Spectroscopy Of The Breast

Peter Stanwell, PhD

Carolyn Mountford, D.Phil

Terminology

Abbreviations

Definitions

Detectable biochemicals are distinguished by their resonance frequency, line shape, line width, phase, and peak area

Anatomy-Based Imaging Issues

Overview

MRS success depends on
- Appropriate pre-acquisition set up
- Acquisition protocol
- Post-processing
- Shimming for MRS acquisition is essential to overcome magnetic field inhomogeneities
MRS pitfalls
- B0 inhomogeneities are often dominant factor limiting successful MRS
- Some breast coils are not optimized for spectroscopy
- Presence of biopsy clips may affect shimming
- Some gadolinium-based MR agents have been shown to affect breast MRS
- Lipids
  - Localized shimming more difficult due to abundance of adipose tissue with different susceptibility from surrounding glandular (and malignant) tissue
  - Impact of unwanted lipid signals can be reduced by decreasing amplitude of lipid signal
  - Techniques for lipid suppression exist for use in MRS

Spectroscopy Acquisition Protocols

Data Acquisition

Parameters set during pre-scan include power calibrations, frequency adjustment, shimming, and water suppression adjustment
- Shimming = process by which B0 field is made as homogeneous as possible, typically using linear x-, y-, and z-gradient coils
- Water suppression = process by which water signal is suppressed to allow detection of important metabolites

Spectroscopy Processing

Processing Steps

Water referencing
- Aligns spectral resonances against water set at 4.74 ppm
- Allows for correction of resonance distortions using water signal as reference
Apodization
- In clinical application, this mathematical function is used to improve SNR
Zero filling
- Method of improving spectral resolution
Fourier transformation
- Converts time domain signal into frequency domain
Phase correction
- Allows for correction of distortions in final spectrum
Baseline correction
- Allows for correction of baseline errors in final spectrum
- Vertical display may need to be increased to see resonances between 3.00 and 3.40 ppm

Spectra Interpretation

Malignant vs. Benign

Key metabolite in breast spectroscopy is choline resonance at 3.23 ppm
Malignant breast cancers show resonance at 3.23 ppm
Benign breast lesions show resonance at 3.28 ppm
- This resonance is reported in breast tissue in lactating and other healthy women

Differential Diagnosis

Invasive Tumor

Ductal Carcinoma in Situ

Early stage DCIS (no microinvasion) has spectral characteristics of benign lesion (in vitro studies)
DCIS with microinvasion has same spectral profile as malignant tumor
- These results need to be verified with larger studies