Improved dynamic susceptibility contrast (DSC)-MR perfusion estimates by motion correction

Optimising MR perfusion imaging: comparison of different software-based approaches in acute ischaemic stroke

OBJECTIVES

Perfusion imaging (PI) is susceptible to confounding factors such as motion artefacts as well as delay and dispersion (D/D). We evaluate the influence of different post-processing algorithms on hypoperfusion assessment in PI analysis software packages to improve the clinical accuracy of stroke PI.

METHODS

Fifty patients with acute ischaemic stroke underwent MRI imaging in the first 24 h after onset. Diverging approaches to motion and D/D correction were applied. The calculated MTT and CBF perfusion maps were assessed by volumetry of lesions and tested for agreement with a standard approach and with the final lesion volume (FLV) on day 6 in patients with persisting vessel occlusion.

RESULTS

MTT map lesion volumes were significantly smaller throughout the software packages with correction of motion and D/D when compared to the commonly used approach with no correction (p = 0.001-0.022). Volumes on CBF maps did not differ significantly (p = 0.207-0.925). All packages with advanced post-processing algorithms showed a high level of agreement with FLV (ICC = 0.704-0.879).

CONCLUSIONS

Correction of D/D had a significant influence on estimated lesion volumes and leads to significantly smaller lesion volumes on MTT maps. This may improve patient selection.

KEY POINTS

• Assessment on hypoperfusion using advanced post-processing with correction for motion and D/D. • CBF appears to be more robust regarding differences in post-processing. • Tissue at risk is estimated more accurately by correcting software algorithms. • Advanced post-processing algorithms show a higher agreement with the final lesion volume.

ASFNR recommendations for clinical performance of MR dynamic susceptibility contrast perfusion imaging of the brain

MR perfusion imaging is becoming an increasingly common means of evaluating a variety of cerebral pathologies, including tumors and ischemia. In particular, there has been great interest in the use of MR perfusion imaging for both assessing brain tumor grade and for monitoring for tumor recurrence in previously treated patients. Of the various techniques devised for evaluating cerebral perfusion imaging, the dynamic susceptibility contrast method has been employed most widely among clinical MR imaging practitioners. However, when implementing DSC MR perfusion imaging in a contemporary radiology practice, a neuroradiologist is confronted with a large number of decisions. These include choices surrounding appropriate patient selection, scan-acquisition parameters, data-postprocessing methods, image interpretation, and reporting. Throughout the imaging literature, there is conflicting advice on these issues. In an effort to provide guidance to neuroradiologists struggling to implement DSC perfusion imaging in their MR imaging practice, the Clinical Practice Committee of the American Society of Functional Neuroradiology has provided the following recommendations. This guidance is based on review of the literature coupled with the practice experience of the authors. While the ASFNR acknowledges that alternate means of carrying out DSC perfusion imaging may yield clinically acceptable results, the following recommendations should provide a framework for achieving routine success in this complicated-but-rewarding aspect of neuroradiology MR imaging practice.

Arterial input function in perfusion MRI: a comprehensive review

Cerebral perfusion, also referred to as cerebral blood flow (CBF), is one of the most important parameters related to brain physiology and function. The technique of dynamic-susceptibility contrast (DSC) MRI is currently the most commonly used MRI method to measure perfusion. It relies on the intravenous injection of a contrast agent and the rapid measurement of the transient signal changes during the passage of the bolus through the brain. Central to quantification of CBF using this technique is the so-called arterial input function (AIF), which describes the contrast agent input to the tissue of interest. Due to its fundamental role, there has been a lot of progress in recent years regarding how and where to measure the AIF, how it influences DSC-MRI quantification, what artefacts one should avoid, and the design of automatic methods to measure the AIF. The AIF is also directly linked to most of the major sources of artefacts in CBF quantification, including partial volume effect, bolus delay and dispersion, peak truncation effects, contrast agent non-linearity, etc. While there have been a number of good review articles on DSC-MRI over the years, these are often comprehensive but, by necessity, with limited in-depth discussion of the various topics covered. This review article covers in greater depth the issues associated with the AIF and their implications for perfusion quantification using DSC-MRI.

