Echo Planar Spectroscopic Im aging ROBERT V.M ULKERN, 1,2 LAW RENCE P. PANYCH 2 1 D epartm ent of R adiology, C hildren’s H ospital, B oston, M assachusetts 2 B righam and W om en’s H ospital, H arvard M edical S chool, B oston, M assachusetts ( ) ABSTRACT: Echo planar spectroscopic im aging EPSI allowsform apping one spatial dim ension and the spectroscopic dim ension in a single readout.M odern clinical m agnetic resonance imaging scanners are becoming widely equipped with echo planar imaging gradient capabilities.Thus there exists an enorm ous potentialfor EPSI to facilitate clinical m agnetic resonance spectroscopy. H ere, a pedagogical analysis of how the spectral and spatialencoding occur w ithin EPSIis provided along w ith dem onstrations ofthe technique and a discussion of the inherent signal-to-noise lim itations.  2001 John W iley& Sons,Inc. Concepts M agn Reson 13:213 237,2001 KEY W ORDS: fast spectroscopic im aging; echo planar; chem ical shift im aging; m agnetic resonance IN TR O D U CTIO N Ž 1 . The proton H nuclear m agnetic resonance Ž . NMR signal from the hum an body hasbecom e an increasingly valuable entity. M anipulated w ith m agnetic field gradients to m ake im ages ofour body parts,the 1 H N M R signal from tissue w ater and adipose tissue, or fat, is the essential ingredi- ent of m odern m agnetic resonance im aging Ž . M R I .In 1994 alone,som e 27 outof1000 people Ž . in the U nited States received an M RIexam 1 . Im ages offat and water,together or separately Ž . 2, 3 ,are not,how ever,the sole source of infor- m ation available from M R I scanners. T he less well-known but older cousin of M RI,m agnetic Ž . resonance spectroscopy M R S , is also playing an R eceived 9 N ovem ber 2000; revised 18 February 2001;accepted 20 February 2001. C orrespondence to: R obert V .M ulkern; E -m ail: m ulkern @ bw h.harvard.edu. Ž . Ž . Concepts in M agnetic R esonance,V ol.13 4 213 237 2001  2001 John W iley & Sons,Inc. increasingly im portant role in the clinical use of Ž . M R I scanners 4 . T he m ost prom inent applica- tion for 1 H M RS isforbrain m etabolite detection Ž . and or quantitation 5 , particularly for brain Ž . tum or treatm ent m onitoring 6, 7 but also for Ž . other brain disorders 8, 9 .T he prostate gland is also a target for clinical 1 H M R S studies, provid- ing an interesting and prom ising adjunct to both im aging and standard clinical assessm ents of the response of prostate cancer to various therapies Ž . 10, 11 . B oth brain and prostate M R S studies focus on the detection and quantitation of low concentra- tion m etabolites for w hich the ubiquitous w ater m olecule is the solvent w hose signal is generally suppressed in order to allow for easier detection of the m etabolites of interest. T here are also potential m edicaluses for M RS applications in w hich the prim ary resonances from w ater and fat are the focus of interest. M easurem ent of overall body fat content with M R is a task em inently suited for proton spectroscopic im aging and of Ž general m edical utility for som e conditions 12, 213