NMR IN BIOMEDICINE NMR Biomed. 2005;18:285–292 Published online 22 June 2005 in Wiley InterScience (www.interscience.wiley.com). DOI:10.1002/nbm.954 Comparison of high-resolution echo-planar spectroscopic imaging with conventional MR imaging of prostate tumors in mice and Gregory S. Karczmar* Department of Radiology, University of Chicago, Chicago, IL 60637, USA Received 27 September 2004; Revised 20 December 2004; Accepted 21 December 2004 ABSTRACT: High spectral and spatial resolution (HiSS) MRI of rodent tumors has previously been performed using conventional spectroscopic imaging to obtain images with improved contrast and anatomic detail. The work described here evaluates the use of much faster echo-planar spectroscopic imaging (EPSI) to acquire HiSS data from rodent tumor models of prostate cancer. A high-resolution EPSI pulse sequence was implemented on a 4.7 T Bruker scanner. Three-dimensional EPSI data were Fourier-transformed along the k-space and temporal (free-induction decay) axes to produce detailed water and fat spectra associated with each small image voxel. The data were used to generate images of spectral parameters, e.g. peak-height images for each small voxel. Two variants of EPSI were performed; gradient-echo or spin-echo excitation with EPSI readout. These imaging methods were tested in commonly used rodent prostate cancers, including seven mice implanted with non-metastatic AT2.1 (n ¼ 3) and metastatic AT3.1 (n ¼ 4) prostate tumors on the hind leg, and 10 mice implanted with LNCaP prostate cancers in situ. The peak-height images derived from EPSI datasets provide more detailed tumor anatomy, improved signal-to-noise and contrast-to-noise ratios compared with the gradient-echo or spin-echo images at all echo times. The results suggest that HiSS MRI data from small animal models of prostate cancer can be acquired using EPSI, and that this approach improves imaging of heterogeneous tissue and vascular environments inside the tumors compared with conventional MR techniques. Copyright # 2005 John Wiley & Sons, Ltd. KEYWORDS: prostate cancer; MRI; echo-planar spectroscopic imaging; tumor; mouse; spectral/spatial imaging INTRODUCTION Prostate cancer is the second leading cause of cancer death in men following lung cancer. 1 With development of better technology, MRI has increasingly been used for its diagnosis. Prostate cancer is usually detected in MR images based on T 2 /T * 2 contrast. 2 Malignancy is seen as an island of low signal intensity surrounded by high signal intensity from benign peripheral tissue. 3 However, T 2 - weighted MR images require a long echo time (TE), in order to obtain sufficient contrast between the tumor and surrounding normal tissues. The resulting low signal-to- noise ratio (SNR) and loss of image definition impede early detection, accurate measurement of tumor volume and the characterization of tumor invasiveness. 4 To improve the morphological imaging of prostate cancers, various MR techniques have been proposed. 5–7 Recently, diffusion MRI has been demonstrated to in- crease tumor-to-normal tissue contrast in rodents 8 and humans, 9 compared with T 2 -weighted MRI. An alterna- tive approach is to use high resolution spectroscopic imaging methods, i.e. sampling the free-induction decay (FID) to produce a detailed proton spectrum of water and fat for each voxel to obtain improved functional and anatomic images. Very high spatial resolution is possible using EPSI because the water and fat resonances are imaged, rather than the relatively low concentrations of metabolites that are more common targets for spectro- scopic imaging. 11,17,23 Sarkar et al. have successfully used echo-planar spectroscopic imaging (EPSI) to im- prove anatomic imaging of the human pelvis and joints. 11 Work from this laboratory demonstrated improved ana- tomical and functional imaging of rodent tumors 12–15 and human breast 16–18 with high spectral and spatial resolu- tion (HiSS). 19,20 Yang et al. demonstrated that both SNR and T * 2 contrast can be increased in MR images of cerebral Copyright # 2005 John Wiley & Sons, Ltd. NMR Biomed. 2005;18:285–292 *Correspondence to: G. S. Karczmar, University of Chicago, Depart- ment of Radiology, MC2026, 5841 S. Maryland Ave, Chicago, IL 60637, USA. E-mail: gskarczm@uchicago.edu Contract/grant sponsor: National Cancer Institute; contract/grant number: 1 R21 CA089408. Contract/grant sponsor: Department of Defense; contract/grant number: DAMD17-02-1-0033. Abbreviations used: gEPSI, gradient echo excitation EPSI readout pulse sequence; sEPSI, spin echo excitation EPSI readout pulse sequence; HiSS, high spectral and spatial resolution; CSI, conventional spectroscopic imaging; PH, peak–height; PF, peak–frequency. Weiliang Du, Xiaobing Fan, Sean Foxley, Marta Zamora, Jonathan N. River, Rita M. Culp