The 39 steps: evading error and deciphering the secrets for accurate dynamic susceptibility contrast MRI

Dynamic susceptibility contrast (DSC) MRI is the most commonly used MRI method to assess cerebral perfusion and other related haemodynamic parameters. Although the technique is well established and used routinely in clinical centres, there are still many problems that impede accurate perfusion quantification. In this review article, we present 39 steps which guide the reader through the theoretical principles, practical decisions, potential problems, current limitations and latest advances in DSC-MRI. The 39 steps span the collection, analysis and interpretation of DSC-MRI data, expounding issues and possibilities relating to the contrast agent, the acquisition of DSC-MRI data, data pre-processing, the contrast concentration-time course, the arterial input function, deconvolution, common perfusion parameters, post-processing possibilities, patient studies, absolute versus relative quantification and automated analysis methods.

Automated vs manual delineations of regions of interest- a comparison in commercially available perfusion MRI software

Background

In perfusion magnetic resonance imaging a manual approach to delineation of regions of interest is, due to rater bias and time intensive operator input, clinically less favorable than an automated approach would be. The goal of our study was to compare the performances of these approaches.

Methods

Using Stroketool, PMA and Perfscape/Neuroscape perfusion maps of cerebral blood flow, mean transit time and Tmax were created for 145 patients with acute ischemic stroke. Volumes of hypoperfused tissue were calculated using both a manual and an automated protocol, and the results compared between methods.

Results

The median difference between the automatically and manually derived volumes was up to 210 ml in Perfscape/Neuroscape, 123 ml in PMA and 135 ml in Stroketool. Correlation coefficients between perfusion volumes and radiological and clinical outcome were much lower for the automatic volumes than for the manually derived ones.

Conclusions

The agreement of the two methods was very poor, with the automated use producing falsely exaggerated volumes of hypoperfused tissue. Software improvements are necessary to enable highly automated protocols to credibly assess perfusion deficits.

Perfusion MRI using dynamic-susceptibility contrast MRI: quantification issues in patient studies

OBJECTIVES

Measurement of perfusion accurately, noninvasively, and with good spatial resolution offers the chance to characterize abnormal tissue in many clinical conditions. Dynamic-susceptibility contrast (DSC) MRI, also known as bolus-tracking MRI, is a dynamic MRI method to measure perfusion and other related hemodynamic parameters. This review article describes the principles involved in perfusion quantification using DSC-MRI as well as discusses the main issues affecting its quantification in patient studies.

CONCLUSIONS

It is shown that DSC-MRI is a very powerful technique that provides important information regarding cerebral hemodynamics. The relatively high contrast-to-noise ratio, fast acquisition, and wealth of information available have made DSC-MRI the most commonly used MRI technique for the rapid assessment of the brain hemodynamics in clinical investigations. While very important advances have been achieved in the last 2 decades, there are still some remaining limitations that users should be aware of to avoid misinterpretation of the findings and to make the most of the invaluable information provided by perfusion MRI.

Deconvolution-Based CT and MR Brain Perfusion Measurement: Theoretical Model Revisited and Practical Implementation Details

Deconvolution-based analysis of CT and MR brain perfusion data is widely used in clinical practice and it is still a topic of ongoing research activities. In this paper, we present a comprehensive derivation and explanation of the underlying physiological model for intravascular tracer systems. We also discuss practical details that are needed to properly implement algorithms for perfusion analysis. Our description of the practical computer implementation is focused on the most frequently employed algebraic deconvolution methods based on the singular value decomposition. In particular, we further discuss the need for regularization in order to obtain physiologically reasonable results. We include an overview of relevant preprocessing steps and provide numerous references to the literature. We cover both CT and MR brain perfusion imaging in this paper because they share many common aspects. The combination of both the theoretical as well as the practical aspects of perfusion analysis explicitly emphasizes the simplifications to the underlying physiological model that are necessary in order to apply it to measured data acquired with current CT and MR scanners.

