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Single Parameter Mapping

For validation of the OpenMRF framework, it is important to compare MRF results with gold-standard reference measurements obtained with dedicated single-parameter mapping sequences.

In this context, it is useful to distinguish between two categories:

  • T1 and T2 mapping, which can be measured using vendor-provided protocols based on spin-echo acquisitions
  • Specialized relaxation parameters such as T1ρ, T2ρ, or adiabatic T1ρ, which are usually not available as standard vendor sequences and are therefore provided by dedicated OpenMRF implementations

T1 and T2 Mapping for OpenMRF Validation

For OpenMRF validation, T1 and T2 reference measurements were acquired using single-parameter spin-echo-based mapping protocols.

T1 Mapping

The T1 mapping protocol for the acquisition of T1-weighted images uses:

  • adiabatic inversion pulse
  • identical spin-echo readouts
  • minimal TE (e.g. 6.4 ms)
  • TR = 5 s
  • inversion delays: 35, 60, 95, 160, 260, 430, 710, 1180, 1940, 3200 ms

T2 Mapping

The T2 mapping protocol for the acquisition of T2-weighted images uses:

  • identical spin-echo sequence without inversion
  • TR = 5 s
  • echo times: 10, 15, 20, 30, 40, 60, 85, 120, 175, 250 ms

Common Imaging Parameters

  • FOV: 250 × 250 mm
  • Matrix: 128 × 128
  • Slice thickness: 6 mm
  • Bandwidth: 281 Hz/pixel

The corresponding reconstruction and mapping code is available in:

include_misc/src_nist_values/reco_spin_echo_T1_T2.m

Note

We are currently working on a Pulseq-based implementation of a standardized spin-echo sequence for T1 and T2 mapping. This will enable vendor-independent, reproducible gold-standard reference measurements within the OpenMRF framework.

Reference Mapping for T1ρ, T2ρ, and Adiabatic T1ρ

Reference measurements for T1ρ, T2ρ, and adiabatic T1ρ are not typically available as vendor-provided product sequences. For these parameters, OpenMRF provides dedicated implementations for both sequence compilation and reconstruction.

The corresponding source code can be found in:

main_sequence/relax_mapping

These reference mapping sequences are based on an interleaved spiral readout.

Default Acquisition Scheme

By default:

  • each preparation time is acquired with an 8× interleaved spiral readout
  • a 5 s recovery delay is applied between different readouts

Rationale for Spiral-Based Reference Mapping

The main advantage of the spiral mapping approach is that the readout is primarily proton-density weighted, while the image contrast is dominated mainly by the preceding T1ρ, T2ρ, or adiabatic T1ρ preparation.

This is important because it supports the use of a monoexponential signal model for fitting, which is typically assumed for these relaxation parameters.