MRI Physics | Magnetic Resonance and Spin Echo Sequences - Johns Hopkins Radiology
Summary
TLDRهذا الملخص يوفر نظرة عامة على التصوير المغناطيسي المغناطيسي (MRI) وسلسلة الإشارة الدوارة الأساسية. يصف النص كيفية توجيه البروتونات داخل الجسد بواسطة المجال المغناطيسي، وتأثير الإشارة الراديوفرقية (RF) على البروتونات لتحويل المغناطيسية الطواعية إلى العموديةية، مما ينتج إشارة يمكن قراءتها بواسطة الماسح الMRI. يناقش النص أيضًا تأثيرات تأثير T2*، وكيفية استخدام الإشارة الدوارة المزدوجة لتحسين الوضوح التصويري.
Takeaways
- 🧲 البروتونات في الجسم يمكنها التصرف مثل ال永久مغناطيس بفضل القوة المغناطيسية الخارجية.
- 🌀 يمكن أن تغير البروتونات في الMRI من التوجيه الطواعدي إلى العكس بسبب التأثيرات المغناطيسية.
- 🌌 الMRI تستخدم المجال المغناطيسي الكبير لوضع البروتونات في الوضع التناوب.
- 🔄 البروتونات تدور حول محورها وتسمى هذه الحركة بالتلاشي النووي.
- 🌀 التلاشي النووي يعتمد على قوة المجال المغناطيسي التطبيق ونسبة الجيروماغنيتية.
- 📡 يمكن تحفيز البروتونات بواسطة الترددات الراديو المطلوبة (RF) للتحول إلى مستوى آخر.
- 🔄 التحول إلى الوضع التناوب يسبب التلاشي الزايدئ للبروتونات، مما ينتج عنه الإشارات الكهربية.
- 📉 تتدهور الإشارات بسرعة بسبب تأثيرات T2*، التي تحدث بسبب عدم ال均质في المجال المغناطيسي.
- 🛡 يمكن مكافحة تأثيرات T2* بتطبيق إشعاعات RF معكوسة بدرجة 180 للبروتونات.
- 🔄 يمكن التقاط عدة عكاسات في الMRI بواسطة تطبيق العديد من الإشارات RF معكوسة.
- ⏱️ يتم استدعاء الزمن الذي يمر بين تكرار الإجراءات بـ TR، وهو الوقت اللازم لإعادة الضبط.
Q & A
ما هي الصورة الMRI وكيف تعمل؟
-الصورة الMRI (Magnetic Resonance Imaging) هي نوع من التقنيات الطبية التي تستخدم المجال المغناطيسي للتصوير الداخلي للجسم. تعمل الMRI بواسطة مصباح ضخم يولد مجال مغناطيسي يمكن للبروتونات الموجودة في الجسم التناوب معه، مما يسمح بإنشاء صورًا高清ة للجسم.
ما هي البروتونات وما دورها في الMRI؟
-البروتونات هي جزيئات من الهيدروجين الموجودة في الدهون والعضلات والسكرات في الجسم، وتعتبر جزءًا كبيرًا من الجسد. يمكن لهذه البروتونات التصرف مثل الماجنتات الباردة عندما توضع في مجال مغناطيسي خارجي.
كيف تغير البروتونات في الوجه الطبيعي؟
-بشكل عادي، تكون توجيه البروتونات عشوائية، ولكن يمكنها التناوب مع الشكل الطبيعي عندما تضع في مجال مغناطيسي خارجي.
ما هو المجال المغناطيسي B0 الذي يشار إليه في النص؟
-المجال المغناطيسي B0 هو المجال المغناطيسي الأساسية الذي يولده مصباح الMRI، ويساعد على توجيه البروتونات في الجسم.
ماذا يسمى الحركة الدوارة للبروتونات؟
-الحركة الدوارة للبروتونات، التي تدور حول محورها مثل الكرات، تسمى ال precession أو nuclear spin.
ما هي معادلة لارمور ودورها في الMRI؟
-معادلة لارمور تحدد سرعة precession (الدورة الدوارة) للبروتونات بناءً على قوتها من المجال المغناطيسي التطبيق. وهي تقول أن سرعة precession مساوية لقوة المجال المغناطيسي المطلوبة والمعامل الجيرومغناطيسي، الذي هو ثابت خاص بكل نواة أو عنصر.
