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《內容》
1. CT 和 MRI 上不同灰階所代表的東西2. 生理性鈣化和 CT density 表3. MRI intensity 表4. MRI 基本原理5. Diffusion-weighted image and ADC
Na), carbon (13 C), potassium (39 K), exogenous noble gases such as helium (3 He) and xenon (129 Xe)等。《Image parameters》● ρ(proton density):
◇ CSF, urine and other fluids>liver>kidney and spleen>grey matter>white matter>articular
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(A)PDI (B)T2WI (C)FLAIR (D)T1WI
cartilage>fibrocartilage>membranes>cortical bone and air.
◇ Only mobile protons which give an MR signal with conventional techniques. When the protons
are in large molecules or immobilized in solids, they give no detectable signal with most MRI
techniques. Thus cortical bone contains protons, but these do not give a detectable signal.
◇ Proton density increased:oedema, infection, inflammation, acute demyelination, acute
haemorrhage, some tumours, cysts and other conditions.
◇ Proton density decreased:scar formation, fibrosis, some tumours, capsule and membrane
formation as well as with calcification.
● T1(longitudinal relaxation time) & T2(transverse relaxation time):◇ The first of these relaxation times, T1, or the longitudinal relaxation time, represents the time
taken by the system of nuclei to return to thermal equilibrium after the RF pulse.
◇ The second or transverse relaxation time, T2, indicates the characteristic decay time of the FID
(free induction decay) and is due to the irreversible dephasing of the initially coherent precession
of nuclei which follows the RF pulse.
◇ A local change in magnetic field homogeneity, e.g. due to local iron or deoxy-hemoglobin
content, causes a reduction in T2 which is called T2*.
◇ Thus liquids have a very long T1 and T2, soft tissues have shorter values of T1 and T2 and solids
have very long T1s and very short T2s. (肝臟含水量不少,但是因為有 organic iron,所以其T1&T2 下降)
◇ In liquids or systems containing mobile protons, T2/T1 is approximately unity, whereas in solids,
T1-weighted spin echo sequence(short TR, short TE)
T2-weighted spin echo sequence(long TR, long TE)
If TI is decreased to 100–150 ms, it is possible to null the signal from fat with the short TI
IR STIR sequence. It is also possible to increase TI in order to null the signal for fluids
(the Fluid Attenuated Inversion Recovery or FLAIR sequence).
● TE(Echo time):Time between center of RF excitation pulse and the center of spin echo formation
(readout period).
● TR(Repetition time):Time between successive excitation of spins. For a gradient echo, it is the
time between successive alpha pulses; for a spin echo it is the time between successive 90° RF pulses.
● Fast pulse sequences:◇ Fast low-angle shot or FLASH sequence:使用 PS,但縮短 TR,而使用小於 90°的 RF 保留
訊號強度◇ Echo-planar imaging (EPI):a train of gradient echoes is obtained after a single 90° RF
excitation,目前最快的成像法,可應用在 fMRI,DWI,PWI,FLAIR 等。(Diffusion-weighted MRI and ADC(apparent diffusion coefficient))
1. 方法:This is achieved by applying a pair of diffusion sensitizing gradients symmetrically around a 180° refocusing RF pulse of a T2-weighted MR sequence.
Mobile molecules acquire phase shifts, which prevent their complete rephasing
and result in signal loss.(因為含 T2 components,所以有些 T2 hyperintense 的lesions 在 DWI 也會 hyperintense,但是 ADC 並不會 hypointense,稱為 T2 shine-
through)
2. Diffusion tension imaging (DTI):new technique(diffusion imaging is done in three(x-y-z)
orthogonal planes and mean diffusion is calculated for each pixel),因為原本是假定 diffusion 在各方向都相同,但是在 in vivo 時卻不是,需要作校正才比較準確。
3. Acute ischemic lesions:cytotoxic oedema , high signal on DWI (‘light bulb sign’) and low signal on ADC
4. Chronic ischemic lesions:low signal on DWI and high signal on ADC
5. Diffusion MRI and ADC 提供大腦組織完整性的資訊,水分子的 diffusion 愈好,ADC 的值愈高。ADC value differences in four conditions(range and mean) x 10-3 mm2/s
Acute infarct(cytotoxic edema) 0.14-0.50(0.32±0.09) low signal
Normal cerebral white matter 0.60-1.05(0.84±0.11) normal signal
Vasogenic edema 1.28-2.20(1.68±0.27) higher signal
就會出現. 在接下來 24 小時,ADC 會進一步下降而 DWI 會繼續增加 intensity. 而在 24~28小時的時候,ADC會下降至最低點,約正常值的 50~60%. This is the same level reached after 20
minutes of cardiac arrest and may be the maximum extent to which reduction of the extracellular
volume can occur (50%). A restricted ADC has always preceded the development of infarction.
