Dr. Rebecca Thornhill

The Ottawa Hospital - Diagnostic Imaging - Magnetic Resonance Imaging
501 Smyth Road - Module N, lab N6
Ottawa, Ontario K1H 8L6
613-737-8899, ext 78171
rthornhillattoh [dot] ca

Research Summary

 MRI is a highly versatile imaging modality, capable of producing images with strong soft-tissue contrast. MRI enables us to map the abundance and behaviour of hydrogen nuclei. This is fortuitous, because living tissues consist mostly of water and fat, which are quite rich in hydrogen. As it turns out, the hydrogen nucleus is comprised of a single unpaired proton, which can be viewed as a small magnetic field. In MR, we can observe a signal by perturbing the protons from their alignment with the scanner’s main magnetic field. Indeed, we can exploit prior knowledge of tissue composition, eg, how ‘watery’ or ‘fatty’ or otherwise proton-dense we expect a given lesion or normal tissue to be in order to make certain structures more conspicuous than its neighbours. However, sometimes the discrepancy between the relaxation times of the pathology and those of normal tissue is too weak to enable a confident distinction. Fortunately, there are paramagnetic contrast agents that can be injected to exaggerate the inherent differences between lesion and healthy tissue. Furthermore, it turns out that we can track the contrast-related changes in MRI signal by acquiring dynamic images (ie, in a time series) and then use this information to estimate the concentration of contrast-agent. This is potentially useful, since we can apply certain mathematical models to the concentration versus time curves to estimate physiologically meaningful parameters like blood flow or vascular ‘leakiness.’

 In fact, contrast-enhanced MRI offers a potential 'one-stop-shop' for the evaluation of cardiovascular disease. In addition to blood flow and related information, my research interests are largely concerned with how MRI can be used to assess cell membrane integrity and cardiac muscle function following a heart attack. More recently, I have become interested in how MRI can be adapted to measure subtle changes in heart function in diabetics. Heart failure is the most common cause of death among diabetics who have suffered a heart attack. In addition to providing clinicians with a sensitive marker of incipient heart failure, MRI strain analysis may offer a non-invasive strategy for delineating incremental benefits from emerging therapies. Another potential surrogate marker for diabetic heart failure could be image texture. I have previously used texture analysis to identify ‘signature’ grey-level patterns within pathologies (eg, stroke lesions, soft-tissue tumours) and I have found it can provide us with an objective yet simple way of analyzing our image data.

Selected Publications

  1. Kassner A, Thornhill RE. Texture Analysis: a review of neurologic MR imaging applications. AJNR Am J Neuroradiol 2010;31:809-816.
  2. Thornhill RE, Chen S, Rammo W, Mikulis DJ, Kassner A. Contrast-enhanced MR imaging in acute ischemic stroke: T2* measures of blood-brain barrier permeability and their relationship to T1 estimates and hemorrhagic transformation. AJNR Am J Neuroradiol 2010;31:1015-1022.
  3. Thornhill RE, Prato FS, Wisenberg G, White JA, Nowell J, Sauer A. Feasibility of the single bolus strategy for measuring the partition-coefficient of Gd-DTPA in patients with myocardial infarction: independence of image delay time and maturity of scar. Magn Reson Med 2006;55:780-789.