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Myocardial Perfusion Mapping with an Intravascular MR Contrast
Agent: Performances of Deconvolution Methods at Various Flows
B. Neyran, M. Janier, C.Casali, D. Revel, E. Canet.
Laboratoire CREATIS, UMR 5515, Hôpital Cardiologique and INSA, Lyon, France.
Introduction
In MR perfusion studies, regional Myocardial Blood
Flow (rMBF), Mean Transit Time (rMTT) and Myocardial
Blood Volume (rMBV) are important physiological parameters
to measure. Thus, mapping of these parameters required a
robust and fast deconvolution technique to process on a pixel
by pixel basis. Here, different mathematical methods were first
compared using numerical experiments. rMBF, rMTT and
rMBV maps of the myocardium were then computed from firstpass T1 images of an isolated pig heart preparation (1) obtained
at myocardial flow levels ranging from 50 to 400 ml/min/100 g.
Methods
From the central volume principle (2), rMBF, rMTT and rMBV
can be computed from arterial input Cin(t) and myocardial
tissue Ctis(t) curves:
C tis (t) = Cin (t) * r(t)
[1]
where * denotes the convolution product and r(t) is the residue
function or remaining fraction, with:
r(0) = rMBF = rMBV
rMTT
∞ r(t)
rMTT = ∫
dt; rMBV = r(0).rMTT
r(0)
0
[2]
rMBF, rMTT and rMBV were computed using [1] and [2] by
four mathematical approaches: two analytical techniques (A-B)
and two model independent methods (C-D). A and B are based
on mathematical descriptions of the indicator first-pass
distribution: the residue is assumed to be an exponential (A)
and a Fermi function (B). For C and D, no assumptions are
made, the residue is identified using the singular value
decomposition (svd) (C) (3) and the discrete time form of an
auto-regressive model (ARMA) (D).
Experimental protocol
Non-beating blood perfused isolated pig hearts were reperfused
in the magnet with the flow level monitored by a calibrated
pump. In some hearts, segmented variation of perfusion was
obtained by occlusion of the left anterior descending artery
(LAD) in the magnet. Hearts were imaged at normal and high
flow before and after occlusion in a 1.5 T whole-body scanner
(Vision, Siemens, Germany). The first-pass of 0.05 mmol of an
intravascular agent (CMD-A2-Gd-DOTA, Guerbet, France)
was followed by T1 TurboFLASH at flows from 50 to 400
ml/min/100g.
Numerical Experiments
Arterial input Cin(t) were simulated using a gamma function,
with respectively MTT=7 sec and 14sec.
The myocardial system was simulated by one single, well
mixed compartment with time constant rMTT and a delay td.
For such a system, the residue function is an exponential (2,3).
Signal noise ratio (SNR) of 20 for the input and 10 for the
tissue was simulated by Gaussian random noise. For fixed
rMBV, respectively 9, 15 and 4.5%, rMBF was varied from O
to 400 ml/min/100g. Mean ± standard deviation (SD) of rMBF,
rMTT and MBV identified by methods A to D over 128 trials
were plotted against true values.
MR Data analysis
Myocardial maps of rMBF (ml/min/100g), rMTT (sec) and
rMBV (in %) were calculated by the four methods for the
different perfusion conditions. The mean (± SD) of the different
parameters was calculated for the perfused myocardium.
Result
For rMBV true values of 9%, 15% and 4.5%, rMBV was well
estimated by all methods at flow values>40 with SNR of 10.
For flows<40, rMBV was always underestimated. At this low
SNR, rMBF determination was highly dependent on
experimental conditions, i.e. often underestimated at high flows
and with the large input MTT of 14 sec. Finally, rMTT was the
most difficult to determined. Maps of rMBV were similar with
all methods, the hypoperfused region being well identified
(Figure). rMBF maps were more variable, very noisy at low
flow and underestimated at any flow with C (Figure).
Discussion-Conclusion
Both simulations and maps showed that rMBV could be used
to detect abnormal perfusion. Evaluation of rMBF was more
variable. In conclusion, for myocardial applications, i.e. flows
from 50-400 ml/100g/min, rMBV mapping is more robust to
detect abnormalities with any technique and to select the
appropriate deconvolution method for rMBF mapping.
References
1. C. Casali. et al. Invest. Radio. Vol 32, No 11. J.F. 713-720
(1997). 2. N.A. Lassen. W. Perl. Tracer kinetic methods in
medical physiology. Raven Press New York. 1979. 3. L.
Ostergaard. et al. Magn. Reson. Med. 36. 715-725 (1996).
Figure
rMBF and rMBF maps of the isolated pig heart perfused at 320
ml/min/100g after LAD occlusion with methods A to D. The
occluded area was well delineated on rMBV maps (arrow).
RV=right ventricular cavity.
A
B
C
D
244±15
5
239±15
8
61±2
8
376±22
8
500
rMBF
ml/100g
/mn
0
20
RV
rMBV
%
0
5.6±2.
1
5.6±2.
2
6.1±3.
4
5.7±2.
5