Magnetic Resonance Imaging

---

Magnet

1.5 Tesla superconducting magnet

---

Superconducting wire

---

Cross sectional view of a superconducting MRI magnet.

---

Gradient Coils

(Z) antihelmholtz coil.

---

(X) figure-8 coils.

---

(X) figure-8 coils.

---

RF Coils

Resonant electrical devicces used to create the B₁ field and detect the signal.

Types: Transmit & Receive, Receive Only, and Transmit Only

n = resonant frequency
L = Inductance
C = Capacitance

B₁ must be perpendicular to B_o.

---

Multi Turn Solenoid

---

Surface Coil

---

Image of the lower human spine obtained with surface coil.

---

Flat circular surface coil with its connecting cable.

---

Surface coil molded to conform to the back of the knee.

---

Bird Cage Coil

---

Picture of the human head in a bird cage coil.

---

Single Turn Solenoid

---

Single Turn Solenoid around the human wrist.

---

Saddle Coil

---

Quadrature Detector

A device that separates the M_x' and M_y' signals from the M_x and M_y signal coming from the RF coil.

A laboratory to rotating frame of reference converter.

Inputs: n and n_o

Outputs: M_X'(t) and M_Y'(t)

---

Doubly Balanced Mixer (DBM)

Two Input Signals: Cos(A) and Cos(B)
Output Signal: 1/2 Cos(A+B) and 1/2 Cos(A-B)

---

---

Safety

No ionizing radiation, but...
strong magnetic fields,
radio frequency energy,
time varying magnetic fields,
cryogenic liquids, and
rapidly changing magnetic fields.

---

The USFDA recommends that the exposure to RF energy be limited.

The specific absorption rate (SAR) is the limiting measure.

SAR = Joules of RF / Second / kg of body weight = Watts/kg

---

MRI Safety Guidelines

Static Magnetic Field ( Bo )
Maximum clinical imager magnetic field	2.0 T
Maximum inadvertant exposure	0.0005 T

Rate of Change of Magnetic Field (dB/dt)
System		< 6 T/s
Axial Gradients	t > 120 ms	< 20 T/s
	12 ms < t < 10 ms	< 2400/t(ms) T/s
	t < 12 ms	< 200 T/s
Transverse Gradients		< 3x above

t = risetime of gradient

RF Power Deposition
Temperature (37o C normal)	Change	< 1 o C local
	Maximum Head	< 38 o C
	Maximum Trunk	< 39 o C
	Maximum Extremities	< 40 o C
Specific Absorption Rate (SAR)	Whole Body	< 0.4 W/kg
	Any 1 gram of tissue	< 8.0 W/kg
	Head (averaged)	< 3.2 W/kg

Acoustic Noise Level
LPeak	200 Pascals
Leq. averaged over 1 Hr	105 dBA

---

Copyright © 2000 J.P. Hornak.
All Rights Reserved.