CONTRAST AND RESOLUTION CONSIDER- ATIONS IN KEYHOLE MRI APPLICATION TO DYNAMIC STUDIES OF C
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∗
ቤተ መጻሕፍቲ ባይዱ
Faculty of Electrical Engineering,
+
J. Stefan Institute, Slovenia
Corresponding author: Jure Mediˇ c Faculty of Electrical Engineering Trˇ zaˇ ska 25 1000 Ljubljana Slovenia E-mail: jure.medic@fe.uni-lj.si
5 MR completely encoded images fn (x, y ), which produced the sequences of post-contrast keyhole images gn (x, y ). Standard deviation σerr of the image noise was calculated as the average value of all σn in original completely encoded images σerr =
Absctract
This paper presents a quantitative evaluation of the effect of the keyhole technique on contrast-enhanced dynamic MRI; specifically, the effect of the observed region of interest on keyhole images compared to a complete set of data was evaluated. The limiting condition of the quantitative use of the keyhole approach was defined and exemplified. Results obtained by simulations, as well on MR Images, indicate that object size and noise-level determine the minimum keyhole size and the maximum size of the region of interest, so that if keyhole parameters are properly selected, a quantitative analysis of MRI gives the same signal intensity as complete data acquisition. In this case the time resolution increases by factor N/L.
1 20 20 n=1
σn . The quantitative values of the selected region of
interest (ROI) rn (x0 , y0 ) around point (x0 , y0 ) were calculated using a moving average filter (MA) [4] on both original completely encoded images f n (x, y ) and all keyhole images gn , (x, y ). If we denote this value as r (x0 , y0 ) then from the signal-processing perspective, r (x0 , y0 ) can be viewed as the convolution of the moving average filter (MA filter) and the MR image, evaluated at the selected point (x0 , y0 ) in the MR image. In the case of the square ROI, the mean value is calculated for M 2 points: 1 rn (x, y ) = ( h(x, y − j )) ∗ fn (x, y ) M j =−M/2
2
where w (E, d, y ) is window-function deffined in image domain 1 if 1 ≤ y ≤ y0 − d/2 w (E, d, y ) = 1/E if y0 − d/2 ≤ y ≤ y0 + d/2 1 if y ≥ y0 + d/2 if fn (x, y ) is the completely encoded post-contrast image and f 1 (x, y ) is the precontrast completely encoded image. The number of complete phase-encoding steps is denoted by N , and the keyhole size by L. The contrast-enhanced object is defined in y direction by its central point y0 , size by d, and contrast enhancement by E . Ideal low-pass and high-pass filters are denoted by hlp and hhp respectively. The temporal resolution of the keyhole image is D= N L [3]
1
Introduction
Magnetic resonance imaging (MRI) has firmly established a leading position among diagnostic imaging modalities, predominantly due to its high tissuecontrast sensitivity; yet in comparison with X-ray computer tomography MRI requires longer imaging time (ordinarily, a few minutes for an imaging sequence). Faster MRI techniques such as FLASH MRI and Echo Planar Imaging (EPI) were developed to overcome this problem (1, 2), but at the expense of a decreased signal-to-noise ratio. Keyhole data acquisition was proposed to permit faster imaging procedures without loss of sensitivity. This can be achieved in dynamic studies, where initial spatial information is combined with spatial information at later points in time. Examples of application are shown to be interventional MRI and contrast mediaenhanced dynamic studies. (3, 4) In keyhole imaging schemes a complete k-space data set is acquired only once, before the administration of a contrast agent. Subsequently, a small, usually central, subset of the k-domain (in the phase-encoding direction) is acquired repetitively. Each such subset is combined with data from the first acquisition which is not included in temporal acquisition. Data is then Fourier-transformed for the production of subsequent images. The subsequent keyhole images gn (x, y ) are defined in image domain as sin( πyL ) iπy/N N gn (x, y ) = (fn (x, y ) − f1 (x, y )) ∗ + f1 (x, y ) πy e sin( N ) and can be aproximated by gn (x, y ) = fn (x, y ) ∗ hlp (y ) + fn (x, y ) · w (E, d, y ) ∗ hhp [2] [1]
The aim of our study was the quantitative evaluation of the effect of the keyhole on contrast in MRI. Specifically, the effect of the keyhole size on signal intensity for the observed region of interest compared to the complete set of data was evaluated. A criterion for defining the limiting conditions of the quantitative use of the keyhole approach based on noise considerations were defined and exemplified on dynamic contrast-enhanced MRI data.
