Lecture7

5 million cross section of tissue using cat cis 350

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Unformatted text preview: Even More Expensive ❚ PET - Positron Emission Tomography ❙ Requires cyclotron on premises ❙ Cost - $2 million ❚ NMR - Nuclear Magnetic Resonance ❙ Cost = $1.5 million Cross section of tissue using CAT CIS 350 Lecture 7 © Roxanne Hiltz and Eli Rohn 14 7 Spring 2000 - Lecture 7 Safety-critical systems: The Therac ❚ “Safety critical” systems are those that can cause death or injury if they fail; health, military, infrastructure such as power plants, airplanes and air traffic control ❚ The Therac-25 was a software-controlled radiation therapy machine used to treat people with cancer CIS 350 Lecture 7 15 Computer Failures- Therac 25 ❚ Between 1985 and 1987, Therac-25 machines at four medical centers gave massive overdoses of radiation to six patients. ❚ Intended dose was 100- 200 rads. Some received 13,000- 25,000 rads. Three deaths and several severe injuries resulted. CIS 350 Lecture 7 © Roxanne Hiltz and Eli Rohn 16 8 Spring 2000 - Lecture 7 Therac 25: What went wrong? ❚ Lapses in good safety design, insufficient testing, bugs in the software that controlled the machines, and an inadequate system of investigating and reporting the accidents all played a part. CIS 350 Lecture 7 17 Therac 25: What went wrong? ❚ Older machines had hardware safety interlock mechanisms, that prevented the beam from firing in unsafe conditions. ❚ Old software from these older systems was reused in the Therac 25, without testing; whereas the bugs in the software had not caused injuries when the physical safety devices were there, they did when the hardware safety devices were removed in the new model. CIS 350 Lecture 7 © Roxanne Hiltz and Eli Rohn 18 9 Spring 2000 - Lecture 7 Therac 25: What went wrong? ❚ Frequent malfunctions and error messages started to be ignored by the operators. ❚ For error conditions, the machine stopped and gave an error message; but the operator could turn it back on with one keystroke. ❚ Overconfidence: first victim told operator she had been burned, he said impossible... CIS 350 Lecture 7 19 Therac 25: What went wrong? ❚ Interesting bug: the flag to show incorrec...
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