Magnetic resonance in surgical oncology: II - literature review

A high spatial resolution in vivo 1H magnetic resonance spectroscopic imaging technique for the human breast at 3 T

PURPOSE

The technical challenges that have prevented routine proton magnetic resonance spectroscopic imaging (1H MRSI) examinations of the breast include insufficient spatial resolution, increased difficulties in shimming compared to the brain, and strong lipid contamination at short echo time (TE) at 1.5 T. The authors investigated the feasibility of high spatial resolution 1H MRSI of human breast cancer in a clinical setting at 3 T.

METHODS

Ten patient studies (eight cancers and two benign lesions) were performed in a 3 T whole-body clinical imager using a pulse sequence consisting of optional outer volume presaturation, optional CHESS pulse for lipid suppression, CHESS pulse for water suppression, and standard 2D/3D PRESS pulse sequence with an elliptical weighted k-space sampling scheme.

RESULTS

All ten studies were technically successful. The spectral quality was acceptable for all cases even the one with a 65 Hz width of water peak at half height. Choline (Cho) signals were clearly visible in malignant lesion areas, while there was no detectable Cho in normal appearing breast or in benign lesions. It was also observed that the distribution of Cho signal can be nonuniform across MRI demonstrated lesions.

CONCLUSIONS

To the author's knowledge, this is the first 2D/3D MRSI study of human breast cancer with short TE (less than 135 ms) at 3 T and the highest spatial resolution (up to 0.25 cm3) to date. In conclusion, the authors have presented a robust technique for high spatial resolution in vivo 1H MRSI of human breast cancer that uses the combined advantages of high field, short TE, multivoxel, and high spatial resolution itself to overcome the major technical challenges and illustrated its potential for routine clinical examination as well as advantages over single-voxel techniques in studying metabolite heterogeneity.

Targeting regions with highest lipid content on MR spectroscopy may improve diagnostic yield in stereotactic biopsy

Gliomas are heterogeneous brain tumors and prognosis and treatment are dependent on the highest histological grade present. Stereotactic biopsy is associated with an inherent risk of sampling error. Magnetic resonance spectroscopy (MRS) uses clinical MR scanners to provide chemical in addition to conventional information. MRS studies in brain tumors have found increased levels of choline-containing compounds (Cho) and decreased levels of N-acetylaspartate (NAA), creatine (Cr) and phosphocreatine (PCr) which are all associated with increased grade of glioma. We propose the use of MRS-guided stereotactic biopsy of astrocytomas to increase diagnostic yield and reduce the sampling error rate. MRS was performed on two patients undergoing stereotactic biopsy for suspected astrocytoma. Spectral peak heights of the metabolites Cho at 3.2 parts per million (ppm) chemical shift, NAA at 2.0 ppm and lipid (Lip) at 1.2 ppm, were measured. Biopsy targets were selected from the voxels with the highest Cho/NAA and normalised against Cho levels in the contralateral normal tissue, as well as those with highest lipid content. The biopsies were taken and tissue diagnosis was obtained via standard histological techniques. Histological grade was found to be different in one case: the region with a high Lip/Cr and Cho/NAA ratios showed glioblastoma, whereas the region with high Cho/NAA but low Lip/Cr ratios showed anaplastic astrocytoma. The second patient had high Cho/NAA ratio but low Lip/Cr ratio in both targets and the histology revealed anaplastic astrocytoma in both samples. MRS is a useful biomedical imaging tool for diagnosing and grading astrocytomas. Targeting regions with highest lipid content can potentially improve the diagnostic yield and minimize sampling error in stereotactic biopsy.

A high spatial resolution 1H magnetic resonance spectroscopic imaging technique for breast cancer with a short echo time

The high sensitivity but low specificity of breast MRI has prompted exploration of breast (1)H MRS for breast cancer detection. However, several obstacles still prevent the routine application of in vivo breast (1)H MRS, including poor spatial resolution, long acquisition time associated with conventional multi-voxel MRS imaging (MRSI) techniques, and the difficulty of "extra" lipid suppression in a magnetic field with relatively poor achievable homogeneity compared to the brain. Using a combination of a recently developed echo-filter (EF) suppression technique and an elliptical sampling scheme, we demonstrate the feasibility of overcoming these difficulties. It is robust (the suppression technique is insensitive to magnetic field inhomogeneity), fast (acquisition time of about 12 min) and offers high spatial resolution (up to 0.6 cm(3) per voxel at 1.5 T with a TE of only 60 ms). This approach should be even better at 3 T with higher resolution and/or shorter TE.

Limits of a localized magnetic resonance spectroscopy assay for ex vivo myocardial triacylglycerol

Localized magnetic resonance spectroscopy (LMRS) promises a powerful non-invasive means to determine myocardial triacylglycerol (TAG) in a clinical setting. Here, the linearity, specificity, robustness, precision, and accuracy of an ex vivo mouse-heart LMRS TAG assay are assessed by quantifying the spatial, spectral, and relaxation-induced uncertainties. The protocol, which is based on localization by adiabatic selective refocusing (LASER) using frequency offset corrected inversion (FOCI) pulses, alternating gradient polarity, and simple post-processing, is shown to have good characteristics. The presented protocol has a benchmark, phantom-based, accuracy of 3%, and when applied to ex vivo mouse hearts the accuracy is 6%, making the LMRS assay comparable to the typical destructive bioanalytical assay.