كيف تؤثر البراكينج الراديو الترددي (RF) على البروتونات؟
-البراكينج الراديو الترددي يمكن أن يغير توجيه البروتونات من المجال المغناطيسي الرئيسي، مما يسبب change في longitudinal magnetization وyields transverse magnetization.
ما هو الفرق بين纵向 magnetization و transverse magnetization؟
-纵向 magnetization هي توجيه البروتونات مع المجال المغناطيسي الرئيسي، بينما transverse magnetization هي توجيه البروتونات في مسار متناغم مع المجال المغناطيسي الرئيسي.
ما هي free induction decay وما هو دورها في الMRI؟
-Free induction decay هو عملية تراجع البروتونات إلى حالتهم الطبيعية بعد التحريك بbraeking RF pulse، مما ينتج إشارة كهربائية يمكن استكشافها بواسطة الMRI.
ما هي الفرق بين T1 time و T2 time في الMRI؟
-T1 time هو الوقت الذي يستغرقه البروتونات لاستعادة 63% من纵向 magnetization، بينما T2 time هو الوقت الذي ينقضي فيه البروتونات في خسارة 63% من transverse magnetization.
ما هي التأثيرات T2 star وما هي الطريقة لمكافحةها؟
-T2 star effects هي تأثيرات تسببت بسبب عدم ال均质في المجال المغناطيسي، مما يؤدي إلى dephasing وانخفاض الإشارات. يمكن مكافئتها بإضافة braeking RF pulse لإعادة ترتيب precession للبروتونات.
ما هو الفرق بين التصوير بالspin echo و fast spin echo imaging؟
-التصوير بالspin echo يستخدم braeking RF pulse واحدة فقط، بينما fast spin echo imaging يستخدم العديد من الbraeking RF pulses لزيادة سرعة التصوير وتقليل تأثيرات T2 star.
Outlines
🧲 MRI基础与质子共振
Dr. Erin Gomez介绍了磁共振成像(MRI)的基本原理,包括质子在身体中的存在,以及它们如何像小磁铁一样响应外部磁场。她解释了MRI扫描仪如何利用质子的这种特性来生成图像。质子在外部磁场中会排列成与磁场平行或反平行的方向,形成净磁化向量。此外,她还讨论了质子的进动(precession)现象,即质子围绕自己的轴旋转,其速度取决于磁场的强度。通过使用射频(RF)脉冲,可以改变质子的进动方向,从而产生可以被MRI扫描仪检测到的信号。
🌀 自旋回波序列与T1和T2时间
Dr. Gomez继续解释了自旋回波序列,这是一种MRI成像技术,它使用90度的RF脉冲将质子的磁化向量翻转到垂直位置,然后通过180度的RF脉冲来重新同步质子的进动,从而产生回波信号。她讨论了T1和T2时间的概念,这些时间反映了质子恢复其原始磁化状态的速度。不同的组织类型具有不同的T1和T2时间,这可以用来区分它们。此外,她还提到了自由感应衰减(FID)的概念,这是一种由于质子的失相位而导致信号快速衰减的现象,以及T2*效应,这是由于磁场不均匀性导致的信号丢失。
🔄 重复序列与信号衰减
最后,Dr. Gomez讨论了在MRI扫描中如何通过重复自旋回波序列来捕获多个回波,以及这些回波如何随着时间的推移而逐渐减弱,直到信号完全消失,序列需要重新开始。她还提到了TR(重复时间)的概念,即序列重复之间的时间间隔。这段视频脚本的总结强调了MRI技术的基础原理,以及如何通过调整扫描参数来优化图像质量。
Mindmap
Keywords
💡بروتون
💡مجال مغناطيسي
💡ال precession النووي
💡ال頻率 الprecessional
💡RF (радиو التردد)
💡ال净磁化
💡الانحراف 90 درجة
💡التدوير الحراري
💡الزمن T1 والزمن T2
💡تأثير T2 النجمي
💡الـ echo ال行星ي
Highlights
Dr. Erin Gomez provides an overview of magnetic resonance and the basic MRI spin echo sequence.
Proton properties and their role in MRI are discussed, including their presence in fat, muscle, and water.
Explanation of how protons act like bar magnets and align with an external magnetic field.
Introduction to the MRI scanner as a giant magnet generating its own magnetic field, B0.
Description of net magnetization vector and its alignment with the MRI scanner's magnetic field.
Precession or nuclear spin of protons and its dependency on the magnetic field's strength.
