mix questions Flashcards
what standard controls patient demography and billing info
Health Level seven (HL7)
Privacy patient controll and anonymization
HIPAA
what is the lower limit for fringe field for MRI
0.5mT
The peripheral magnetic field around a main MR scanner is called fringe field, and it can affect magnetically sensitive devices. (5G or 0.5 mT) & below is considered as safe level exposure to general public. [Huda, p198-199] (Ken) 1T = 104 G
2T field, 10 ppm what’s noise in Hz
42.58 MHz/T
(2T) => 85.16 MHz (10 p/M) =>851.6 Hz
(Ken)Additional info from Bushberg p.453 - 454, chemical shift refers to the slightly different precessional freq. of protons in different materials or tissues. The shifts (in parts per million, ppm) are referenced to water.
1.5 T field, 3 ppm chemical shift between fat and tissue, what’s noise in Hz;
1.5 * 42.58MHz * 3 ppm = 191.61 Hz
More info taken from the web: Although the majority of the MR signal comes from the hydrogen nuclei of water molecules, lipid protons in fatty tissues can also contribute. Fat and water protons do not resonate at exactly the same frequency. Fat protons resonate at a slightly higher frequency, the difference of which is usually described as a fraction of the resonant frequency. Since the difference in frequencies (or the “chemical shift”) is very small, it is expressed as “parts per million” or “ppm”. Fat protons resonate at 3.5 ppm, a higher frequency than water protons (figure 8). The absolute frequency difference depends on the strength of the main magnetic field (which determines the Larmor frequency). Thus at 0.35 Tesla (15 MHz resonant frequency), a 3.5 ppm chemical shift will result in fat resonating at a frequency of 52.5 Hz (15 MHz x 3.5 ppm) higher than water. At 1.5 Tesla (64 MHz) fat resonates at a frequency of 224 Hz higher than water.
given E(keV)=1.24/lamda(nm), given gyromagnetic ratio and B in Tesla, given Lamor equation, calculate E for proton state inversion
Bushberg, p 382
Assume gyromagnetic ratio (r) in units of MHz/T, ex: Proton r = 42.58 MHz/T
Lamor fequency w0(MHz) = rB, and c = lambda*w0
So, E(keV) = 1.24/(c/rB in nm)
What’s the Larmor frequency under 1T.
For proton w0 = 42.58 MHz/T x 1T = 42.58 MHz
Bright on T1
fat subacute hemorrhage melanin protein-rich fluid slowly flowing blood paramagnetic substances: gadolinium, manganese, copper calcification (rarely) laminar necrosis of cerebral infarction
Dark on T1
increased water, as in edema, tumor, infarction, inflammation, infection, hemorrhage (hyperacute or chronic)
low proton density, calcification
flow void
Bright on T2
increased water, as in edema, tumor, infarction, inflammation, infection, subdural collection
methemoglobin (extracellular) in subacute hemorrhage
Dark on T2
low proton density, calcification, fibrous tissue
paramagnetic substances: deoxyhemoglobin, methemoglobin (intracellular), iron, ferritin, hemosiderin, melanin
protein-rich fluid
flow void
T1 represents
longitudinal relaxation time.
The time constant which determines the rate at which excited protons return to equilibrium. It is a measure of the time taken for spinning protons to realign with the external magnetic field
T2 represents
transverse relaxation time.
The time constant which determines the rate at which excited protons reach equilibrium or go out of phase with each other. It is a measure of the time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field.
Why is copper added to beam?
For electron beam, the high z material (copper or lead) is used as a scattering foil to induce scattered electron to achieve large treatment field size, while the pencil beam electron hits on the scattering foil. (Podgorsak p124)
what’s specificity given TN, TP, FP, FN;
TN/(all negative)=TN/(TN+FP)
what’s SENSITIVITY given TN, TP, FP, FN;
TP/(all positive)=TP/(TP+FN)
50 cm SID, 20 cm thick patient, entrance exposure is 1000 mR, if SID changes to 100 cm and keep the radiography exit exposure the same, what’s the entrance exposure?
If patient is close to imager, we can write down
1000 *(30/50)^2 = y *(80/100)^2–> Entrance exposure (y) = 562.5 mR
focal spot is 2.4 mm, 100 cm source to film distance and 20 cm source to object distance, what’s the image unsharpness
image unsharpness: f.spot(M-1)= 2.4mm((100/20)-1)=9.6 (Ken) additional info: Bushberg p 147-148
Increasing window width would
decrease contrast
Calculate the magnification given source to image distance (SID) and object to image distance (OID).
M = SID/(SID – OID)
SID-OID =SOD (Source object distance)
Bushberg 147
