Physics of MRI in cancer (contrast enhancement, DWI) Flashcards
Complete the sentences.
Diffusion is the random motion of molecules.
- In water there is high/low diffusion, in solid tissue there is high/low diffusion.
- The MRI signal decreases/increases by diffusion.
- Water has high/low diffusion and is therefore bright/dark on diffusion-weighted MRI.
- Cell swelling or a tumour has high/low diffusion and is therefore bright/dark on diffusion-weighted MRI.
Diffusion is the random motion of molecules.
- In water there is high diffusion, in solid tissue there is low diffusion.
- The MRI signal decreases by diffusion.
- Water has high diffusion and is therefore dark on diffusion-weighted MRI.
- Cell swelling or a tumour has low diffusion and is therefore bright on diffusion-weighted MRI.
How would you identify the earliest signs of an acute stroke?
A stroke can be caused by hypoxia, which causes cell swelling and consequently restricted diffusion.
On DWI, restricted diffusion can be identified by an increased signal, since diffusion normally blocks the DWI signal.
What determines the sensitivity for diffusion and how is the senstivity expressed?
- The sensitivity of diffusion is determined by the settings of the MRI machine.
- The sensitivity is expressed in the b-value, the larger the b, the larger the senstivity for diffusion.
While the b-value is used to express the sensitivity of the MRI for diffusion, the b-value is dependent on what kind of tissue is analysed (e.g. T2 shine through). What is a solution to this problem?
Finding a tissue parameter that is not dependent on the b-value. For this the apparent diffusion coefficient (ADC) is used.
How is the apparant diffusion coefficient (ADC) calculated?
- The MRI signal at different b-values is measured.
- The plot that is calculated from this is used to calculate ADC.
- A larger b gives more signal loss due to diffusion.
What hint does an diffusion-weighted image in acute stroke give us?
A hint about the earliest signs of ischaemia.
How would this picture translate itself to an ADC image?
The area that is bright on the diffusion-weighted image denotes the presence of an ischaemic area that is bright because there is less signal loss due to decreased diffusivity. On the ADC image, this bright area would display itself as dark, since the ADC is decreased.
What is the relationship between cancer and diffusion?
A key property of cancer is that cells are more compact together (i.e. increased cellularity), which causes the displacement distribution of water molecules and subsequently leads to restricted diffusion.
How will tumors show up in:
- diffusion weighted image
- ADC map
- diffusion weighted image → bright color, high signal (since less diffusivity and thus less signal loss)
- ADC map → dark color, low value (since lowered ADC value)
Diffusion-weighted (DW) MRI is a tool to diagnose cancer.
Is the DW-MRI signal intensity in tumors increased or decreased, as compared with healthy tissues.
Explain your answer.
In tumors, cell density is increased and diffusion is restricted. Thereby in tumors, the diffusion-weighting imaging has less effect of lowering the MRI signal intensity (compared with healthy tissue).
“Diffusion kills the MRI signal”
What indicates an increasing ADC value during treatment?
It indicates an early treatment response (even while tumor size could be the same).
What is angiogenesis? And how does it relate to cancer?
- It is the physiological process through which new blood vessels form from pre-existing vessels, formed in the earlier stage of vasculogenesis. Angiogenesis continues the growth of the vasculature by processes of sprouting and splitting.
- Like healthy tissue, tumours also need to provided with oxygen. They do this by promoting angiogenesis.
Angiogenesis is related to perfusion. Even though healthy tissues as tumours both use angiogenesis to form new blood vessels, the perfusion of healthy blood vessels and tumour blood vessels is different. Explain this.
- Normal blood vessels have controlled angiogenesis, which cause healthy tight blood vessels to form with supporting cells and maturation factors surrounding the vessels.
- Tumor blood vessels on the other hand have a lot of growth factors to enhance even more aniogenesis, have fewer supporting cells and are leaky. Tumours thus have increased perfusion of their blood vessels compared to normal blood vessels.
What special tool in MRI can be used to measure the perfusion of tissues?
Dynamic Contrast-Enhanced Magnetic Resonance Imaging (DCE-MRI)
How is DCE-MRI able to measure perfusion in tissue?
So with the use of DCE-MRI, a gadolinium contrast agent is administered intravenously. Gadolinium follows the path of the blood vessels. Where there is healthy tissue, gadolinium passes the blood vessels with no problem. However, as has been discussed in the previous question, tumors have a more rapid blood supply, more leakage and thus more perfusion. As the contrast agent passes through a tumour, the contrast agent will leak through the tumour vessel walls into th extravascular space due to the increased permeability. This can be visualized with DCE-MRI.
Is the tumour visible on the T2-weighted image?
No, as also seen in the description of this figure: the lesion is isointense on the T2-weighted imaging.