Search for a map and threshold in perfusion MRI to accurately predict tissue fate: a protocol for assessing lesion growth in patients with persistent vessel occlusion

BACKGROUND

The MRI-based mismatch concept has been used to estimate the risk of infarction in ischemic stroke. Based on multiple studies on magnetic resonance perfusion imaging, it seems unlikely that any perfusion parameter threshold will provide a reliable prediction of radiological or clinical outcome for all patients. The goal of our study was to find a minimally biased yet maximally useful perfusion postprocessing protocol which would offer the treating physician a useful estimate of tissue fate.

METHODS

One hundred and forty-five acute ischemic stroke patients, admitted within 24 h after stroke to the Charité-University Medicine Hospital in Berlin between March 2008 and November 2009, were included in this study. Using three different software packages (Perfscape/Neuroscape, PMA and Stroketool), maps of mean transit time, cerebral blood flow (CBF) and T(max) were created. Three different thresholds were applied on each parameter map and subsequent volumes of hypoperfused tissue were calculated.

RESULTS

Overall, the maps and thresholds giving the least amount of overestimation of the final infarct volume were T(max) 8 s in Perfscape/Neuroscape, CBF 20 ml/100 g/min in PMA and CBF 15% (of the highest value on the scale for a given patient) in Stroketool. In patients with persistent vessel occlusion, a CBF map with a restrictive threshold showed volumes of tissue at definite risk of infarction in up to 100% of patients. The additional use of a CBF map with a high threshold enabled identification of patients without penumbras.

CONCLUSIONS

No combination of software, map and threshold was able to give a reliable estimate of tissue fate for either all patients or any subgroup of patients. However, in patients with vessel occlusion, combination of a CBF map with a low and a high threshold can enable calculation of the minimum volume of brain tissue that will inevitably be lost if the occlusion persists.

Vessel size imaging using dual contrast agent injections

PURPOSE

To investigate the feasibility of a vessel size imaging (VSI) technique with separate contrast agent injections for evaluation of the vessel caliber in normal tissues and in brain tumors.

MATERIALS AND METHODS

Computer simulation was first performed to assess the potential errors in the estimation of vessel caliber that could result from time shifts between the dual contrast agent injections. Eight patients (four female, four male, 37-77 years old) with brain tumors (three high-grade gliomas, two low-grade gliomas, and three meningiomas) were recruited for clinical study. Dynamic susceptibility contrast magnetic resonance imaging (MRI) using gradient echo (GE) and spin echo (SE) echo-planar imaging sequences were performed separately with a 10-minute interval on a 3.0T scanner. Vessel caliber maps were calculated and analyzed in regions of interest at cortical gray matter (GM), thalamus, white matter (WM), and tumors.

RESULTS

From the computer simulation, the error of vessel caliber measurement was less than 8% when the difference between the time-to-peak of the GE and the SE studies was 1.5 seconds, and reduced to within 5% when the difference was 1 second. From the patient datasets of a 64 x 64 matrix, the estimated vessel calibers were 37.4 +/- 12.9 microm for cortical gray matter, 20.7 +/- 8.8 microm for thalamus, and 15.0 +/- 5.1 microm for white matter, comparable to results in the literature. Two patients had a VSI with 128 x 128 matrix and showed similar results in vessel calibers of normal tissues. All the tumors had larger mean vessel diameter than normal-appearing tissues. The difference in vascular size between normal tissue and tumor was demonstrated clearly in both the VSIs of regular and high spatial resolution.

CONCLUSION

This study suggests that VSI with a dual injection method is a feasible technique for estimating microvascular calibers of normal tissues and brain tumors in clinical scanners.