Cardiovascular diseases are the leading cause of death in the industrialized world and contribute to roughly a third of global deaths.  The predominant form of acquired cardiovascular disease, atherosclerosis, results from the chronic buildup of fatty material in the inner layer of the arteries supplying the heart, brain, kidneys, digestive system, and lower extremities.  Progressive coronary artery disease restricts blood flow to the heart, presenting as chest pain during physical exertion, referred to as chronic stable angina, or when the patient is at rest, known as unstable angina.  More severe manifestation of disease may lead to myocardial infarction, or heart attack.  Patients presenting with chest pain are usually subject to a range of currently available noninvasive tests, including ECG, treadmill tests, SPECT, PET, and CT—none of which measure blood flow and provide only anatomic information or indirect indications of disease.  Due to the lack of accurate functional information provided by current noninvasive tests, many patients require invasive catheter procedures to assess coronary blood flow.  There is a pressing need for a noninvasive means to quantify blood flow in the human coronary arteries to assess the functional significance of diffuse and focal coronary artery disease.  Additionally, there is a need to achieve rapid assessment of blood flow to enable use in emergency rooms, in-patient treatment, and onsite hospital use.  In addition to non-invasive use, there is a need within invasive imaging, such as coronary angiography, to quickly estimate functional metrics without the need for pressure or flow wires or special medication.  Such a technology is also applicable to preventing, diagnosing, managing and treating disease in other portions of the cardiovascular system including the arteries of the neck, e.g. the carotid arteries, the arteries in the head, e.g. the cerebral arteries, the arteries in the abdomen, e.g. the abdominal aorta and its branches, the arteries in legs, e.g. the femoral and popliteal arteries. 
A functional assessment of arterial capacity is important for treatment planning to address patient needs.  Recent studies have demonstrated that hemodynamic characteristics, such as Fractional Flow Reserve (FFR), are important indicators to determine the optimal treatment for a patient with arterial disease.  Conventional assessments of these hemodynamic characteristics use invasive catheterizations to directly measure blood flow characteristics, such as pressure and flow velocity.  However, despite the important clinical information that is gathered, these invasive measurement techniques present severe risks to the patient and significant costs to the healthcare system. 
To address the risks and costs associated with invasive measurement, a new generation of noninvasive tests have been developed to assess blood flow characteristics.  These noninvasive tests use patient imaging (such as computed tomography (CT)) to determine a patient-specific geometric model of the blood vessels and this model is used computationally to simulate the blood flow using computational fluid dynamics (CFD) with appropriate physiological boundary conditions and parameters.  Examples of inputs to these patient-specific boundary conditions include the patient's blood pressure, blood viscosity and the expected demand of blood from the supplied tissue (derived from scaling laws and a mass estimation of the supplied tissue from the patient imaging).  Although these simulation-based estimations of blood flow characteristics have demonstrated a level of fidelity comparable to direct (invasive) measurements of the same quantity of interest, physical simulations demand a substantial computational burden that can make these virtual, noninvasive tests difficult to execute in a real-time clinical environment.  Consequently, the present disclosure describes new approaches for performing rapid, noninvasive estimations of blood flow characteristics that are computationally inexpensive.