The Larmor equation's role in expressing the relationship between precession frequency and magnetic field strength.
Use of radio frequency (RF) pulses to influence protons and alter their longitudinal magnetization.
The concept of transverse magnetization and its creation through RF pulses.
Explanation of how protons recover to their original state after being influenced by RF pulses.
Introduction to the spin echo sequence, starting with a 90-degree RF pulse.
Discussion of free induction decay and its role in generating electrical signals from proton alignment changes.
Definition of T1 and T2 times in relation to proton recovery and their significance in MRI imaging.
The impact of magnetic field inhomogeneity on proton dephasing and the appearance of T2 star effects.
How a 180-degree refocusing RF pulse combats T2 star effects and improves MRI signal quality.
The process of capturing echoes through multiple 180-degree pulses to enhance MRI imaging.
Explanation of the time to echo (TE) and time to repetition (TR) in MRI sequences.
Conclusion of the overview with a summary of the basic MRI spin echo sequence.
Transcripts
hello my name is dr erin gomez and this
is a brief overview of magnetic
resonance and a basic mri spin echo
sequence
let's talk about protons
we have protons in the fat muscle and
sugars within our body and of course
within water
remember that a significant portion of
our bodies consists of water and that a
hydrogen atom is just a proton one
positron and one electron with a
positive and a negative pole
because of this each of these protons is
capable of acting like a bar magnet
usually the orientation of these protons
is random but they can be influenced by
an external magnetic field
at the most basic level an mri scanner
is a giant magnet and generates its own
magnetic field which we can call b0
when protons are placed within this
magnetic field they'll line up parallel
or anti-parallel to the primary magnetic
field with a small majority aligning
with the direction of the primary
magnetic field just going with the flow
this generates what is referred to as
the net magnetization vector
we can imagine this net magnetization
along the z axis the long axis or length
of the patient's body
in addition to aligning with the
magnetic field produced by the mri
scanner the protons in your body are
also spinning along their axes like
little tops or globes this is called
precession or nuclear spin
the speed or frequency of this axial
spin depends on the strength of the
applied magnetic field and can be
expressed by the larmor equation
simply put this equation states that the
precession frequency of a particle is
equal to the strength of the magnetic
field applied and the gyromagnetic ratio
which is a constant that is unique to
each specific nucleus or element
with the protons aligned with the main
magnetic field we can influence them
using externally applied radio frequency
or rf pulses when this happens the
protons are knocked down into an
alternate plane and also precessed
together in phase
the angle depends on the strength and
duration of the rf pulse
knocking the protons down into another
plane is a change in their longitudinal
magnetization
normally the majority of protons are
going with the flow and following the
direction of the external magnetic field
but with a little extra energy which we
can call excitation protons have the
ability to go against the current and
instead orient themselves in the
opposite direction against that of the
magnetic field this is called
anti-parallel
that's not all that happens with some
energy applied in the form of the rf
pulse the protons will also process
together in phase we can think of this
brief synchronization as the transverse
magnetization of the protons
to recap we've put some energy into the
system and temporarily convinced each of
these protons to sit down and get it
together
this doesn't last long much as if you
were knocked off of your feet or if i
yelled at my wild little children as
they ran haphazardly around their
playroom recovery is imminent
they'll behave for a short time but
they'll soon return my energy back to me
as the baseline state of disorder is
restored
much like my children the protons will
recover or return to their original
state of orientation with the magnetic
field and asynchronous procession
now that we've gone over what can happen
when we administer an rf pulse let's
talk specifically about what happens
during a typical spin echo sequence
remember the flip angle induced by an rf
pulse depends on the strength and
duration of the pulse
the thing being flipped is the net
magnetization vector at the beginning of
a standard spin echo sequence we apply a
90 degree pulse
this means that after the rf pulse has
been applied the net magnetization
vector is perpendicular to its original
orientation
this orientation is achieved by
eliminating longitudinal magnetization
and generating a transverse
magnetization vector by synchronizing
proton precession
during recovery longitudinal
magnetization increases and transverse
magnetization decreases the protons d
phase
this looks like a spiraling of the net
magnetic vector along the z axis
this spiraling of the net magnetization
vector induces an electrical signal by a
process called free induction decay
which is really just a throwback to the
high school physics principle of
