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LetterLETTER

  
      

  
      
Reply
  
    	
Elias R.  Melhem

  
    	
American Journal of Neuroradiology July 1999,  20 (7) 1382-1383; 

  
  
  



  


  
  



			

		

	
	
 	
	  
  

		
		

			

			  

  
      
  
  

    
  


  
  



  
      
  
  

    

  

    

  
      
  
  

    
I would like to thank Drs. Patel and Klufas for their interest and comments regarding our recent publication in the AJNR (1).
We agree with Drs. Patel and Klufas regarding the issue of paramagnetic brain lesion demonstration as a function of the MR imaging technique used. We have demonstrated that GRASE imaging performs better than fast spin-echo imaing in this respect. This can be explained by two factors. First, despite the implementation of multiple 180º refocusing RF pulses, the effects of static magnetic field inhomogeneity in GRASE imaging are not minimized to the same degree as in fast spin-echo imaging. This is related to the off-resonance echoes used to fill the periphery of k-space (2). The greater the number of these off-resonance echoes, the greater is their representation in the center of k-space and, hence, the greater is their contribution to the overall image signal. Second, the effects of time-varying magnetic field variability are more pronounced in GRASE compared to fast spin-echo techniques. This is related to differences between the two techniques regarding the shortest achievable time interval between the Hahn spin-echo. In GRASE imaging, this interval has to be longer in order to accommodate the multiple gradient-echo reversals (3–5).
Formal comparison between GRASE imaging and conventional spin-echo and gradient-recall echo imaging, as related to the demonstration of paramagnetic brain lesions, is lacking. We believe, however, that both conventional spin-echo and optimized gradient-recall echo readouts are superior for the demonstration of paramagnetic brain lesions to the GRASE technique currently in question. As a result of much longer echo-times (echo-spacing), selective T2 relaxation enhancement from molecular diffusion through regions of variable magnetic field is much greater in conventional spin-echo imaging than in GRASE imaging. The effects of accumulated phase and chemical shifts in the few non-Hahn echoes acquired in our GRASE technique are probably not enough to outweigh the differences in echo-times (echo-spacing). This, however, may not be true for GRASE techniques implementing a greater number of gradient echoes per 180º–180º interval. Also, in gradient-echo recall imaging, with relatively long echo-times optimized for paramagnetic brain lesion detection, both static and time-varying magnetic field inhomogeneities contribute to selective T2 relaxation enhancement, resulting in exaggeration of signal loss compared to GRASE imaging (6).
Hyperintense brain lesion demonstration on MR images depends on contrast resolution, spatial resolution, signal-to-noise, and artifacts related to human and technical factors. When contrast resolution, spatial resolution, and signal-to-noise are identical across the different MR imaging techniques, and the human factor is eliminated, conventional spin-echo techniques probably will be superior to the different fast hybrid spin echo–based techniques. This is because of the inherent technique-related artifacts. In both fast spin-echo and GRASE imaging, there is further modulation in signal along the phase-encoding direction related to T2-decay resulting from variable echo times. This modulation affects the spatial-encoding process in the phase direction and results in ghosting and blurring artifacts. These artifacts are proportional to the number of echoes per TR interval, the echo-spacing, and the scheme used to fill k-space. Shorter echo-spacing (between the echoes within a 180º–180º interval) and nonsequential (interleaved) phase-encode ordering through the echo train in GRASE imaging actually may reduce signal modulation between segments of k-space compared to fast spin-echo techniques (7).
It is important to emphasize that, with fast hybrid MR imaging techniques, high temporal resolution can be exchanged for better spatial resolution, signal-to-noise, and contrast resolution (longer TR). In our study, a longer TR was used with the  faster GRASE technique whereas the scan time per sequence similar remained the same. This may explain partially the better demonstration of hyperintense brain lesions on GRASE images. Also, fast imaging reduces the chance of human-related image artifacts. The effect of these artifacts commonly outweigh differences in inherent technique-specific artifacts.
We agree with Drs. Patel and Klufas that the advantages and disadvantages of fast MR imaging techniques compared to conventional spin-echo have to be tested in the clinical setting. In addition to differences in MR imaging hardware and software available at various clinical sites, practicing radiologists have to factor in the type of patients being imaged.
Continual improvements in MR hardware performance and pulse sequence design has and will continue to reduce the need for T2-weighted conventional spin-echo techniques. I also would like to point out that optimized high resolution (256 × 512 matrix), T2-weighted GRASE imaging already has replaced both conventional and fast spin-echo techniques for routine brain imaging at several clinical sites in Europe and the United States.
References
  1. 7.
    Rockwell D, Melhem ER, Bhatia R.  GRASE (gradient- and spin-echo) MR imaging of the brain.. AJNR Am J Neuroradiol 1997;18:1923-1928
    Abstract
  2. 8.
    Mugler JP III, Brookeman JR,  Off-resonance image artifacts in interleaved-EPI and GRASE pulse sequences.. Magn Reson Med 1996;36:306-313
    PubMed
  3. 9.
    Feinberg DA, Oshio K.  GRASE (gradient- and spin-echo) MR imaging: a new fast clinical imaging technique.. Radiology 1991;181:597-602
    PubMed
  4. 10.
    Oshio K, Feinberg DA.  GRASE (gradient- and spin-echo) MR imaging: A novel fast MRI technique.. Magn Reson Med 1991;20:344-349
    CrossRefPubMed
  5. 11.
    Feinberg DA, Berthold K, Litt AW.  Dual contrast GRASE (gradient-spin echo) imaging using mixed bandwidth.. Magn Reson Med 1994;31:461-464
    CrossRefPubMed
  6. 12.
    Luedeke KM, Roeschmann P, Tischler R.  Susceptibility artifacts in NMR imaging.. Magn Reson Imaging 1985;3:329-343
    CrossRefPubMed
  7. 13.
    Melhem ER, Patel R, Rockwell D, Whitehead RE,  Bhatia R, Jara H.  Utility of dual-echo gradient- and spin-echo (GRASE) MR imaging of the brain: a comparison to fast spin-echo.. AJR Am J Roentgenol 1998;171:797-802
    PubMed
  


  
  



  
      
  
  

    
  


  
  



  




  


  
  



  
      
  
  

    
  


  
  



  
      
  
  

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Elias R.  Melhem

  
    	
American Journal of Neuroradiology Jul 1999, 20 (7) 1382-1383; 

  
  
  



  


  
  



  
      
  
  

    

  
    
  
  
    
  
  
    
  


  


  
  



  
      
  
  

    
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