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Imaging, Vol. 10, pp. 887-892, All rights reserved.

1992 Copyright

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0730-725X/92 $5.00 + .OO 1992 Pergamon Press Ltd.

0 Original Contribution OUTFLOW REFRESHMENT ANGIOGRAPHY: A BRIGHT BLOOD, BRIGHT STATIC TISSUE TECHNIQUE MARK DOYLE, SUSAN A. MULLIGAN, * TETSUYA MATSUDA, AND GERALD M. POHOST Division of Cardiovascular Disease, *Department of Radiology, University of Alabama at Birmingham, Birmingham, AL 35294, USA A new “bright blood” strategy, outflow refreshment imaging, is introduced in which a number of overlapping slices are excited in rapid succession. Flowing spins that refresh each overlapped slice portion contribute a bright signal. Additionally, static tissue in each non-overlapped slice portion also yields a bright signal. However, the flow/static contrast is comparable to that produced in inflow refreshment images, and angiograms can be generated by conventional maximum intensity projection processing. The dual ability to visualize angiograms and static tissue images is a major benefit of the strategy. Computer simulations of flow sensitivities and in vivo results are presented which compare the outflow and inflow refreshment imaging strategies.

Keywords: NMR angiography;

Artifacts;

Imaging, angiography;

Short TE, thin slices.

contrast, a condition which does not necessarily require that static tissue signals be suppressed. The acquisition parameters for O/R can be chosen to produce similar flow-static contrast to that of I/R strategies. Thus O/R images allow angiograms to be simply generated by MIP processing, in addition to yielding bright cross-sectional images of static tissue regions.

INTRODUCTION Thin slice inflow refreshment methods, when applied to predominantly straight vessels, display excellent sensitivity to flowing spins. l-5 The combination of rapid repetition times and high flip angles result in the static tissue signals being highly suppressed, and consequently the images afford good flow/static contrast. Angiograms are generated by maximum intensity projection (MIP) processing and have proven useful in identifying stenotic and occluded vessels.6 The source images, typically in the transverse plane, yield cross-sectional views of the vessels and are often referred to prior to making a diagnosis. However, since the source images generally present low intensity static tissue regions their diagnostic value is limited, especially in regions where the flow signal is lost (e.g., due to turbulence or high velocity jets). Presented here is a new bright blood imaging strategy termed outflow refreshment imaging. The outflow refreshment (O/R) approach retains the good flow sensitivity feature of thin slice inflow refreshment (I/R) methods, but additionally yields images with bright static tissue signals. As with I/R techniques, blood contrast in O/R relies on the time of flight phenomena. To allow successful depiction of vessels by MIP processing the images must possess good flow-static

METHODS All images were obtained using a Philips Gyroscan (Philips Medical Systems, Shelton, CT), 1.5 T, l-m bore imaging system with unshielded gradients of maximum strength 10 mT/m and a minimum rise time of 1.5 msec. The O/R Imaging Method The O/R method employs a multiple slice gradient echo sequence in which the selected slice advances rapidly along the subject. For maximum flow sensitivity, the slice is advanced into the oncoming flow.’ Upon excitation of the last slice (e.g., after 64 slices) the acquisition slice is reset to the first slice position, and the phase-encoding gradient advanced in preparation for the next set of excitations. This contrasts with the I/R methods, which perform all phase-encoding steps on each slice before advancing to the next slice. Thus in

RECEIVED 2/11/92; ACCEPTED S/11/92. Address correspondence to Mark Doyle, PhD, Cardiac

NMR Lab., 828 8th Court South, University of Alabama at Birmingham, Birmingham, AL 35294. 887

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O/R imaging, a number of 2D slices are encoded over a period of several minutes. O/R Flow Contrast Flow contrast is achieved when slices are acquired in an overlapped manner, that is, when the slice advancement amount is less than the slice thickness. Employing a high flip angle (e.g., 90”) in combination with rapid slice advancement (e.g., 60 msec), results in suppression of the static tissue of each slice’s “trailing edge” with a suppression efficiency of more than 90%.* During the inter-slice advancement interval, flowing spins that refresh each partially suppressed slice portion contribute to the flow/static contrast; that is, due to flow refreshment, the flow signal effectively originates from a wider slice than the static signal (Fig. 1). Consequently, the source images present bright static tissue signals with notably brighter flow signals. The term outflow refreshment was adopted because flow/static contrast arises from flowing spins which enter the “trailing edge” of the overlapped slices (Fig. 1). Flow Simulation To compare the flow sensitivities of the inflow and outflow strategies a computer simulation was conducted. Conditions that were considered in the simulation were: the T, of the flowing liquid was 1000 msec, transverse signal was assumed to be in a completely dephased state after each measurement, the equations for plug flow were used, and the selection slice profile was rectangular. For the I/R case the slice thickness was 2.5 mm, and the slice repetition interval was 60 msec. For the O/R case the slice thickness was 5 mm, the slice overlap was 2.5 mm, and the slice ad-

