A. Rationale for the Workshop
- In the last several years contrast-enhanced MR angiography (CE-MRA) has
become a commonly used, commercially available method. In parallel with this,
dynamic contrast-enhanced MR imaging (DCE-MRI) has emerged as a promising method
for diagnosis and prognosis of cancer. However, widespread use of DCE-MRI is
limited by the need for further technical improvements.
- DCE-MRI and CE-MRA share the need for flexible acquisition methods to permit
trade-off between temporal and spatial resolution. Hence, some of the
considerable effort invested on CE-MRA acquisition methods should be translated
to DCE-MRI.
- The focus of CE-MRA is on visualization of small vessels with demands for
temporal resolution dictated by the desire to separate arteries and veins, while
focus of DCE-MRI is on contrast kinetics with demands for spatial resolution
dependent on the specific application. Consequently, translation will require
consideration of these differing needs.
- Quantitative DCE-MRI, which is needed to permit its widespread use, requires
special purpose data acquisition techniques and analysis software not normally
included by the instrument manufacturers. Although precedents for development of
advanced analysis package include functional neuro MRI (fMRI) and prostate MR
spectroscopic imaging (MRSI), additional capabilities will be necessary to
permit accurate generation of quantitative data.
B. Purpose of Workshop
The workshop brought together a limited number of researchers who are
experienced in MRI pulse sequence techniques, image analysis, or applications in
either CE-MRA or DCE-MRI with representatives from the MRI industry. The purpose
was to identify desirable performance features and a possible framework of
future pulse sequence and image analysis strategies which might be used for
dynamic contrast-enhanced imaging of cancer to ensure that advances in this area
can be widely implemented.
C. Workshop Program
The program for the workshop is given in Appendix A. As shown, the program
consisted of two parts, the first consisting of individual presentations on the
meeting topic with several preceding presentations by NIH officials, and the
second a discussion session. The list of all participants, including speakers
and observers, is listed in Appendix B.
The workshop brought together MRI researchers who collectively represented a
variety of backgrounds. This included individuals with a clinically based
understanding of the requirements for DCE-MRI, others experienced in the
physics-based development of MR acquisition techniques which have been used for
CE-MRA, others with firsthand experience in DCE-MRI techniques as applied
clinically to the imaging of cancer, and representatives from industry. Speakers
were asked to present: (i) a review of their own work in CE-MRA or DCE-MRI with
emphasis on the technical aspects and how they impact clinical utility; and (ii)
a statement of their view of the future technical needs of DCE-MRI and how these
might be met by innovations in acquisition and analysis methods. Detailed notes
of the individual presentations and discussion as made on site by NIH staff are
presented in Appendices C and D.
D. Major Points / Issues (not listed in any priority)
- For DCE-MRI to be useful in a routine clinical setting, including clinical
trials of new cancer therapies, acquisition and processing must be
straightforward providing a 3D representation of the relevant quantitative
information.
- Properties of successful DCE-MRI methods include: cover entire tumor with
best possible spatial and temporal resolution; flexible specification of FOV;
results independent of system and contrast dose; free from spurious
correlations; free from motion artifacts, free from errors due to tissue
properties; include estimate of error; and highly reproducible.
- Many CE-MRA techniques are relevant to DCE-MRI, but in general they are not
directly transferable "off the shelf" from their current embodiments on
commercial MRI machines.
- Many meritorious technical directions are possible for attempting to get
high 3D spatial and good temporal resolution. However, there is a need for more
MRI sequence development both in general and for application specifically in
DCE-MRI.
- Most commercial MRI manufacturers already provide to their respective
academic collaborators some level of capability to develop acquisition
techniques of their own. In general this requires a technical knowledge of the
MRI machine which is much deeper than that required for routine clinical
operation.
- Translation of new methods developed at one institution to other
institutions for use in clinical trials of new cancer therapies requires
personnel dedicated to supporting implementation at sites with considerably less
technical knowledge of the MRI machine.
- Requirements on temporal resolution may vary over the course of a
contrast-enhanced examination, with resolution the order of seconds targeted for
the initial contrast enhancement phase and becoming progressively longer to tens
of seconds during later phases which may occur ten minutes later. This may vary
according to the organ under study and is also subject to the preferences of the
individual directing the examination.
- A variety of the technical directions for research involve the acquisition
of MR data in non-standard formats, using non-standard k-space trajectories and
view orders, and with various ways to combine and filter data between different
time frames as part of the image reconstruction. To study these effectively will
require familiarity with details of the specific commercial MRI system used.
- The application of DCE-MRI in an individual patient will likely require a
greater degree of analysis than CE-MRA, as kinetic parameters must be estimated
quantitatively. There is need for more research and development in this area of
study.
- Because of the manner in which k-space is sampled one trajectory (line, ray,
or spiral) at a time, and because of the presence of time-varying signal levels
in DCE-MRI, the very meanings of temporal and spatial resolution may need
definition, and standard tests may need to be identified.
- There is a need for parallel developments in humans as well as in animal
models.