M/2
[4]
where M is the size of ROI in one dimension. The response of the MA filter in k-domain is a sinc-like function which represents a low-pass filter. MR Imaging MRI was performed on a Siemens Magnetom SP 63 operating at 1.5 T. T1weighted two-dimensional (2D) spoiled gradient-refocused acquisition (FLASH) images were obtained with the following parameter settings: TR 66 ms, TE 6 ms, section thickness 5 mm, FOV 350 mm, 256-phase encoding, acquisition time 11 seconds. Experiments were performed on a patient with spontaneous liver tumours. Gd-DTPA was administered with a dose of 0.1 mmol Gd/kg . A non-enhanced image-set was obtained before contrast medium administration. Subsequently, serial MRI was performed at one-minute intervals for 7 minutes to
Methods
Simulations All calculations were made with Matlab 4.2 (maths works) running on a PC. Simulations were performed on a sequence of all 6 MR images of the liver. First, a pre-contrast image f1 (x, y ) was selected as the reference image. A keyhole technique based on Eq. [1] with different sizes (L) was then performed on all 3
CONTRAST AND RESOLUTION CONSIDERATIONS IN KEYHOLE MRI: APPLICATION TO DYNAMIC STUDIES OF CONTRAST MEDIA KINETICS
Jure Mediˇ c∗, Saˇ so Tomaˇ ziˇ c∗, Igor Serˇ sa+ , Franci Demsar+
ቤተ መጻሕፍቲ ባይዱ
Faculty of Electrical Engineering,
+
J. Stefan Institute, Slovenia
Corresponding author: Jure Mediˇ c Faculty of Electrical Engineering Trˇ zaˇ ska 25 1000 Ljubljana Slovenia E-mail: jure.medic@fe.uni-lj.si
5 MR completely encoded images fn (x, y ), which produced the sequences of post-contrast keyhole images gn (x, y ). Standard deviation σerr of the image noise was calculated as the average value of all σn in original completely encoded images σerr =
Absctract
This paper presents a quantitative evaluation of the effect of the keyhole technique on contrast-enhanced dynamic MRI; specifically, the effect of the observed region of interest on keyhole images compared to a complete set of data was evaluated. The limiting condition of the quantitative use of the keyhole approach was defined and exemplified. Results obtained by simulations, as well on MR Images, indicate that object size and noise-level determine the minimum keyhole size and the maximum size of the region of interest, so that if keyhole parameters are properly selected, a quantitative analysis of MRI gives the same signal intensity as complete data acquisition. In this case the time resolution increases by factor N/L.
