Future Interventional Neuroimaging
A decade ago, at RSNA 2004, the cone beam flat detector CT (DynaCT) was introduced on clinical C-Arm systems. Over the years, a number of significant enhancements have entered clinical practice, like cone beam reconstruction algorithms beyond Feldkamp and others.
Current developments are along two lines: further enhancements of morphological imaging and the exploration of functional imaging based on DynaCT.
Sophisticated algorithms have been developed and tested clinically for metal artifact correction (MAC). Together with high resolution DynaCT, the MAC is very useful for stent-assisted coiling in reconstructing the parent vessel and the stent struts in the vicinity of metals.
The development of imaging techniques that focus on small volumes of interest (VOI) has dramatically reduced the dose to the patient. For many situations in interventional imaging, those techniques allow for repeated 3D-updates to access the progress and the final results of the procedure.
The first step towards functional imaging was the measurement of Cerebral Blood Volume (CBV) using native and enhanced DynaCT scans and tailored injection protocols. The results of preclinical and clinical studies revealed essential equivalence to CBV from MSCT.
Figure 1: Perfusion maps of a stroke case: (upper row) MDCT before, (middle row) DynaCT before, (lower row) DynaCT after recanalization of the occluded MCA.
Recently the technology and application have been extended to dynamic perfusion using repeated DynaCT scans and refined reconstruction algorithms. Animal studies in comparison to MSCT perfusion have been published; clinical studies are currently ongoing. Figure 1 shows perfusion maps (CBF, CBV, MTT, TTP) of a stroke case—(upper row) MDCT before, (middle row) DynaCT before, and (lower row) DynaCT after recanalization of the occluded MCA. Each enhanced DynaCT scan may be reconstructed separately, resulting in a 4-dimensional DynaCTA, which may, for example, be useful to assess the collateral blood supply. Bringing these elements together is a new comprehensive concept for treatment of ischemic stroke that could save time and patient transports.
An exciting technology to study dynamic blood flow—4D-DSA—has been developed most recently. The underlying theory is to use dynamic information about inflow and outflow from 2D-DSA and backproject that into a static 3D-DSA volume reconstructed during steady state. Extended rotational DynaCT protocols have been developed to acquire all of this information during one scan. In this way a 3D volume is reconstructed for each point in time during inflow and outflow, which allows 3D viewing at any point in time. Early clinical studies indicate very useful applications to obtain detailed information about arterial inflow into AVMs, intranidal aneurysms, venous drainage, and fistulas.
Since the contrast enhancement curve is known at every voxel of the vascular tree, 4D-DSA has the potential to quantify blood flow in a volumetric manner. Quantitative evaluations of blood velocities and flow rates are expected to be subjects of further clinical studies.