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Fig. 1. For flow in a pipe the first, second, and third slice positions are shown advancing opposite to the flow direction. The slice advancement amount is less than the slice thickness, resulting in overlapping slices. In the overlapped regions, static tissue signal is suppressed (light shading). During the slice advancement interval (e.g., 60 msec), flowing spins refresh the overlapped slice portion, yielding a bright signal. The dark shading indicates suppressed flow spins that exit the slice.

vancement interval was 60 msec. Simulation of the O/R case was run with a 90” flip angle; and two I/R simulations were run, one with a 90” and the other with a 45” flip angle. In Vivo Study Outflow refreshment and inflow refreshment images of a volunteer’s legs were acquired and MIP angiograms generated. To allow direct comparison of the two techniques the slice advancement amount for the O/R acquisitions was set equal to the slice thickness of the I/R acquisitions (2.5 mm). Pairing these parameters, together with employing the same number of slices (64 slices), ensured that the same spatial extent was imaged in each case (16 cm). Additionally, the slice advancement interval for the O/R case was made equal to the slice repetition interval for the I/R case (60 msec), allowing the O/R and I/R acquisitions to be completed in approximately the same total scan time. The I/R acquisitions employed the SLIP (spatially separated lipid presaturation) lipid suppression strategy, allowing suppression of both the venous and lipid signals with a single presaturation slab.9 To observe O/R angiographic appearances for a range of slice overlap amounts, six sets of images were acquired with slice thickness varying from 12.5 to 2.5 mm, and the slice overlap ranging from 80% to 0% (overlap amounts ranged from 10 to 0 mm), respectively. For each case the choice of slice thickness and overlap amount ensured that the absolute slice advancement was 2.5 mm. Thus, the spatial region covered in each acquisition was similar. The following parameters were used for all acquisitions: the field of view was 320 mm, the number of slices was 64, the slice selection and measurement gradients were flow compensated up to first order, lo the matrix resolution was 256 x 256, and a reduced number of phase-encoding steps were employed (corresponding to a 40% reduction). Two I/R acquisitions were performed, one with a 90” flip angle and one with a 45” flip angle, in each case the TE was 19 msec (representing the shortest available echo time), a 40-mm wide venous presaturation pulse was applied immediately prior to each slice excitation, and the acquisition time was 10.5 min. For the O/R acquisitions, a 90” slice selection flip angle was used and the TE was 13 msec (representing the shortest echo time for this sequence). Between excitation of the last slice and re-initiation of excitations at the first slice position, a delay of 1000 msec was introduced to allow flow to refresh the scanned region. Thus the O/R acquisition time of 13 min was slightly longer than that of the I/R case (10.5 min). The different TEs employed for I/R and O/R slightly favor image quality in the O/R sequence.

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However, since both TEs are quite long, image quality should not be significantly different. From both the O/R and I/R image sets MIP angiograms were generated. RESULTS Simulated flow sensitivity curves for O/R and I/R sequences are shown in Fig. 2. The 90” O/R flow curve resembles the 90” I/R flow curve, the major difference being that the O/R curve is elevated on a large base line reflecting the bright appearance of static tissue. Cross-sectional inflow and outflow refreshment images of a volunteer’s legs from approximately the same region are compared in Fig. 3. Notice that the I/R images display multiple ghost vessels emanating from the genuine vessel, but the O/R images are not marred by these obtrusive artifacts. From the set of six O/R acquisitions (covering a range of slice thicknesses and overlap amounts), and the two I/R acquisitions (90’ and 4.5” flip angles) a series of MIP angiograms were produced (Fig. 4). DISCUSSION The static tissue signal contributing to outflow refreshment images is spin density weighted and conse-

