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

What We’ll Cover Today• Review some issues from HW 3• Review important points from each modality• What is bright in what modality?• Brain development review

Common issues in HW 3• Make sure to use sufficient significant figures when solving math

problems. Premature rounding can lead to errors in your final answer

• When calculating the intensity of reflected ultrasound, account for forward attenuation, reflection amount, then return attenuation• When working with dB, add the values. When working with ratios, multiply

the values• We loosely refer to dB in class with or without the minus sign, but the energy

is only decreasing after being transmitted from the transducer, so each effect will be –dB (ratio <1)

• Bright areas in fetal brain ultrasound: Bone, Choroid Plexus, Sulci• Dark areas: Ventricles (CSF), Sinuses (blood), areas far from

transducer (attenuation), areas beyond bond (no energy)

MRI – Important topics (page 1)• Proton density, M0, B0, Mz, Mxy

• B1 field (RF pulse): = M𝝉 0 x B1

• Larmor frequency: f0 = gamma_bar * B0 = 42.58 MHz/T*B0

• How signal is measured• Image contrast: T1, T2, T2* (1/T2* = 1/T2 + 1/T2’)• Off-resonance• Pulse sequences: TE, TR, TI, what parameters for given

contrast, spin echoes vs gradient echoes

MRI – Important topics (page 2)• Mxy = M0(1-exp(-TR/T1))exp(-TE/T2)• Gadolinium, and how it affects measurement• Magnetic gradients• Diffusion-Weighted MRI• How we collect multiple lines of k-space, how many slices

can we collect at once? (Tscan = Ny * TR)• Noise dependencies: SNR ~ f(TE,TR,T1,T2,M0)B0(voxel

volume)*sqrt(Tsampling)

• Know that bandwidth and sampling time are inversely related

X-Rays: Radiography• X-ray energy: E(keV) = 1.24/λ(in nm), c = λ*f (f=3x108m/s)• X-ray interactions: Compton Scattering, Photoelectric Effect, (absorption)• Linear Attenuation Coefficient (units are cm-1)

• Mass attenuation coefficient * density = LAC• Proportional to Z, density, energy

• Film convention: Dark = high x-ray exposure• X-ray resolution (5-10 line pairs/mm for non mammography, better for

mammography)• X-ray tubes: kVp, mAs, effect of each on the x-ray spectrum and image

quality• Dangers of x-ray exposure, how much comes from natural vs man-made

sources, beam hardening• Image formation, magnification• Scatter, effect of SPR, ways to improve it• Angiography: Iodine contrast, DSA

X-Rays: CT• 360 degrees of radiographs are taken and backprojected to form

2d image• These are stacked to form a full 3d volume (modern collection actually uses

a helix pattern)• Filtered backprojection corrects for blurriness of simple recon

• Units are normalized to water LAC: HU = (mu – muwater)/muwater * 1000• LACs depend on x-ray energy

• Be familiar with general ranges of HU for various tissues• Windows and levels for CT give range of visible contrast

• Bone window: W = 2500, L = 1000• Soft tissue window: W = 600, L = -100

• Contrast is already normalized: C = A - B• Typical resolution: 1-2 mm x 1-2 mm in plane• X-Ray dose is much higher than for radiograph (why?)

Angiography• Iodine contrast for CT, Gadolinium contrast for MRI• MRI can also use time of flight non-contrast

• Understand presaturation• Understand flow voids

Ultrasound• Transducer transmits oscillating pressure wave (1-10MHz) into body• Duration of sine wave is limited to control axial resolution• Soft tissue speed of sound is 1540 m/s

• Air much lower, bone twice as high• Speed of sound is independent of applied frequency

• To image: send pulse, listen to echos, draw that “line”, move to next line and send new pulse

• At boundaries between different acoustic impedances, energy is reflected: % reflected = (Z2-Z1)2/(Z2+Z1)2

