Data Acquisition

Perfusion-weighted images can be obtained with "bolus-tracking techniques" that are sensitive to the passage of contrast material through a capillary bed (13,14).

Pre contrast Nadir 100 seconds

Figure 2 Typical T2*-weighted DCE-MRI study of a patient with a malignant astrocytoma. 30 mL of IV contrast Gd-DTPA was given after the 10th data point. First pass T2* susceptibility effects cause marked darkening of the tumor periphery. Darkening of the grey matter of the brain is greater than the less vascular white matter. The first pass and recirculation phases are indicated. Signal intensity changes for four regions of interest are shown in the insert [subtraction T2* image of the nadir point for the tumor regions of interest (ROI)]. An anatomic T2-weighted image at the same slice position is also shown for reference.

Pre contrast Nadir 100 seconds

Figure 2 Typical T2*-weighted DCE-MRI study of a patient with a malignant astrocytoma. 30 mL of IV contrast Gd-DTPA was given after the 10th data point. First pass T2* susceptibility effects cause marked darkening of the tumor periphery. Darkening of the grey matter of the brain is greater than the less vascular white matter. The first pass and recirculation phases are indicated. Signal intensity changes for four regions of interest are shown in the insert [subtraction T2* image of the nadir point for the tumor regions of interest (ROI)]. An anatomic T2-weighted image at the same slice position is also shown for reference.

A decrease in signal intensity of tissues caused by susceptibility occurs due to the presence of concentrated contrast media within vessels (Figs. 2 and 3). The degree of observed signal intensity loss is dependent on the type of sequence used, on vascular concentration of the contrast agent, and microvessel size and density (15). The signal to noise ratio (SNR) of dynamic susceptibility contrast-enhanced (DSC)-MR images can be enhanced by using higher doses of contrast medium (i.e., >0.2mmol/kg body weight) (16).

The typical imaging strategy is to collect data using a fast imaging technique to produce a temporal resolution of approximately two seconds. During this short acquisition window it is usually possible to acquire multislice data at a matrix resolution of 128 x 128 or greater, depending on scanner specifications. High specification, echo-planar-capable MRI systems allow 5 to 15 slices to be acquired. However, echo-planar sequences have limited applications in extracranial tissues because of greater intrinsic sensitivity to susceptibility-inducing environments (e.g., highly concentrated contrast media and bowel gas/tissue boundaries), which can result in spatial misregistration of major vessels during the first passage of the contrast agent

Figure 3 Typical T2*-weighted DCE-MRI study of a patient with an invasive ductal cancer of the breast. A patient with breast cancer (same patient depicted in Figures 5, 6, and 8) was given 22 ml of IV contrast Gd-DTPA after the 10th data point. First pass T2* susceptibility effects cause marked darkening of the tumor with no alteration in signal intensity of fibro-glandular breast parenchyma (normal tissue) or fat. The first pass and recirculation phases can clearly be seen. Insert shows a subtraction T2* image of the nadir point for the tumor regions of interest.

through the vessels (17). Standard spoiled gradient-echo sequences on conventional MRI systems can also characterize these effects but are usually limited to a single slice (Figs. 2 and 3). It has been noted that susceptibility-weighted, spin-echo sequences are more sensitive to capillary blood flow but the signals obtained are of lower magnitude compared with gradient-echo sequences, which incorporate signals from larger vessels (18). It is unclear whether there are significant advantages of using spin-echo sequences, but there are certainly significant costs in terms of SNR.

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