8. Time course of the ADC in human stroke. 在人類 ischemic stroke, ADC的下降最早在發生後105 minutes出現.. 在接下來48小時 lesions會變得更hyperintense,而ADC會進一步下降. ADC值的下降總是在 infarction發生前出現. The ADC has been found to have a two-phased time course with an
initial decrease that is followed by a return to normal in the subacute to
chronic phases termed pseudonormalization because the tissue is
infarcted. In the chronic phase the ADC subsequently became high due
to increased water diffusion in the residual stroke cavity. The period
during which the ADC remains restricted has varied between
laboratories. In some the ADC had normalized by 48 hours while in
others the ADC pseudonormalized between 4 and 10 days.
Stage T2WI DWI ADC
Hyperacute(0-6
hours)
Normal Increased Decreased
Acute(6-96 hours) Normal to
increased
Increased Decreased
Subacute(4-
10days)
Increased Normal to
increased
Decreased/normal
Chronic Increased Decreased to
increased
Increased
9. Abscess:high signal on DWI and low signal on ADC
10.Tumors with central necrosis (both primary and metastatic):low signal on DWI and high
signal on ADC(DWI 可用來 DD ring-enhancing lesion)
11.Abscess 的 ADC 值為 0.21 to 0.34 310-3 mm2/s
An infarct 8 to 24 hours old:0.61 ± 0.14 x 10-3mm2/s
An infarct 1 to 8 days old:0.51 ± 0.18 x 10-3mm2/s
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(MR perfusion imaging)1. 方法:During the first pass of an intravenously injected gadolinium-based
contrast agent, the contrast medium causes a transient signal drop on T2*-
weighted (susceptibility-weighted) MRI. MR perfusion imaging is, however, at
present only semiquantitative and cannot provide absolute values[8] . In the
absence of absolute quantification of the CBF, comparison with the contralateral
hemisphere provides the easiest way to analyse MR perfusion images. This
becomes, however, problematic if the perfusion of the contralateral hemisphere
is not normal, as in the presence of bilateral carotid artery disease.2. PWI may show hypoperfusion in a much larger area of tissue than shown by the DWI. This indicates a much
larger area of tissue is at risk for infarction, a "diffusion-perfusion mismatch", indicating a threatened portion
of the brain that is still salvageable
Relative cerebral blood volume (rCBV), mean transit time (rMTT), and relative cerebral blood flow (rCBF)
DW MR Imaging Characteristics of Various Disease Entities
Disease
MR Signal Intensity
ADC CauseDW
Image
ADC
Image
Acute Stroke High Low Restricted Cytotoxic edema
Chronic Stroke Variable High Elevated Gliosis
Hypertensive encephalopathy Variable High Elevated Vasogenic edema
Cyclosporin toxicity Variable High Elevated Vasogenic edema
Table 37C-2. Segments of the internal carotid artery and associated branchesFischer segment
Boundary Branches and vascular territory
C5 "gasserian segment"
Endocranial opening of carotid canal to the beginning of the first (posterior) ICA genu
Meningohypophyseal artery (posterior trunk) (near C4 and C5) Inferior hypophyseal (pituitary gland) Marginal tentorial or Bernasconi and Cassinari (tentorium) Clival dural branch (cavernous sinus, cranial nerves III through VI)
C4 "cavernous segment"
End of ascending portion and beginning of the horizontal segment
Inferolateral trunk-ILT (lateral mainstem artery) supplies cranial nerves III, IV, VI, and gasserian ganglion (cranial nerve V) and cavernous sinus dura, foramen of rotundum
C3 "carotid knee"
Posterior 90-degree bend to anterior 90-degree bend
○ MCA:most of the lateral surface of the hemisphere,insula 和 anterior and lateral aspects of the temporal lobe M1 or sphenoidal segment(origin to limen insulae)
