Synopsis
This
talk will describe the basics of dynamic contrast-enhanced (DCE) MRI with focus
on the acquisition of the data. Key prerequisites for accurate quantification
of perfusion parameters such as Ktrans are sufficiently high spatial
and temporal resolution but also accurate quantification of the contrast agent (CA)
concentration. The basics of how this can be achieve will be covered in the
talk.
Introduction
In
DCE-MRI a CA is injected intravenously and the uptake and washout is
monitored using a high temporal resolution MRI scan that is sensitive to the
change in T1 relaxation time caused by the CA (1). From the temporal dynamics, in each voxel or
in a region of interest, physiological information can be extracted regarding
e.g. blood volume or leakiness of the capillary system (2). Applications are found in several areas, for
example assessment of chemoradiotherapy (3). To accurately quantify perfusion parameters,
high quality DCE-MRI data is required. However this is difficult to obtain due
to several conflicting demands regarding temporal resolution, spatial
resolution and accuracy. Requirements from physiology
The requirements on the imaging is set by the physiology of
the imaged subject and the information that is to be extracted from the data.
Typically this implies that the kinetics of the CA needs to be monitored for
several minutes (up to about 15 min for very slowly enhancing tissues (4)) and a sample is needed at
least every 4 seconds [5] for Ktrans
quantification. If blood flow and capillary transit time is to be quantified
the sampling rate must be even higher and a sampling time of 1.5 s has been
suggested [13]. Meeting the temporal requirements would be simple if it were
not for the spatial requirements that must be fulfilled simultaneously. In for
instance cancer imaging one is often interested in tumor heterogeneity, and not
acquiring slices covering all of the tumor/tumors is not a good option since
that could lead to information being missed (5), hence both a large imaging field-of-view (FOV) and
high spatial resolution are typically desired. Acquiring the blood plasma CA concentration, the arterial input function (AIF), is essential for most
analysis of DCE-MRI data. It is also very demanding considering that the AIF
changes rapidly, the vessels are usually small and a large FOV might be required if no
suitable vessel is close to the investigated site. Quantifying the contrast agent concentration
Not
only is it enough that images can be obtained with sufficient spatio-temporal
resolution and FOV, it must also be possible to infer the CA concentration from
the images. This implies that the T1 relaxation time must be measured at a very
high rate (0.2-1 Hz) in the entire FOV. To achieve this a minimum repetition
time 3D spoiled gradient Echo (3D-SPGR) sequence is most often used (6). However, this sequence is T1-weighted and does
not in it-self give the T1 time and the CA concentration. For that, additional
measurements are required, i.e. a baseline signal and a T1 map without the CA
present. The baseline is obtained simply by monitoring the signal before the
injection and the T1 map is typically acquired using the variable flip angle
(VFA) method (7), also before the injection of CA. The CA
concentration estimation is very sensitive to the pre-contrast T1 and
unfortunately it has been shown that the SPGR sequence used in DCE-MRI and in the
VFA method is sensitive to variations in B1 and also sensitive to the spoiling
scheme used (8). This implies that B1 ideally also should be
measured, using e.g. actual flip angle imaging (9) or a Bloch-Siegert shift approach (10), in particular at high fields (> 1.5 T).
Effects of insufficient spoiling is normally not compensated for, although it
is possible by adjusting the signal equation (11).Measuring the arterial Input function
Measuring the AIF is the most challenging part of DCE-MRI. Not
only in terms of spatial and temporal resolution, but also regarding accuracy
of the estimated CA concentration. The steady state property of the
3D SPGR sequence implies that CA quantification in flowing blood will fail
unless the blood has been flowing for several cm in the imaged region before it
reaches the site where the AIF is sampled (11,12). This may increase the required
size of the imaged region. An important consideration when setting up a DCE-MRI
exam is to determine the flip angle of the 3D SPGR sequence used for CA
monitoring. A moderate flip angle around 10° at TR = 5 ms will give a sequence most sensitive to the CA concentration levels in tissue, however, the signal
from blood will in this case saturate making it impossible to obtain the AIF.
Hence, the use of a measured AIF will require a larger flip angle (around
20°-30°) and this results consequently in a trade-off in the measurement precision
in tissue. It may also lead to difficulties with SAR at higher field strengths. Advanced reconstruction techniques for DCE-MRI
Simultaneously meeting all requirements regarding
spatio-temporal resolution and accurate CA quantification without compromises
has until recently been impossible. However, with the introduction of
compressed sensing (CS) this is changing. Whole brain, high resolution
monitoring of CA concentration in DCE-MRI has recently been achieved with a
temporal resolution of 4.1 s using CS (13), and CS technology is now
becoming available on standard clinical systems. Another interesting development
where CS is used is to resolve motion (e.g. XD-GRASP (14)) in free breathing abdominal
DCE-MRI. Finally, the latest development with model based DCE-MRI (15) could provide further improvement
of the temporal resolution. We are therefore at a very exciting time
when no compromises might be needed. This could make it
possible to finally reach a long sought goal of standardizing the DCE-MRI data
acquisition.Acknowledgements
No acknowledgement found.References
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