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quently of high intensity. At the echo time employed in this study (13 msec), muscle regions appear brighter than lipid regions. Thus it was not necessary to employ lipid suppression techniques such as SLIP to improve angiographic contrast. The O/R angiograms are almost tri-level in intensity: the brightest signal representing flowing blood, next brightest is muscle, with lipid being the least intense. On the display monitor, angiogram image contrast can be interactively adjusted to view predominantly blood, or blood and static tissues combined. This feature may aid in vessel identification by allowing anatomical landmarks to be superimposed on the angiogram. The bright static tissue signal in O/R images allows cross-sectional anatomy to be viewed directly. Additionally, since slices are acquired in a contiguous manner, it should be feasible to reformat the data to allow assessment in other orientations.” The ability to render bright static tissue images may aid in assessing vascular patency even in instances when the flow signal is lost, that is, the position and extent of the lumen could be determined by the signal void shown against the bright static tissue background. The bright static tissue signal of O/R images presents a limitation for MIP processed angiograms since there is a possibility that low intensity vessels will be

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Fig. 2. Computer simulations of flow sensitivities for the outflow refreshment (O/R) and inflow refreshment (I/R) strategies (see text for details). The outflow refreshment curve was obtained with a 90” pulse, and the inflow refreshment curves were obtained with 90” and 45” pulses as indicated. Plotted on the abscissa is the mean flow velocity (m/set), and plotted on the ordinate is the signal intensity in arbitrary units.

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B Fig..3. Cross-sectional images of a volunteers’ legs at the mid-thigh level obtained with (A) the outflow refreshment technique (basic slice thickness 5 mm, slice overlap amount 2.5 mm, flip angle 90”, and slice advancement time 60 msec), and (B) the lipid suppressed inflow refreshment technique (slice thickness 2.5 mm, flip angle 90”, and TR 60 msec). It is evident that while vessels are seen well with both techniques, anatomical landmarks can no longer be easily recognized in the inflow refreshment image.

lost to the angiogram. l2 Vessels that either possess slow flow or have a significant in-plane segment (resulting in poor flow refreshment) fail to generate maximum contrast. These constraints are common to both O/R and I/R methods, but it is apparent that in the lipid suppressed I/R angiograms of Fig. 4, certain small vessels are visible that are not seen as clearly in the O/R angiograms. This sensitivity loss for the O/R angiograms is attributable to two sources: (1) inhomogeneities in the receiver/transmitter coil result in artifactually bright image regions (e.g., observe in Fig. 2, in the upper left hand corner the leg appears to be significantly brighter than other image regions); and (2) since O/R images are spin-density weighted, static tissues which possess higher spin densities than blood may result in a reduction in blood/static contrast compared with I/R images. Thus even vessels that appear

bright, displaying image contrast over a local region, sometimes fail to appear in the MIP angiogram. Better angiographic visualization is expected from algorithms for vessel rendering that identify vessels on the basis of local, rather than global, flow/static contrast. For a 90” pulse, the flow sensitivity curve for I/R reaches a plateau when the flow velocity is sufficient to totally refresh the thin slicer3 (Fig. 2). The same plateau is seen for the O/R case, but in this instance it is reached when the flow velocity is sufficient to refresh the overlapped slice portion. While the shapes of the flow sensitivity curves are similar, the O/R curve is elevated on a high baseline, reflecting the high sensitivity to static tissue. However, from inspection of the flow sensitivity curves it can be seen that flow/ static contrast for O/R and I/R are similar when 90” pulses are used. For I/R imaging a flip angle less than 90” is often used to improve sensitivity to slow flow. However, use of a 45” pulse (a commonly employed angle) in I/R reduces the flow/static contrast compared to the 90” case. In the O/R case suppression of the overlapped slice portions is achieved with fewer pulses than in the I/R case. Thus for O/R, use of selection pulses of less than 90” results in poorer flow/ static contrast compared with I/R. During the O/R acquisition, arterial flow excited at the last slice position will advance towards the initial slice position. Thus, there is a possibility that this “tagged” flow may be re-excited in the early slices before it has relaxed sufficiently to contribute a bright signal. Consequently, in the in vivo example presented here, a deliberate time delay of 1000 msec was allowed to elapse between excitation of the last slice and reinitiation of excitations at the first slice. This time interval allowed the tagged blood to either exit the region of interest or relax sufficiently to contribute a large signal when re-excited. This delay slightly increased the scan time for the O/R acquisition (13 min) compared to the I/R acquisition (10.5 min). In O/R imaging intrinsic venous signal presaturation is achieved when thick slices and large slice overlap amounts are employed (Fig. 4). This arises since the wide slice excitation suppresses a section of venous blood, which upon flowing into the adjacent plane yields a low signal when re-excited. Conversely for thin slices combined with low or nominally zero overlapping amounts, the venous flow contributes significantly to the images (Fig. 4). The O/R images do not display coherent pulsatile artifacts that are commonly seen in I/R images (Fig. 3). This feature is related to the dissociation of the acquisition time per slice from the subject’s heart rate.14 In I/R methods the acquisition time per slice is short

