21ST CENTURY MEDICINE: Image-Guided Surgery
Lynda Tolve has a brain tumor. The question for neurosurgeon Dr. Peter Black is, where is the tumor located? If it's too close to regions of the brain that control motor skills, the tumor cannot be removed. But how to find the tumor without cutting open the patient's skull? Radiologists at Boston's Brigham and Women's Hospital have pioneered a radical technique that maps the brain before the operation. Join FRONTIERS as the surgical team peers inside the brain and performs this procedure.
Background Information: About MRI
Activity 1: Inducing Magnetic Properties
Activity 2: Measuring Magnetic Force
BACKGROUND INFORMATION: ABOUT MRI
MRI (magnetic resonance imaging) uses a strong magnetic field and radio waves to create images of body tissues. This sophisticated noninvasive medical diagnostic technique utilizes the interaction between living tissues and magnetic fields. A patient undergoing MRI lies inside a cylinder that contains a powerful electromagnet. Within the magnetic field of the MRI, body tissues take on temporary magnetic properties. The magnetic field causes the nuclei in certain atoms inside the body to line up. Radio waves are then directed at the nuclei; each kind of tissue emits characteristic signals from the nuclei of its atoms. Radiologists measure the signals produced by the different tissues and use computers to translate these measurements into cross-sectional images. As you see on FRONTIERS, MRI is part of the diagnosis that helps surgeons locate the brain tumor.
ACTIVITY 1: INDUCING MAGNETIC PROPERTIES
- construction paper
- 2 strips of insulated bell wire: 15 cm and 2 m in length
- wire strippers
- knife switch
- lantern battery (6-volt cell)
- paper clip and other objects to magnetize (coins, nails, etc.)
Observe how magnetic properties can be induced.
- Cut an 8 1/2" x 11" sheet of construction paper in half lengthwise. Roll one section into a small, tight tube about 1 cm in diameter. Secure with tape.
- Strip both ends of the two wires.
- Wrap the longer wire around the tube. Keep about 20 cm free on both ends of the wire. As you form this tight coil, be careful not to crush the tube.
- Attach the smaller wire between the dry cell and one terminal of the knife switch. Connect one end of the longer coil wire to the unattached end of the knife switch and the other to the cell.
- To magnetize a paper clip, place it inside the tube. Close the circuit (switch) for no more than 10 seconds. Remove the clip. Test its magnetism. Then try magnetizing a variety of objects including steel nails, bobby pins, coins, various metals, etc.
- Why is it important to close the switch for only 10 seconds?
- How would you test the way the number of windings affects the induced magnetic properties?
- When you close the switch, you create a circuit without a resistor, producing a short circuit, allowing the current to increase and generating heat in the wire and battery, causing it to overheat. Both the wire and the battery can become dangerously hot; depending on the condition of the dry cell, if the switch is closed for too long, the cell's terminals can melt.
- Magnetize identical objects in the same way, but vary the number of windings (try 10 and 20). Then test and compare the magnetic force of these objects (ACTIVITY 2).
ACTIVITY 2: MEASURING MAGNETIC FORCE
- plastic straw
- steel washer
- paper clip
- index card
- protractor or compass
- magnetized objects from ACTIVITY 1
Measure the force of temporary magnets.
- NOTE: You may want to assemble this measuring device before making the temporary magnets in ACTIVITY 1.
- Cut a small section of straw about 5 cm long.
- Carefully make two small snips (about 0.5 cm deep) with scissors on one end of the straw.
- Insert the washer into these cuts; it should fit snugly.
- Use the compass point to poke a hole in the middle of the straw at right angles to the plane of the washer.
- Straighten one of the bends of a paper clip. Tape the clip to the edge of a table so that the straight portion extends off the table edge.
- Attach the straw assembly to the paper clip by slipping the paper clip through the hole in the middle of the straw. The straw assembly should swing freely.
- Tape an index card to the side of the table and trace out the arc formed by the washer movement.
- Use a pencil and a compass to mark this arc into 0.5 cm lengths.
- To measure the force of a magnet, place it next to the washer. Slowly move the magnet along the indicated arc. Record the position on the arc when the washer's attraction to the magnet is overcome by the weight of the washer. NOTE: If the magnet is too strong, add weights to the washer or increase the distance between the magnet and washer.
- Plot and compare relative strengths of the objects you magnetized in ACTIVITY 1. How long do various objects hold their temporary magnetism?
- How would you test the duration of the magnetic properties induced in a temporary magnet?
- Carpenters often magnetize the shaft and tip of their screwdrivers. Why? Would technicians who repair computers take advantage of this technique?
- Retest the strength of the induced magnet at regular time intervals. The information obtained will illustrate the relationship between time and loss of magnetic properties.
- So screws stay attached to the tip without having to be held, thus freeing up one hand. No; magnetic fields may damage computer software and electronic parts because they are magnetically sensitive.
- PHYSICS: Can you use this device to determine the actual force produced by the magnet? Yes. Using vector analysis, you'd have to determine what part of the washer's weight is directed away from the magnet. When the washer drops, the force of the magnet is overcome by the weight vector of the washer.
- DESIGN CHALLENGE: Using what you have learned about balance and magnetic attraction, can you build a device that detects steel or iron objects? (Hint: You'll need to assemble a delicate balance and magnetized pointing needle.)
- MEDICINE: Find out more about new advances in MRI technology. For example, one development is the use of real-time MRI during brain surgery.
- INQUIRY CHALLENGE: Design a method of inquiry that can be used to explore the polarity of the induced magnets. This inquiry should include relevant questions and controlled investigations into the orientation of induced poles, the effects of coil-direction wrapping and the shape of the induced magnetic field.
Scientific American Frontiers
Fall 1990 to Spring 2000
Sponsored by GTE Corporation,
now a part of Verizon Communications Inc.