Method and apparatus for anatomically tailored k-space sampling and recessed elliptical view ordering for bolus-enhanced 3D MR angiography

Abstract

Current bolus chase magnetic resonance angiography is limited by the imaging time for each station. Tailoring the density of k-space sampling along the anterior-posterior direction of the coronal station allows a substantial decrease in scan time that leads to greater contrast bolus sharing among stations and consequently a significant improvement in image quality. Fast arterial-venous transit in the carotid arteries requires accurate, reliable timing of the acquisition to the bolus transit to maximize arterial signal and minimize venous artifacts. The rising edge of the bolus is not utilized in conventional elliptical-centric view ordering because the critical k-space center must be acquired with full arterial enhancement. The invention provides a recessed elliptical-centric view ordering scheme is introduced in which the k-space center is acquired a few seconds following scan initiation. The recessed view ordering is shown to be more robust to timing errors in a patient studies.

1 . A method of MR imaging a region of interest in a body containing arterial features comprising: 
 a) applying a static magnetic field to the region of interest;    b) applying a magnetic gradient along at least first and second dimensions to produce a spatially distributed magnetic response of varying frequency and varying magnitude;    c) sampling the magnetic response at a spacing interval corresponding to a sampling rate which is less than a Nyquist rate;    d) performing a Fourier transform on the result of step (c) to provide a first imaging result;    e) providing a duplicate of image information of the first imaging result to provide a second imaging result; and    f) combining the first and second imaging results to isolate arterial features in the region of interest.    
     
     
         2 . The method of  claim 1 , wherein step (b) further comprises applying a magnetic gradient along a third dimension, and said step (c) comprises sampling at a sub-Nyquist rate along a thickness dimension of said first, second and third dimensions.  
     
     
         3 . The method of  claim 1 , wherein said step (e) comprises repeating steps (a)-(d) in the field of interest to provide the second imaging result, and introducing a contrast agent to the region of interest when repeating steps (a)-(d).  
     
     
         4 . The method of  claim 3 , wherein only a single dose of a contrast agent is introduced when repeating steps (a)-(d).  
     
     
         5 . A method according to  claim 1 , further comprising: 
 performing MR imaging at a first station using k-space sampling prior to performing said steps (a)-(f), wherein sampling at the first station is delayed such that sampling of a center of the k-space is timed to coincide with a contrast agent passing through the region of interest.    
     
     
         6 . The method according to  claim 5 , wherein said k-space sampling at the first station is performed at the Nyquist rate and spacing corresponding to the Nyquist rate.  
     
     
         7 . The method according to  claim 5 , wherein said k-space sampling at the first station is performed at less than the Nyquist rate and at a spacing interval larger than a spacing interval of the Nyquist rate.  
     
     
         8 . The method according to  claim 1 , wherein said step of combining the first and second imaging results comprises reconstructing portions of the first imaging result and the second imaging result that correspond to arterial features to isolate the arterial features in the region of interest.  
     
     
         9 . The method according to  claim 3 , wherein said step of combining the first and second imaging results comprises determining a background area from one of said first and second imaging results and overlapping the background area over the other imaging result to isolate arterial features in the region of interest in the other imaging result.  
     
     
         10 . A method of MR imaging a region of interest in a body containing arterial features comprising: 
 a) applying a static magnetic field to the region of interest;    b) applying a magnetic gradient along at least first and second dimensions to produce a spatially distributed magnetic response of varying frequency and varying magnitude;    c) sampling the magnetic response at a spacing interval which corresponds to a sampling rate which is less than a Nyquist rate, but taking a number of samples that corresponds to the Nyquist rate, thereby expanding a sampling range of the frequency;    d) performing a Fourier transform on the result of step (c) to provide a first imaging result;    e) providing a duplicate of image information of the first imaging result to provide a second imaging result; and    f) combining the first and second imaging results to isolate arterial features in the region of interest.    
     
     
         11 - 19 . (canceled)  
     
     
         20 . The method of  claim 5 , wherein the sampling of the k-space center is timed to coincide with a substantial peak of the contrast agent passing through the region of interest.  
     
     
         21 - 22 . (canceled)  
     
     
         23 . A method of MR imaging a region of interest comprising: 
 administering a contrast agent to the region of interest;    applying a static magnetic field to the region of interest;    applying a magnetic gradient along at least one of first and second dimensions to produce a distributed magnetic response having a spatial distribution in k-space; 
 sampling the magnetic response, wherein sampling is performed such that sampling of a center of the k-space is delayed from a start of sampling of the magnetic response, such that sampling of the center of the k-space is timed to coincide with a substantial peak of the contrast agent passing through the region of interest.  
   
