to noise levels. The signal of cysts depends on the diffusion of their content. When the content is serous, the signal intensity is low because of the low restriction on diffusion. When the content is mucinous, the signal intensity increases according to the extent of restriction on diffusion. The DWI signal of fibroadenoma depends on cellularity. When diagnosing tumors, morphologic and other data are required in addition to DWI data.
Breast cancer imaging capability
A total of 54 cases were studied: 22 cases of papillotubular carcinoma, 22 cases of solid-tubular carcinoma, and 10 cases of scirrhous carcinoma. The cancers could be detected by the DWI–STIR method in all 54 cases. No significant differences were observed in the percentages of correct diagnosis between different pT categories or different mammary gland densities.
A total of 30 cases were studied, and cancers could be detected as malignant lesions by the DWI–STIR method in 26 cases (87%). Seven cases were identified as mammography category 1 or 2, and five cases (71%) could be diagnosed as malignant lesions by the DWI–STIR method. When the background mammary gland was classified into two categories, fatty and fibroglandular breast (10 cases) and heterogeneous and dense breasts (20 cases), their percentages of correct diagnosis were 80 and 90%, respectively. The percentages were high, with no significant differences between different mammary gland densities.
Invasive lobular cancer
A total of 30 cases were studied, and cancers could be detected as malignant lesions by the DWI–STIR method in 28 cases (93%). Of the 30 cases, 6 were identified as mammography category 1 or 2, and 5 cases (83%) could be diagnosed as malignant lesions by the DWI-STIR method. Excluding the pT4 category (2 cases), the percentages of correct diagnosis for the pT1 category (12 cases) and the pT2 and pT3 categories (16 cases) were 92 and 94%, respectively. When the background mammary gland was classified into two categories, fatty and fibroglandular breast (6 cases) and heterogeneous and dense breasts (24 cases), their percentages of correct diagnosis were 100 and 92%, respectively. The percentages were high, with no significant differences between different pT categories or different mammary gland densities.
Evaluation of neoadjuvant chemotherapy
Neoadjuvant chemotherapy (NAC) has been found to be effective for breast cancer and is being applied to an increasing number of cases. NAC is found to correlate with prognosis only when the therapeutic effect is judged as a pathologically complete response (pCR). Although diagnosis of pCR is required in diagnostic imaging, the therapeutic effect is difficult to judge in many cases because of the additional effects of NAC.
Evaluation of neoadjuvant chemotherapy (NAC) for Gd-dynamic MRI alone and Gd-dynamic MRI combined with DWI
Dyn + DWI
Application to cancer screening
The ultimate objective of cancer screening is to improve the vital prognosis of individual cancer patients and reduce the mortality of the population. Accurate and timely detection of cancer is crucial in situations where lives can be saved. The conditions for the use of DWI for cancer screening are that the target cancer can be detected in a timely manner and that a reasonable level of cost effectiveness can be achieved compared with other examination methods. Appropriate selection of patients to be examined also must be carefully considered; high-risk groups are likely to be given priority. At present, we do not have sufficient objective data to evaluate the effectiveness of using DWI for cancer screening, and future studies are warranted.
Measurement of ADC
In the conventional MRI diagnosis of mammary gland tumors, benign and malignant tumors have been distinguished mainly on the basis of morphology and blood flow data. As DWI has become more widely employed, a new approach for distinguishing between benign and malignant tumors through ADC measurements is being pursued. Compared with normal mammary gland and benign tumors, cancers cause low ADC values because of significantly reduced diffusion. However, cancers cannot be distinguished completely from benign tumors because there is an overlap between their ADC ranges. Moreover, MRI equipment is not yet precise enough to be used as a measuring instrument in routine clinical practice, and problems exist in the measurement methods.
To measure ADC, the region of interest needs to be set by the operator, which is arbitrary and adds to the above problems. It is prudent to use ADC only for reference purposes because of difficulty in achieving universality in ADC measurement.
Through the progress of recent MR technique, application to breast cancer of DWI has established. To interpret DWI, we can use molecular information for diagnosis of breast cancer and evaluation of neoadjuvant chemotherapy, as well as conventional morphologic and vascular information. In the future, a further study of the diffusion phenomenon in vivo and optimization of the scan parameter in the higher magnetic fields is expected.