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Fig. 4. Frontal view MIP angiograms of a volunteer’s thighs. Six outflow refreshment and two inflow refreshment angiograms are shown. The outflow acquisitions employed 90” pulses, arId indicated are the slice thickness, percentage slice-overlap, and numerical slice-overlap values. The inflow refreshment angicIgrams were obtained using a slice thickness of 2.5 mm and flip angles of 90” and 45” as indicated (see text for details).

compared to the heart cycle time, resulting in strong pulsatile artifacts. In the O/R method, data from each slice are acquired over the total acquisition time. Thus although on average pulsatile flow signal disruption is comparable in both cases, in the O/R case the signal is randomly distributed over the image as unobtrusive speckling.

CONCLUSIONS A new bright blood imaging strategy, outflow refreshment (O/R), has been introduced. This approach has several advantages over previously reported inflow refreshment (I/R) methods, including: 1) Bright static tissue signals are generated, allowing soft tissue struc-

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tures to be examined in the cross-sectional images. This allows for the possibility, that even in regions where the blood signal is lost, the vessel lumen size and patency may be assessed from the cross-sectional images. 2) The O/R images do not generate obtrusive pulsatile blood flow artifacts, thus averaging is not required to improve the signal/artifact ratio.

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REFERENCES 1. Laub, G.A.; Kaiser, W.A. MR angiography with gradi-

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3.

4.

5.

6.

ent motion refocusing. J. Comput. Assist. Tomogr. 12: 377; 1988. Ruggieri, P.M.; Laub, GA.; Masaryk, T.J.; Medic, M.T. Intracranial circulation: Pulse-sequence considerations in three-dimensional (volume) MR angiography. Radiology 171:785-791; 1989. Groen, J.P.; de Graaf, R.G.; van Dijk, P. MR angiography based on inflow. In: Book of abstracts: Seventh Annual Meeting of the Society of Magnetic Resonance in Medicine. San Francisco: SMRM; 1988:906. Edelman, R.R.; Wentz, K.U.; Mattle, H.; Zhao, B.; Liu, C.; Kim, D.; Laub, G. Projection arteriography and venography: Initial clinical results with MR. Radiology 172:351-357; 1989. Keller, P.J.; Drayer, B.P.; Fram, E.K.; Williams, K.D.; Dumoulin, C.D.; Souza, S.P. MR angiography with two-dimensional acquisition and three-dimensional display. Radiology 173:527-532; 1989. Rossnick, S.; Laub, G.; Braeckleet, R.; et al. Three dimensional display of blood vessels in MRI. Proceed-

9.

10.

11.

12.

13.

14.

ings of the IEEE Computers in Cardiology Conference. New York: Institute of Electrical and Electronic Engineers; 1986:193-196. Whittemore, A.R.; Bradley, W.G.; Jinkins, J.R. Comparison of concurrent and countercurrent flow-related enhancement in MR imaging. Radiology 170:265-271; 1989. Matsuda, T.; Doyle, M.; Pohost, G.M. Slice thickness reduction by partial overlapping presaturation. Magn. Reson. Med. 24:358-363; 1992. Doyle, M.; Matsuda, T.; Pohost, G.M. SLIP, a lipid suppression technique to improve image contrast in inflow angiography. Magn. Reson. Med. 21:71-81; 1991. Haacke, E.M.; Lenz, G.W. Improve MR image quality in the presence of motion by using rephasing gradients. AJR 148:1251-1258; 1987. Gullberg, G.T.; Wehrli, F.W.; Shimakawa, A.; Simon% M.A. MR vascular imaging with a fast gradient refocusing pulse sequence and reformatted images from transaxial sections. Radiology 165:241-246; 1987. Anderson, CM.; Saloner, D.; Tsuruda, J.S.; Shapeero, L.G.; Lee, R.E. Artifacts in maximum-intensity-projection display of MR angiograms. AJR 154:623-629; 1990. Wehrli, F.W.; Shimakawa, A.; Gullberg, G.T.; MacFall, J.R. Time-of-flight MR flow imaging: Selective saturation recovery with gradient refocusing. Radiology 160: 781-785; 1986. Doyle, M.; Matsuda, T.; Pohost, G.M. A new acquisition mode for 2D inflow refreshment angiography. Magn. Reson. Med. 18:51-62; 1991.

Outflow refreshment angiography: a bright blood, bright static tissue technique.

A new "bright blood" strategy, outflow refreshment imaging, is introduced in which a number of overlapping slices are excited in rapid succession. Flo...
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