inducing a current by rotating a
magnetic field search the depths of your
mind for the right hand rule
a few additional terms to note the
recovery of the longitudinal
magnetization of a proton occurs
exponentially
the point at which 63 percent of the
longitudinal magnetization has been
recovered is called the t1 time
the time at which 63 percent of the
transverse magnetization has been lost
is called the t2 time
the t1 and t2 time is unique to each
tissue type image
think about a class of children running
a foot race each will recover to their
baseline heart rate at a slightly
different time depending on their
physical fitness
we can take advantage of these unique
tissue properties and alter the mri
sequences to highlight them this is
called waiting and discussion of this is
for another time
that wasn't so bad was it seemed too
good to be true in a way it is there are
a few caveats and drawbacks to the
concept of free induction decay
number one it only applies to 90 degree
pulses
number two the signal decays very
rapidly and requires a very fast scanner
to detect
number three the dephasing of protons
occurs at a speed known as the t2 star
constant
this exponential decay in the
synchronization of proton spins is due
to the fact that each proton experiences
the magnetic field at a slightly
different strength meaning there is
never true uniformity in precession
these differences in precession end up
compiling leading to increasingly
asynchronous spins because each proton
already experiences the magnetic field
differently than its neighbors any in
homogeneity in the magnetic field makes
de-phasing and thus signal drop out even
worse
these are called t2 star effects
on mr imaging these t2 star effects can
appear as diffuse loss of signal or
black holes in areas where the magnetic
field is particularly distorted
because these effects are due to an
inhomogeneous magnetic field
we can liken them to distractions in a
child's environment
t2 star effects seem terrible isn't
there any way to fight them
fret not the answer is yes
the good news is that we can combat t2
star effects and their resulting signal
decay with the addition of another rf
pulse
to understand this we must remember that
although magnetic field in homogeneity
is inconvenient it is manageable in the
sense the differences in precession
speed that they cause are fixed and
predictable
as some protons lag behind their faster
counterparts we can apply a 180 degree
refocusing rf pulse that instructs all
of the protons to turn around and
process in the opposite direction
much like the classic tail of the
tortoise and the hair though the
tortoise is far behind the rabbit if we
ask both to turn around and head back to
the starting line of the race they'll
catch up to each other and arrive at the
same time due to the differences in
their speeds the crowd goes wild it's a
tie
when the proton procession sinks up
following the 180 degree pulse more
energy is released back into the system
this is called an echo and it is the
information collected by the mr scanner
which will eventually generate a medical
image
we can liken the 180 degree refocusing
pulse and the synchronous procession it
creates to an elementary school class
photo shoot
the teacher may need to raise her voice
in order to get the class to focus its
attention on the photographer and
achieve a yearbook worthy shot the echo
we can apply additional 180 degree
pulses to achieve multiple echoes photo
after photo after photo to continue
decreasing the t2 star effects
eventually however the students have
nothing left to give less and less
energy is yielded back with each echo
eventually dephasing occurs completely
and the echo dies out
once that happens the sequence must be
restarted again with another 90 degree
pulse
imaging in this manner is called spin
echo or fast spin echo imaging
we can use universal diagrams to depict
what happens with specific mr sequences
let's use one to recap the basic spin
echo sequence that we've just discussed
protons are aligned with the main
magnetic field b0 and are processing
randomly
a 90 degree rf pulse is applied
eliminating longitudinal magnetization
and producing a transverse magnetization
vector as protons process in phase
longitudinal recovery and transverse
decay occur producing a signal by a free
induction decay which is susceptible to
t2 star effects
a 180 degree refocusing pulse
temporarily rephases proton precession
producing an echo which can be read out
by the mr scanner
the moment that the echo is produced is
called the te or time to echo
we can apply multiple refocusing pulses
in an attempt to capture as many echoes
as possible the echoes become
successively weaker until the signal
dies out completely and the sequence
must be restarted
the time between repetition of sequences
is called the tr or time to repetition
that's all for now this concludes our
overview of magnetic resonance and the
basic mri spin echo sequence
تصفح المزيد من مقاطع الفيديو ذات الصلة
LX5V PLC MODBUS Communication (RS485) With VFD
The applications of eigenvectors and eigenvalues | That thing you heard in Endgame has other uses
Atomic Spectroscopy Explained in 9 Slides
"okay, but I want GPT to perform 10x for my specific use case" - Here is how
كيف تنجح وأنت متخاذل ضعيف الإرادة؟
ازاي تقبض ١٠٠ الف جنيه في الشهر من البيت ( 5 شغلانات)
5.0 / 5 (0 votes)