Hyperpolarized MR imaging using pulse sequence with progressively increasing flip angle

Abstract

A method and apparatus is provided for generating an MR image of a structural portion of a subject, the portion lying within a hypothetical imaging volume. A specified level of hyperpolarized magnetization is provided within the imaging volume, and a series of MR pulse sequences are applied to the imaging volume to generate respective corresponding MR data signals, each pulse sequence comprising a number of magnetic field gradient pulses and an excitation pulse, and having an associated flip angle determined by the excitation pulse. The associated flip angles of successive pulse sequences are progressively increased to maintain the signal strength of the corresponding MR data signals at a substantially constant level. Acquired MR data signals are cumulatively processed to provide an image of the structural portion.

What is claimed is:  
     
       1. A method for providing a magnetic resonance (MR) image of a structural portion of a subject, the portion lying within a hypothetical imaging volume, said method comprising the steps of: 
       introducing a quantity of magnetically hyperpolarized agent into said subject proximate to said structural portion;  
       successively applying a series of MR pulse sequences to said imaging volume over a specified period of time, to generate respective corresponding MR data signals, wherein each sequence comprises a number of magnetic field gradient pulses and an excitation pulse, has an associated number representing its position in said series, and has an associated flip angle determined by its excitation pulse;  
       computing the value of a first flip angle associated with the first pulse sequence in said series;  
       non-recursively computing the associated flip angle of each successive pulse sequence in said series, following said first sequence, from said first flip angle value and from a non-linear function of said sequence position numbers, each of said non-recursively computed flip angles being successively increased to maintain the signal strength of said corresponding MR data signals at a substantially constant level over said specified time period; and  
       processing said corresponding MR data signals to provide said image of said structural portion.  
     
     
       2. The method of claim  1  wherein: 
       the first pulse sequence in said series has a flip angle θ 1  of specified value, and θ(i), the flip angle associated with the ith pulse sequence in the series, has a value given by the relationship θ(i)=tan 1  (tan θ 1 {square root over (1+L −(i−1+L )tan 2  +L θ 1 +L ))}, where i is a positive integer between 2 and N, and N is the total number of pulse sequences in the series.  
     
     
       3. The method of claim  1  wherein: 
       said structural portion comprises structure within said subject proximate to a cavity disposed to receive said hyperpolarized agent.  
     
     
       4. The method of claim  3  wherein: 
       said structural portion comprises structure of the lungs of said subject.  
     
     
       5. The method of claim  3  wherein: 
       said structural portion comprises cardiac structure of said subject.  
     
     
       6. The method of claim  1  wherein: 
       said hyperpolarized agent is the noble gas Helium-3.  
     
     
       7. The method of claim  1  wherein: 
       said hyperpolarized agent is the noble gas Xenon-129.  
     
     
       8. The method of claim  1  wherein: 
       said hyperpolarized agent comprises a quantity of blood which has been removed from said subject, hyperpolarized externally from said subject, and then reinserted into said subject at a specified location.  
     
     
       9. Apparatus for providing a magnetic resonance (MR) image of a structural portion of a subject, the portion lying within a hypothetical imaging volume, said apparatus comprising: 
       means for introducing a quantity of magnetically hyperpolarized agent into said subject proximate to said structural portion;  
       means for successively applying a series of MR pulse sequences to said imaging volume over a specified period of time, to generate respective corresponding MR data signals, wherein each sequence comprises a number of magnetic field gradient pulses and an excitation pulse, has an associated number representing its position in said series, and has an associated flip angle determined by the its excitation pulse;  
       means for computing the value of a first flip angle associated with the first pulse sequence in said series;  
       means for non-recursively computing the associated flip angle of each successive pulse sequence in said series, following said first sequence, from said first flip angle value and from a non-linear function of said sequence position numbers, each of said non-recursively computed flip angles being successively increased to maintain the signal strength of said corresponding MR data signals at a substantially constant level over said specified time period; and  
       means for processing said corresponding MR data signals to provide said image of said structural portion.