Parametric Maps derived from CTP

Parametric maps derived from CTP series allow doctors to discover rapidly if a patient has an ischemic problem. The parametric maps are formed by evaluating different sections of the brain during the injection of the contrast agent to highlight if there are penumbra areas and core infarction areas in the analyzed brain. The passage of the contrast agent is recorded over time, through time versus contrast concentration curve [30]; this curve is referred to as a time density curve (TDC). The curve represents the intensity of each pixel of the image during the passage of the contrast agent over a period of time [14].

Fig. 2.3 displays an example of a set of images, after performing pre-processing steps, of the same brain section during the injection of the contrast agent, over time. Several TDC equal to the number of pixels in the first image are created.

An example of TDC is presented in Fig. 2.4; thex-axis displays the time elapsed after the start of the scanning (in seconds), they-axis denotes the relative enhancement level

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Figure 2.4: The CTP Time Density Curve; it shows the different measurements for the creation of the parametric maps. The figure is reprinted in unaltered form from:

“Radiological imaging in acute ischaemic stroke” [14] under the consensus of the author.

measured in Hounsfield Units (HU) [31]. Generally, TDC is generated from images acquired in quick succession. Analyzing the TDC in different ways, various measures of perfusion can be calculated for each image pixel. The ratios derived include a variety of color-coded parametric maps; these maps are meant to help visualize an acute stroke [32];

the comparison of these maps helps understand the area affected by a stroke if present.

Time-To-Peak

(a) TTP (b) TTP on the TDC

Figure 2.5: Different visualizations of TTP.The figures are reprinted from: “Radiological imaging in acute ischaemic stroke” [14] under the consensus of the author.

Time-To-Peak (TTP) represents the time from the start of the contrast injection to the peak of enhancement in the tissue. It shows immediately if the patient has an ischemic problem. An example of this representation is given in Fig. 2.5. Fig. 2.5(a) shows the TTP of all pixels from a section of the brain during the injection of a contrast agent, while Fig. 2.5(b) is the representation of one pixel over the TDC. The black

Tomasetti Luca 11 area inside the brain (Fig. 2.5(a)) symbolizes an ischemic area, that may contain both penumbra and infarct core: the contrast agent reaches the affected area later compared to a non-damaged part of the brain; hence, it is not adequately shown in the image.

Cerebral Blood Volume

(a) CBV (b) CBV on the TDC

Figure 2.6: Different visualizations of CBV.The figures are reprinted from: “Radiological imaging in acute ischaemic stroke” [14] under the consensus of the author.

Cerebral Blood Volume (CBV) is defined as the volume of the blood per unit of brain tissue. It is measured as milliliters of blood per 100g of brain tissue (ml/100g). In non-damaged brain tissue, the CBV should be approximately around 4-5ml/100g; however, if the blood volume results below 2.5 ml/100g, it indicates infarcted tissue [33]. While it is shallow in the core area, there is a compensatory increase in the penumbra zone. The CBV is represented by the integral of the TDC, which produces the area below the TDC.

Fig. 2.6 shows both its representation: all its pixels of the brain section (2.6(a)) and its visualization over the TDC (2.6(b)). The violet area inside the brain in Fig. 2.6(a) defines a possible section for an infarcted area.

Cerebral Blood Flow

Cerebral Blood Flow (CBF) represents the volume of blood flow per unit of brain tissue per minute. It is commonly measured in milliliters of blood per minute per 100g of brain tissue (ml/100g/min). Normally, in a patient without any ischemic problem, the value of CBF is around 50-60ml/100g/min [34]. However, if the brain has an ischemic problem, the flow is reduced in both penumbra and core areas, as it is possible to observe from Fig.

2.7. If CBF is reduced and CBV is normal or slightly reduced, the tissue ischemia is likely to be reversible; if CBF and CBV are markedly reduced or if TTP is not measurable, the tissue may be infarcted [35].

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(a) CBF (b) CBF on the TDC

Figure 2.7: Different visualizations of CBF.The figures are reprinted from: “Radiological imaging in acute ischaemic stroke” [14] under the consensous of the author.

Mean Transit Time

Mean Transit Time (MTT) is the time taken by the contrast agent to pass through the brain tissue. It is described as the average transit time of blood through a brain region, estimated in seconds. Fig 2.8(a) shows the representation of MTT over the TDC. If the MTT is raised as compared to the healthy side, ischemia/infarction is present [35].

(a) MTT on the TDC (b) Parametric map of

TMax.

Figure 2.8: MTT on the TDC and a parametric map of TMax. The figures are reprinted from: “Radiological imaging in acute ischaemic stroke” [14] under the consensous of the

author.

TMax

Time-to-maximum (TMax) displays the time taken by a contrast agent to reach and traverse areas of the brain. Together with the TTP, it is a good measure of contrast arrival time to the tissue. “The tissue time-enhancement curves are deconvolved with the arterial input function (AIF) by using the single value decomposition (SVD) method to produce an impulse residue function (IRF)” as stated by [36]. TMax is calculated from

Tomasetti Luca 13 the TTP of the IRF curve, where TMax = 0 reflects normal blood supply in normal tissue without delay. Fig2.8(b)shows an example of a TMax map.