您是否知道 MRI 最初稱為 NMR 或核磁共振成像。
他們不得不放棄稱呼"核"部分，因為這讓人們感到害怕，他們認為自己受到了輻射，但事實並非如此。
與 X 光和 CT不同，MRI 使用我們周圍隨處可見的磁力，並且在穿過時不會損壞組織。
我叫 Pauline，即將成為核子醫學住院醫師，在這段簡短的影片中，我將解釋 MRI 掃描的基礎知識。
MRI的磁場
核磁共振掃描儀本質上是一塊巨大的磁鐵。
磁鐵的強度以特斯拉為單位來測量。
醫院使用的大多數 MRI 掃描儀都是 1.5 或 3 特斯拉，從長遠來看，樹特斯拉 MRI 掃描儀的磁場比地球磁場強約 60 000 倍。
磁場越強，影像就越詳細，這意味著可以以更高的解析度或相同的解析度但更快地對身體進行成像。
MRI 使用磁場和無線電波來測量身體不同組織中的水分含量，然後利用這些資訊產生詳細的影像。
水分子中的氫原子的自旋磁矩
圖像是如此詳細。
因為我們的身體幾乎完全由淋巴結和血管組成，甚至我們的骨頭都浸有水分子。
水分子由兩個氫原子和一個氧原子組成。
氫原子是使水對 MRI 感興趣的部分。
如果我們更詳細地觀察氫，我們會發現它的中心核含有一個稱為質子的正電荷。
每個旋轉的氫質子就像是自行旋轉的小磁鐵。
這種旋轉運動稱為進動。
在正常情況下，所有數十億個氫質子都處於隨機位置，繞著其軸旋轉。
可以把它想像成一群孩子在操場上亂跑，毫無秩序感。
然而，當我們將人體置於非常強的磁場中時，就像指南針的北極和南極與地球磁極對齊一樣，這種隨機性就會改變。
在存在強磁場的情況下，水中的每個質子都會扭曲其方向，以便與磁場對齊。
現在想想那些沒有秩序的孩子，當老師叫喊時，每個孩子要如何排隊。
射頻脈衝
但是，雖然磁場使所有質子想要排列起來，但 MRI 機器會透過發送額外的較弱電磁場（稱為射頻脈衝）的短暫脈衝來有意破壞該磁場。
這種較弱的脈衝指向與磁場不同的方向，因此它會擾亂質子，使它們與常數不一致。
一旦射頻關閉，質子仍然回到原來的位置。
當它們這樣做時，它們會以電磁輻射的形式釋放能量。
為了測量這種發射的能量，需要一些稱為線圈的特殊設備，該設備放置在身體部位周圍。
我們將質子成像到不同類型的組織中，所有組織都發出不同量的能量，這就是為什麼電腦能夠區分不同類型的組織之間的差異。
非常感謝您觀看此視頻，如果您喜歡它並且想觀看更多我的教育視頻，請考慮訂閱我的頻道。
共振原理
核磁共振造影是隨著電子計算機、電子學、電路學、超導體等技術的發展而迅速發展起來的一種生物磁學核自旋成像技術。醫生考慮到患者對「核」的恐懼心理，故常將這門技術稱為「磁共振造影」。
核磁共振成像的「核」指的是氫原子核，因為人體大約70%是由水組成的，MRI即依賴水中氫原子。
當把物體放置在磁場中，用適當的電磁波照射它，以改變氫原子的旋轉排列方向，使之共振，然後分析它釋放的電磁波，由於不同的組織會產生不同的電磁波訊號，經電腦處理，就可以得知構成這一物體的原子核的位置和種類，據此可以繪製成物體內部的精確立體圖像。
原子核在進動中，吸收與原子核進動頻率相同的射頻脈衝，即外加交變磁場的頻率等於拉莫頻率，原子核就發生共振吸收，去掉射頻脈衝之後，原子核磁矩又把所吸收的能量中的一部分以電磁波的形式發射出來，稱為共振發射。共振吸收和共振發射的過程叫做「核磁共振」。
How does an MRI work? | MRI basics explained | Animation -- Dr. Pauline Moyaert
Did you know that MRI was originally called NMR or nuclear magnetic resonance imaging.
They had to drop the nuclear part because it scared people they thought they were being irradiated which is not the case at all.
Unlike X-ray and CT MRI uses a magnetic force which is found all around us and does not damage the tissue as it passes through.
My name is Pauline, I'm about to nuclear medicine residents and in this short video I will explain the basics of an MRI scan.
The MRI scanner is essentially a giant magnet.
The strength of the magnet is measured in a unit called Tesla.
Most MRI scanners used in hospitals are 1.5 or 3 Tesla, putting that into perspective, a tree Tesla MRI scanner is around 60 000 times stronger than the Earth's magnetic field.
The stronger the magnetic field the more detailed the images become, which means the body can be imaged either at a greater resolution or at the same resolution but faster.
MRI uses magnetic fields and radio waves to measure how much water there is in different tissues of the body and then uses this information to generate a detailed image.
The images are so detailed.
Because our bodies are pretty much entirely made of our lymph nodes are blood vessels and even our bones are soaked with water molecules.
A water molecule is made up of two hydrogen atoms and one oxygen atom.
Hydrogen atoms are the part that makes water interesting for MRI.
If we look at hydrogen in more detail we see that it has a central nucleus containing a single positive charge called a proton.
Each spinning hydrogen proton is like a tiny magnet that spins around on its own.
This spinning motion is known as precession.
Under normal conditions all the billions of hydrogen protons are in random positions spinning on their axis.
Think of it as a group of children on a playground running around with no sense of order.
However this Randomness changes when we place a human body into a very strong magnetic field just like the North and South poles of a compass sent to align with the magnetic poles of the Earth.
In the presence of a strong magnetic field each proton in water twists its orientation so that it aligns with the field.
Now think of those same children with no order and how each one will line up when the teacher calls.
But while the magnetic field makes all the protons want to line up, the MRI machine intentionally disrupts this field by sending a brief pulse of an additional weaker electromagnetic field called a radio frequency pulse.
This weaker pulse points in a different direction than the magnetic field and so it disrupts the protons so that they become misaligned with the constant.
As soon as the radio frequency pulls a Switched Off the protons still back to their original position.
And as they do they give off energy in the form of electromagnetic radiation.
To measure this emitted energy requires some special equipment called a coil that is placed around the body part.
We are Imaging protons into different kinds of tissues all give of different amounts of energy this is why computers are able to tell difference between various types of tissues.
Thank you so much for watching this video if you liked it and you want to see more of my educational videos please consider subscribing to my channel.
授課教師
陳永忠 ycchen@thu.edu.tw