1 20 20 n=1
σn . The quantitative values of the selected region of
interest (ROI) rn (x0 , y0 ) around point (x0 , y0 ) were calculated using a moving average filter (MA) [4] on both original completely encoded images f n (x, y ) and all keyhole images gn , (x, y ). If we denote this value as r (x0 , y0 ) then from the signal-processing perspective, r (x0 , y0 ) can be viewed as the convolution of the moving average filter (MA filter) and the MR image, evaluated at the selected point (x0 , y0 ) in the MR image. In the case of the square ROI, the mean value is calculated for M 2 points: 1 rn (x, y ) = ( h(x, y − j )) ∗ fn (x, y ) M j =−M/2
2
where w (E, d, y ) is window-function deffined in image domain 1 if 1 ≤ y ≤ y0 − d/2 w (E, d, y ) = 1/E if y0 − d/2 ≤ y ≤ y0 + d/2 1 if y ≥ y0 + d/2 if fn (x, y ) is the completely encoded post-contrast image and f 1 (x, y ) is the precontrast completely encoded image. The number of complete phase-encoding steps is denoted by N , and the keyhole size by L. The contrast-enhanced object is defined in y direction by its central point y0 , size by d, and contrast enhancement by E . Ideal low-pass and high-pass filters are denoted by hlp and hhp respectively. The temporal resolution of the keyhole image is D= N L [3]
1
Introduction
Magnetic resonance imaging (MRI) has firmly established a leading position among diagnostic imaging modalities, predominantly due to its high tissuecontrast sensitivity; yet in comparison with X-ray computer tomography MRI requires longer imaging time (ordinarily, a few minutes for an imaging sequence). Faster MRI techniques such as FLASH MRI and Echo Planar Imaging (EPI) were developed to overcome this problem (1, 2), but at the expense of a decreased signal-to-noise ratio. Keyhole data acquisition was proposed to permit faster imaging procedures without loss of sensitivity. This can be achieved in dynamic studies, where initial spatial information is combined with spatial information at later points in time. Examples of application are shown to be interventional MRI and contrast mediaenhanced dynamic studies. (3, 4) In keyhole imaging schemes a complete k-space data set is acquired only once, before the administration of a contrast agent. Subsequently, a small, usually central, subset of the k-domain (in the phase-encoding direction) is acquired repetitively. Each such subset is combined with data from the first acquisition which is not included in temporal acquisition. Data is then Fourier-transformed for the production of subsequent images. The subsequent keyhole images gn (x, y ) are defined in image domain as sin( πyL ) iπy/N N gn (x, y ) = (fn (x, y ) − f1 (x, y )) ∗ + f1 (x, y ) πy e sin( N ) and can be aproximated by gn (x, y ) = fn (x, y ) ∗ hlp (y ) + fn (x, y ) · w (E, d, y ) ∗ hhp [2] [1]
The aim of our study was the quantitative evaluation of the effect of the keyhole on contrast in MRI. Specifically, the effect of the keyhole size on signal intensity for the observed region of interest compared to the complete set of data was evaluated. A criterion for defining the limiting conditions of the quantitative use of the keyhole approach based on noise considerations were defined and exemplified on dynamic contrast-enhanced MRI data.
M/2
[4]
where M is the size of ROI in one dimension. The response of the MA filter in k-domain is a sinc-like function which represents a low-pass filter. MR Imaging MRI was performed on a Siemens Magnetom SP 63 operating at 1.5 T. T1weighted two-dimensional (2D) spoiled gradient-refocused acquisition (FLASH) images were obtained with the following parameter settings: TR 66 ms, TE 6 ms, section thickness 5 mm, FOV 350 mm, 256-phase encoding, acquisition time 11 seconds. Experiments were performed on a patient with spontaneous liver tumours. Gd-DTPA was administered with a dose of 0.1 mmol Gd/kg . A non-enhanced image-set was obtained before contrast medium administration. Subsequently, serial MRI was performed at one-minute intervals for 7 minutes to
Methods
Simulations All calculations were made with Matlab 4.2 (maths works) running on a PC. Simulations were performed on a sequence of all 6 MR images of the liver. First, a pre-contrast image f1 (x, y ) was selected as the reference image. A keyhole technique based on Eq. [1] with different sizes (L) was then performed on all 3
CONTRAST AND RESOLUTION CONSIDERATIONS IN KEYHOLE MRI: APPLICATION TO DYNAMIC STUDIES OF CONTRAST MEDIA KINETICS
Jure Mediˇ c∗, Saˇ so Tomaˇ ziˇ c∗, Igor Serˇ sa+ , Franci Demsar+