- In order to have adequate patient statistics, the clinical study of a given
new technique may well require the collaboration of investigators from multiple
institutions. Recruitment of such collaborators may likely involve sites which
focus primarily on clinical MRI vs. those with extensive technical research
capability. Further, not all collaborators will necessarily be located near each
other. In the short term, given the problems identified in #6, it may be best to
specify minimum requirements, rather than specific sequence to facilitate
cooperation from more clinically-oriented site.
- The acquisition of high spatial resolution information over multiple
timepoints will require the handling of large amounts of data. For example, a
single contrast-enhanced 3D run with the order of 10-20 timepoints requires the
order of several 100 MB of memory.
- There are a variety of organs which appear appropriate for study by DCE-MRI.
Although there is a common desire for high spatial and temporal resolution,
there are organ-specific requirements as well. Nominal, targeted specifications
on imaging parameters and contrast delivery for four specific organs are:
- Brain parameters: 2-3mm (cubed) isotropic voxels, whole brain FOV, T1
map in 1-2 minutes, 2 sec temporal resolution in first minute, slower out to 10
min. Measure of arterial function not affected by inflow. Fast bolus @3-5 ml/sec
administration with saline chaser.
- Breast parameters: single or both whole breast FOVs; 10 sec temporal
resolution at 2 mm cubed; with 0.5x 0.5x 3mm resolution in first 1-2 minutes;
fat suppressed for CNR; run washout to 8 minutes; prior T1 map; 2-3 ml/sec
contrast administration; measure arterial input function.
- Liver parameters: visualize >2cm lesions; FOV = 25 x 35 x 18 cm;
continuous with acquisition-based motion correction; 5-10 sec temporal at
spatial 2x2x7 mm spatial resolution; T1 map up front; 1-3 ml/sec bolus delivery;
want arterial phase input function if feasible.
- Lung parameters: visualize >2cm lesions; FOV = 25 x 35 x 18 cm;
continuous with acquisition-based motion correction; 5-10 sec temporal at
spatial 2x2x5 mm spatial resolution; T1 map up front; 1-3 ml/sec bolus delivery;
want arterial phase input function if feasible.
E. Recommendations
- It is recommended that a research priority be the development of pulse
sequences for magnetic resonance imaging of the contrast enhancement of organs
for detection and characterization of cancer. The general goal is to provide
high spatial resolution images of a volume containing the organ of interest and
with temporal resolution adequate to characterize contrast pharmacokinetics.
Among the possible areas of investigation are:
- novel k-space trajectoreis (e.g. spiral, projection reconstruction,
echo-planar, etc.)
- novel ways for temporal sampling of k-space trajectories (e.g. phase
encoding view order, view sharing, temporal filtering, interleaved spiral or
projection reconstruction samples)
- ways for characterization of the temporal response and spatial resolution of
pulse sequences which can possibly be used for DCE-MRI (e.g. frequency-spatial
resolution analysis, identification of standardized tests of spatial and
temporal resolution appropriate for DCE-MRI)
- means for dynamic adjustment of the tradeoff between spatial and temporal
resolution during the course of an DCE-MRI run (e.g. dynamic adjustment of
projection reconstruction sampling, dynamic adjustment of k-space sampling and
extent of coverage)
- new ways for providing improved spatial resolution per unit time as might be
applied to DCE-MRI (e.g. multiple coil techniques such as sensitivity encoding
"SENSE" and simultaneous acquisition of spatial harmonics "SMASH," effective
utilization of high capability gradient technology)
- It is recommended that a research priority be the quantitative evaluation of
DCE-MRI time series of images, with development of techniques for determination
of pharmacokinetic parameters characterizing tumor vascularity, with allowance
for the potential application of these techniques using large data sets
consisting of high resolution 3D volumes sampled at multiple timepoints.
- It is recommended that a research priority be the application of current,
emerging, and new DCE-MRI techniques in the imaging of cancer in humans for
purposes of determination of the sensitivity, specificity, and utility of these
techniques.
- It is recommended that the NCI explore how it would encourage
industry/academic partnerships to support and/or promote the effective
distribution to less-experienced MRI sites of new methods developed by academic
investigators for acquisition and quantitation of DCE-MRI time series images.
- It is recommended that academic investigators with specific MR pulse
sequences that meet, in part, the requirements for quantitative DCE-MRI
acquisition in brain, breast, liver or lung (as described above) send their
software or description to MRI industry to evaluate the ability of their system
to implement the sequences.
- It is recommended that a means should be explored for the major industry
players to develop a process for evaluation of both acquisition and analysis
software and make appropriate selections. (This may be problematic since they
will each make their own decisions in their own way, with some influence from
the direction others are taking).
- It is recommended that a research priority be the development and study of
animal models of cancer in which DCE-MRI may be evaluated
Summary of Appendices
- Appendix A: Meeting Program
- Appendix B: List of Participants
- Appendix C: Notes by NIH Staff of Individual Presentations
- Appendix D: Notes by NIH Staff of Discussion Period
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