• Soft tissue impedance = 1.62, air = .0004, bone = 7.8 (these numbers should be given to you)• Tissue is made up of many small reflectors, these reflect signal to transducer

• “speckle” results from distribution of micro scatterers• Attenuation occurs in both directions, depends on tissue type and depth of

penetration• Soft tissue is 1 dB/cm/MHz, water is very low, bone is 20 times higher

• We measure relative sound intensity, generally compared to transmitted energy• Relative sound intensity (dB) = 10 * log(I / I0)

• Time gain control amplifies signal to offset expected attenuation• Axial Resolution = Spatial Pulse Length / 2 = (n * lambda) / 2• Lateral resolution = beam width, smaller with increasing frequency for fixed

transducer, but higher frequency has less penetration (higher attenuation)

What’s bright?• Radiographs: Bright means less film exposure

• More attenuation has occurred, either from going through more tissue, or higher Z material (bone, metal, etc)

• CT: Bright means high LAC• Cortical bone will always be bright, Iodine is bright because of high

Z• Ultrasound: Bright means high scattering or reflection

• Sulci, Bone, Choroid plexus all have high scattering• Interface between tissue and air or tissue and bone will have very

high % reflection

What’s dark?• Radiographs: Dark means more film exposure

• Less attenuation has occurred, from going through mostly air• CT: Dark means low LAC

• Low density (low Z) materials will be darker, tissues are mostly similar to water, fat is slightly darker

• Ultrasound: Dark means low scattering or reflection• Fluid (CSF, blood) have fewer scatterers than soft tissue• Anything beyond a highly reflecting interface will look dark, since

little energy reaches it• Anything far from the transducer will appear dark, because most of

the energy has been attenuated

MRI: What’s bright/dark?• MRI: Brightness depends on contrast of scan• More “rigid” materials generally have shorter T1 and T2

• In proton density map, brightness is similar across many tissues (WM is ~20% darker)

• In T1 weighting, anything with short T1 will recover signal more quickly and will be brighter -> more “rigid” things will be brighter -> white matter brighter than gray matter

• In T2 weighting, anything with short T2 will decay more quickly and be darker -> more “fluid” things will be brighter -> CSF is brightest, gray matter is brighter than white matter

• In a FLAIR sequence, the CSF signal is nulled, and appears dark. Gray matter is brighter than white matter between we still use T2 weighting

• Fat hasn’t been discussed much yet in lecture, but it has a short T1 and T2, with a higher PD than other tissues

• Cortical bone has T2 that’s so short it always decays completely before we can make a measurement -> always dark

BRAIN DEVELOPEMENTReview of Brain Lecture 7

Possible test material• Neural plate• Neural tube• Rhombencephalon• Mesencephalon• Diencephalon• Telencephalon• Cavum septum pellucidum• Anterior fontinelle• Posterior fontinelle• Caudothalamic groove• Germinal matrix

hemorrhage

• Hydrocephalus• Anencephaly• Holoprosencephaly• Agenesis of the corpus

callosum• What adult structures derive

from pre-natal structures?• Order of sulcal development• Order of myelination of white

matter?• Where are the fontinelles,

what surrounds them, when do they close?

VISUAL AND AUDITORY SYSTEMSReview of Brain Lecture 8

Visual SystemRetina Optic nerves chiasm optic tracts LGN Visual cortex

Visual System

Visual System

How do we image the auditory system?

• Sensorineural: MRI• “Retrocochlear lesions”• Presence of CN 8

(Auditory nerve)• Masses• Infarcts

• Conductive: CT• Temporal bone• Very high resoluBon,

axial reconstruct into coronal plane

• Visualize• Tympanic membrane• Ossicles• OBc capsule

Possible test material• Retina• Optic nerve• Optic chiasm• Optic tracts• Lateral geniculate nucleus

• Optic radiations

• External auditory canal• Tympanic membrane• Middle ear ossicles• Oval window• Cochlea• Semicircular canals• Internal auditory canal