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Reduction of CSF Artifacts on FLAIR Images by Using Adiabatic Inversion Pulses

Joseph V. Hajnala, Angela Oatridgea, Amy H. Herlihya and Graeme M. Byddera

a From the Robert Steiner Magnetic Resonance Unit, MRC Clinical Sciences Centre, Imperial College School of Medicine, Hammersmith Campus, Du Cane Rd, London W12 0HS, UK.



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  		FIG 1. Operation of signal-nulling IR sequences. Normalized signal plotted against inversion time.

A, The longitudinal magnetization of the target tissue to be nulled must pass through zero when the 90° pulse is applied (curve I). Tissues with other T1 values (eg, curve II) produce signals at this time. For example, at 100 ms (when curve I passes through zero), curve II yields a signal of -0.63.

B, An incorrect flip angle of the inverting pulse produces only partial inversion of the longitudinal magnetization and now results in a recovery curve that has already passed through zero when the 90° pulse is applied (curve I) so that the target tissue is no longer nulled. The signal from the other tissue at this time is changed from -0.63 to -0.43.



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  		FIG 2. A and B, Signals (normalized to value at full relaxation) for FLAIR (A) and STIR (B) sequences plotted against RF flip angle (degrees) of the initial "inversion" pulse. Sequence parameters have been set at {infty}/90 (TR/TE) and TI = 2100 for the FLAIR sequence and {infty}/30, TI = 130 for the STIR sequence. 1.0-T values for tissue T1/T2 (ms) were used as follows: brain (mixed gray and white matter) = 700/80, CSF = 4500/4500, fat = 185/80, and muscle = 500/40. The desired performance (which is an effective flip angle of 180°) is shown at the extreme right-hand end of the graphs. For brain and CSF with the FLAIR sequence, there is a gradual rise of CSF signal as the flip angle is decreased (ie, shifts to the left). Equal signals are seen at 112°. With the STIR sequence, fat increases its signal as the flip angle is reduced. Muscle shows a reduced signal magnitude and is nulled at a flip angle of 108°. It has a higher signal than fat at flip angles less than 75° (B)



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  		FIG 3. Graph of the normalized signal obtained after magnetization inversion followed by a 90° pulse plotted against RF amplitude for the conventional inversion pulse (triangles), the 10-ms adiabatic pulse (circles), and the 20-ms adiabatic pulse (squares). The adiabatic pulses provide full inversion over wider ranges of RF amplitudes (or effective flip angles) than does the conventional pulse



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  		FIG 4. A–D, Comparison of FLAIR images of the brain in a healthy male volunteer aged 53 years. At the thalamic level, transverse slices show comparable CSF suppression and SNR for the conventional (A) and 20-ms adiabatic (B) pulse versions. At the level of the pons, the conventional sequence fails to suppress CSF anterior to the pons and cerebellum (C, arrows), whereas the 20-ms adiabatic version provides good CSF suppression (D)



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  		FIG 5. Comparison of coronal STIR sequences acquired through the lumbar spine and pelvis of a healthy male aged 65 years.

A, The conventional version only produces effective fat suppression in the central part of the image. Superiorly, fat has a high signal (upper arrows). CSF in this region has a low signal. The conventional sequence suppresses muscle at the level of the mid-shaft of the femur (lower arrows) and fat has a high signal inferior to this.

B, The 20-ms adiabatic version produces effective fat suppression and appropriate signals from muscle over the full FOV. CSF also has its normal high signal (arrow).



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  		FIG 6. Hydrocephalus with edema around the fourth ventricle in a patient with a high-grade glioma.

A and B, Sinc (A) and 10-ms adiabatic (B) FLAIR sequences (8142/135, TI = 2200). CSF signal is markedly reduced in B.



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  		FIG 7. Patient with probable meningioma.

A and B, Sinc (A) and 10-ms adiabatic (B) FLAIR sequences (8142/135, TI = 2200). The tumor is more readily seen in B (arrow), without the high signal from CSF seen in A.





Copyright © 2009 by the American Society of Neuroradiology. Print ISSN: 0195-6108 Online ISSN: 1936-959X