     
     
         24 . The method of  claim 23 , wherein the sampling of the k-space center is preceded by sampling of a recessed-edge of k-space.  
     
     
         25 . The method of  claim 24 , wherein the sampling of k-space center is followed by sampling of an edge of k-space substantially corresponding to a maximum spatial frequency of the magnetic response in said region of interest.  
     
     
         26 . The method of  claim 25 , wherein a first time interval separates sampling of the k-space center and the recessed edge of the k-space and a second time interval separates sampling of k-space center and the edge of the k-space, wherein during the first time interval, sampling occurs in decreasing order of k-space radius from the recessed edge to the k-space center while sampling every Mth point (M>1), and wherein during the second time interval, sampling occurs in increasing order of k-space radius to sample each k-space point not sampled during the first time interval.  
     
     
         27 . The method of  claim 23 , wherein sampling the magnetic response produces N points, said method further comprising: 
 forming an array [K] of said N points ordered in said array in order of ascending radius from the center of the k-space using index N;    setting a recessed radius k R  having a corresponding index N R −1 in said array to start said sampling;    setting a recess time T recess  corresponding to a time between start of sampling and peak contrast agent in the region of interest;    setting a sequence repetition time T R  as a time to acquire one sample;    wherein sampling order of the k-space prior to sampling of the k-space center is determined according to      Index= N   R −1−( N   R /( T   recess   /TR ))* n  for  n,  0≦ n <(T recess   /TR ).    
     
     
         28 . The method of  claim 27 , wherein (NR/(T recess /TR))=2.  
     
     
         29 . A method of MR imaging a region of interest comprising: 
 a) administering a contrast agent to the region of interest;    b) applying a static magnetic field to the region of interest;    c) applying a magnetic gradient along at least one of first and second dimensions to produce a distributed magnetic response having a spatial distribution in k-space including a low spatial frequency, an intermediate spatial frequency and a high spatial frequency;    d) sampling the magnetic response in order of the intermediate frequency, the low spatial frequency and the high spatial frequency, said high spatial frequency corresponding to a maximum radius value of the magnetic response in the k-space for said region of interest.    
     
     
         30 . The method of  claim 29 , wherein a first time interval separates sampling of the intermediate spatial frequency and the low spatial frequency, and a second time interval separates sampling of the low spatial frequency and the high spatial frequency, wherein during the first time interval, sampling occurs in decreasing order of spatial frequency from the intermediate spatial frequency to the low spatial frequency, sampling every Mth point (M>1), and wherein during the second time interval, sampling occurs in increasing order of spatial frequency to sample each point not sampled during the first time interval.  
     
     
         31 . The method of  claim 29 , wherein the sampling of the low spatial frequency substantially corresponds to a peak of the contrast agent passing through the region of interest.  
     
     
         32 . The method of  claim 29 , wherein sampling the magnetic response produces N points, said method further comprising: 
 forming an array [K] of said N points ordered in said array in order of ascending radius from the center of the k-space using index N;    setting a recessed radius k R  having a corresponding index N R −1 in said array to start said sampling;    setting a recess time T recess  corresponding to a time between start of sampling and peak contrast agent in the region of interest;    setting a sequence repetition time T R  as a time to acquire one sample;    wherein sampling order of the k-space prior to sampling of the k-space center is determined according to      Index= N   R −1−( N   R /( T   recess   /TR ))* n  for  n,  0< n <( T   recess   /TR ).    
     
     
         33 . The method of  claim 32 , wherein (NR/(T recess /TR))=2.  
     
     
         34 - 36 . (canceled)  
     
     
         37 . The method of  claim 25  wherein said first and second time intervals correspond to a time for imaging during application of a single magnetic gradient.  
     
     
         38 . The method of  claim 30  wherein said first and second time intervals correspond to a time for imaging during application of a single magnetic gradient.  
     
     
         39 . The method of  claim 27 , wherein after sampling of the k-space center, sampling occurs in increasing order of k-space radius to sample each point not sampled during said interval 0<n<(T recess /TR).  
     
     
         40 . The method of  claim 32 , wherein after sampling of the k-space center, sampling occurs in increasing order of k-space radius to sample each point not sampled during said interval 0<n<(T recess /TR).