Lateral lenticulostriate arteries:basal ganglia(caudate nucleus) and the anterior limb of the internal capsule
Anterior temporal branches(有時從 proximal M2 分出):temporal tip cortex M2 or insular segment(runs along the insula)
Anterior cortical branches:Lateral orbitofrontal,operculofrontal(ascending frontal or
candelabra branch)和 central sulcus arteries(precentral or prerolandic 和 central or rolandic branches)
Posterior cortical branches:anterior and posterior parietal,angular,和 posterior temporal arteries
M3 or opercular segment(operculum superior to the insula) M4 or terminal segment(convex surfaces)
M4 superior:orbitofrontal,prefrontal,precentral,postcentral,anterior and posterior
parietal,和 angular arteries
M4 inferior:temporal lobe and part of the occipital lobe,包括temporopolar,anterotemporal,middle temporal,posterotemporal 和 temporo-occipital arteries.
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Lateral view
○ ACA:anterior two thirds of the medial portions of the cerebral hemispheres and 約 1cm of the superolateral surface of the brain convexity A1 or horizontal segment(origin to AComA)
Medial lenticulostriate arteries(head of the caudate nucleus and the anterior limb ofthe internal capsule,hypothalamus,optic chiasm 和 infundibulum)
AComA(Anterior communicating artery):lamina terminalis and hypothalamus, anterior commissure, fornix, septum pellucidum, paraolfactory gyrus, the subcallosal region, the anterior part of the cingulated gyrus, the head of the caudate nucleus(basal ganglia)
A2 segment(AComA to its bifurcation into pericallosal and callosomarginal arteries)recurrent artery of Heubner(50%在 A2,44%在 A1,不常在 AComA):caudate
nucleus,the rostral putamen 和 anterior limb of the internal capsuleOrbitofrontal and frontopolar arteries
A3 segment(cortical suppliers)Callosomarginal a.Anterior,middle,posterior(internal) frontal 和 paracentral a.
Pericallosal a.parietal(internal) superior and inferior 和 splenial arteries
anterior spinal cord),posterior spinal artery(rare and may arise from PICA),PICA(posterior
inferior cerebellar artery),which runs around the medulla and over the tonsil and supplies the
inferior vermis,the choroid plexus of the fourth ventricle 和 inferior surface of the cerebellum.
○ BA(Basilar artery): AICA(anterior inferior cerebellar arteries) that course around the pons and toward the
cerebellopontine angle and the internal auditory canal meatus to supply the anterior cerebellar hemispheres,CN VII 和 VIII,和 lateral pontine structures
Labyrinthine artery(15%會從 basilar artery直接分出) Small pontine perforators SCA(superior cerebellar artery) that runs around the brainstem in the pontomesencephalic
groove in the perimesencephalic cistern below the oculomotor and trochlear nerves and above the trigeminal nerve to supply the superolateral surface of the cerebellar hemisphere 和 lateral pontine structures
Anteriorview
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○ PCA(Posterior cerebral artery):basilar artery 在 pontomesencephalic junction, which is superior to
the oculomotor nerve and the tentorium,分出 2條 PCA,供應diencephalon,midbrain,posterior one third of the medial hemisphere surface 和 occipital lobe
P1 or peduncular segment(basilar top to the PComA):Thalamoperforating arteries:diencephalon 和 midbrain
P2 or ambient segment(runs in the ambient cistern from the PComA to the posterior aspect of the midbrain):Posterior thalamoperforating and thalamogeniculate arteries:thalamus,geniculate
body,posterior limb of internal capsule 和 optic tract