BIOE 220/RAD 220 REVIEW SESSION 3 February 6, 2011

What We’ll Cover Today Review some issues from HW 2 Review ultrasound Anatomy Review advanced MRI material (depending on time)

Common issues in HW 2 Pay attention to units when computing linear attenuation coefficient If you’re given a value (from the table) in cm^2/g and a density in g/cm^3, multiply the two together to get a cm^-1 LAC LAC depends on energy of x-rays Calculating Hounsfield Units requires looking up the LAC for water at that energy level Lower energy x-rays get absorbed more -> give higher dose CT contrast is C = A – B… values in HU are already normalized In radiographs, objects closer to the source appear larger

Ultrasound Basics In ultrasound, pressure (sound) waves, generally 1- 10MHz, are transmitted into the body using a transducer By measuring the received signal, we can map reflections to depths within the body, based on time of round trip flight Brightness of objects is determined by the amount they reflect, and by how much sound energy is reaching them Sulci (fibrous tissue) and choroid plexus are bright, fluid is dark Axial resolution is determined by length of sound pulse ½ factor introduced because of round trip measurement Lateral resolution is determined by beam width Depends on frequency, geometry, depth

Ultrasound Basics Speed of sound is dependent on the medium through which it is traveling For soft tissue, it is 1540 m/s Much faster for bone, much slower in air Have to wait for all of the transmitted energy to return before we can transmit a new line (data collection rate is limited by depth and speed of sound) Boundaries between different materials cause much larger reflections Reflection is larger when acoustic impedances are less similar % reflected = 100 * (Z 2 – Z 1 ) 2 / (Z 2 + Z 1 ) 2 % transmitted = %reflected Air has very small Z, soft tissue is 1.62, bone is much larger

Ultrasound Reflectors Tissue itself can be thought of as containing many tiny reflectors

Tissues attenuate signal As energy is reflected (which makes it visible in recording) or absorbed, less is available to penetrate deeper Time Gain Control (TGC) - Inverse scaling is used to correct for attenuation by depth, assuming a consistent tissue type If more/less energy has been reflected by a certain depth than expected, that depth will appear darker/brighter than if everything were homogenous

Surface Anatomy

Deep Gray Structures

Ventricular System

Surface Anatomy

Mid-Sagittal Anatomy

Deep Structures

Level of Anterior Limb of the Internal Capsule

Surface Anatomy

Cerebral Vasculature UHM…WOAH

Circle of Willis

Lateral Cerebrum

Mid Sagittal Cerebrum

Inferior Cerebrum

Deep Structures

Right Carotid Angiogram

Vertebrobasilar Angiogram

Internal Carotid Angiogram (Venous Phase)

Advanced MRI Topics Though they aren’t on this week’s HW, we’ve seen more MRI concepts in class in the past couple weeks I’ll review some of them now, with additional focus when there’s relevant homework

Gadolinium Contrast - MRI Adding Gadolinium to the blood drastically reduces T1 (see slides 5-7 of lecture 4 for why)

In Healthy Brain, Blood Brain Barrier Prevents Large Molecules from Entering Brain

Diffusion Weight MRI Gradients

Diffusion Weighting Can Help Visualize Stroke

1D Spaital Localiation in MRI Gradients encode position as frequency, by altering the Larmor frequency Using the Fourier Transform allows us to recover the frequencies in the recording, which in turn tell us the amount of signal at various positions along the gradient direction

CNR is the Important Metric in MRI In the first homework, we ignored noise and optimized pulse sequences based on contrast In reality, noise interferes with our ability to distinguish different tissues, so contrast to noise is much more important Noise in MRI is generally additive, so SNR and CNR will vary together MR noise is zero mean, so recording for longer improves our SNR (by sqrt(sampling time)) Adjusting scan timing to have more magnetization in the transverse plane also improves SNR Measuring larger voxels means that we’re recording more total signal