Unformatted text preview: Even
❚ PET - Positron Emission Tomography
❙ Requires cyclotron on premises
❙ Cost - $2 million ❚ NMR - Nuclear Magnetic Resonance
❙ Cost = $1.5 million
Cross section of tissue
CIS 350 Lecture 7 © Roxanne Hiltz and Eli Rohn 14 7 Spring 2000 - Lecture 7 Safety-critical systems:
❚ “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
❚ The Therac-25 was a software-controlled
radiation therapy machine used to treat
people with cancer
CIS 350 Lecture 7 15 Computer Failures- Therac
❚ Between 1985 and 1987, Therac-25
machines at four medical centers gave
massive overdoses of radiation to six
❚ Intended dose was 100- 200 rads. Some
received 13,000- 25,000 rads. Three
deaths and several severe injuries
CIS 350 Lecture 7 © Roxanne Hiltz and Eli Rohn 16 8 Spring 2000 - Lecture 7 Therac 25: What went
❚ 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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- Spring '14