When more is less: Higher magnetic fields and their limited impact on signal-to-noise ratio per unit of acquisition time in unlocalized and single-voxel magnetic resonance spectroscopy

Magnetic resonance spectroscopy (MRS) offers significant diagnostic potential but is inherently constrained by a low signal-to-noise ratio (SNR). While increasing the main magnetic field strength <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:msub> <a:mi>B</a:mi> <a:mn>0</a:mn> </a:msub> </a:math> is theoretically linked to increased SNR, practically obtained gains in SNR from <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:msubsup> <c:mi>B</c:mi> <c:mn>0</c:mn> <c:mrow> <c:mn>7</c:mn> <c:mo>/</c:mo> <c:mn>4</c:mn> </c:mrow> </c:msubsup> </c:math> to <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:msub> <e:mi>B</e:mi> <e:mn>0</e:mn> </e:msub> </e:math> , depending on the domination of thermal noise at high <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:msub> <g:mi>B</g:mi> <g:mn>0</g:mn> </g:msub> </g:math> , are not always realized. Especially in clinical settings, the maximum reachable SNR is further constrained by the total available acquisition time (TA) and the regulatory limits on maximum tolerable specific absorption rate (SAR). This work attempts to derive mathematical expressions that enable systematic analysis of the theoretically achievable SNR gain. One important notion in this context is the SNR gain per unit of measurement time as a function of the main magnetic field <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:msub> <i:mi>B</i:mi> <i:mn>0</i:mn> </i:msub> </i:math> strengths in the case of unlocalized X-nuclei and localized (1H and X-nuclei) single-voxel spectroscopy (SVS) pulse sequences. Our findings indicate that, under given fixed total amount of (patient acceptable) measurement time TA and maximum tolerable SAR limitation, together with conditions that ensure the adiabaticity of specific sequences, there exists an optimal magnetic field strength <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:msub> <k:mi>B</k:mi> <k:mn>0</k:mn> </k:msub> </k:math> that maximizes SNR per unit of measurement time ( <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:msub> <m:mi>SNR</m:mi> <m:mi>t</m:mi> </m:msub> </m:math> ). Beyond this optimal <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"> <o:msub> <o:mi>B</o:mi> <o:mn>0</o:mn> </o:msub> </o:math> , further increases in field strength do not yield proportional improvements in <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"> <q:msub> <q:mi>SNR</q:mi> <q:mi>t</q:mi> </q:msub> </q:math> . Key factors are identified, including rf-pulse bandwidth scaling with <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"> <s:msub> <s:mi>B</s:mi> <s:mn>0</s:mn> </s:msub> </s:math> and longitudinal relaxation time ( <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"> <u:msub> <u:mi>T</u:mi> <u:mn>1</u:mn> </u:msub> </u:math> ) dependencies, that impact the net gain as well. Our theoretical analysis emphasizes critical considerations for optimizing SNR per unit time in clinical MRS, even challenging the presumption that higher magnetic fields <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"> <w:msub> <w:mi>B</w:mi> <w:mn>0</w:mn> </w:msub> </w:math> always yield improved SNR per unit of measurement time performance.