Computers, Digital Radiography, PACS
1. A pixel is:
A. a picture element
B. one cell in a matrix of rows and columns that represent a digital image
C. a digital number representing the brightness of an image at one point in the image
D. none of the above
E. a, b and c
2. A CT scan is performed using a 256 x 256 matrix and 256 shades of gray. Approximately ______ bits of storage
are required for each slice:
3. Information will be destroyed in/on a ______ when the computer power is turned off.
A. Floppy disc
B. Hard disk
C. Magnetic tape
4. The minimum storage capacity required to store the information in a digital radiographic exam that consists of a
1024 – 1024 matrix, 12 bit deep, acquired at 30 frames per second for 10 seconds is _____ megabytes.
5. CT, digital radiography, and nuclear medicine image post-processing software can do all the following, except:
A. Reduce noise.
B. Enhance borders.
C. Produce artifacts.
D. Reduce motion blur.
E. Extend dynamic range.
6. An image matrix in a digital radiographic system is changed from 512 2 to 10242. Which of the following is true:
A. Fine vessel resolution will improve by a factor of 4 if the initial II, TV camera and monitor, focal spot, and
exposure times are appropriate.
B. Patient exposure per frame will increase by a factor of 2 to maintain the same level of quantum mottle.
C. The maximum allowed frame rate will decrease by a factor of 4.
D. System image data storage requirement will increase by a factor of 2 for the same number of images.
7. How many megabytes (MB) of disk space are needed to store a single raw image file consisting of 512 x 512
pixels with each pixel having a 16-bit pixel depth?
B. 0.50 MB
C. 0.75 MB
D. 1.00 MB
E. 1.25 MB
Medical Physics/computers dig rad pacs physics exam 2003/pf Page 1
8-11. For the following questions, select one of the computer storage devices (answers may be used more than once).
B. Magnetic tape
C. Hard disk
E. Optical disk
8. Which device has the longest access time?
9. Which device has the largest capacity?
10. Which storage device will lose information when the power goes off?
11.. Which device can you generally not write over again?
12. In order to transmit data over telephone lines to another computer, a computer system must have:
A. An icon.
B. Parallel processing.
D. A modem.
E. A CD-ROM.
13. The spatial resolution of a digital subtraction angiography system is better than:
B. Image intensifiers.
C. 100 mm spot films.
D. Fluoro TV systems.
E. CT scanners.
14. One byte of computer memory can store how many different possible integer values?
15. Digital images acquired from a 23 cm field-of-view fluoroscope are in 512 x 512 format. The image pixel size is:
A. 0.0196 mm
B. 0.22 mm
C. 0.45 mm
D. 22.3 cm
E. 23 cm
16. In picture archiving and communication systems (PACS), large matrix (digitized radiographic films and
computed radiography) display stations include all of the following except:
A. A magnification function.
B. Interactive window and level functions.
C. Ability to accurately measure the linear attenuation coefficient.
D. Ability to invert the gray-scale values of the displayed image.
E. Ability to rotate and flip the displayed images.
17-21. Concerning digital subtraction angiography: (Answer A for True and B for False)
17. X-rays exiting the patient are normally captured in a cesium iodide II tube to produce a visible image.
18. The TV cameras used for DSA normally have a signal-to-noise ratio of about 100:1.
19. The video signals from the TV camera are processed in a digital-to-analog converter to obtain a matrix of
numbers corresponding to the image.
Medical Physics/computers dig rad pacs physics exam 2003/pf Page 2
20. Systems with the greatest capability for artifact correction for re-masking are those which store the subtracted
image in analog form on a video disk.
21.. Compared with film subtraction images, electronically subtracted images normally have somewhat poorer spatial
resolution, but better iodine contrast resolution.
22. On a frame-by-frame basis, digital systems deliver about ______ times the cine dose.
A. 1/50 (very much less)
C. 1 (the same)
E. 50 (very much more)
23. Going from a 256 x 256 image to a 512 x 512 image, which of the following may be true?
Resolution Pixel noise Storage bytes
A. Decreases Decreases Increase x2
B. Increases Decreases Increase x2
C. Increases Increases Increase x4
D. Increases Increases Increase x2
24. A 256x256 matrix nuclear cardiology image displays 128 colors. How many bytes of storage are needed?
A. 32 KB
B. 64 KB
C. 128 KB
D. 512 KB
E. 1 MB
25. Concerning digital computers, all of the following are true, except:
A. ROM stands for Random Order Memory.
B. A Word is a set of consecutive bits treated as an entity, and occupying one storage location in memory.
C. A byte contain 8 bits.
D. A modem converts a digital signal into a frequency-coded signal for transmission over a telephone line.
26. The number of 512 x 512 images with 12 bit pixel values that can be stored on a 2 gigabyte optical disk is
approximately _______. (Assume integer byte values at each pixel location.)
27. In digital imaging, all of the following are true except:
A. 7 bits can represent 512 shades of gray.
B. The resolution of a 512 x 512 display matrix applied to a 10 inch II is 1.0 lp/mm.
C. A 256 x 256 image has 64 K pixels.
D. 1 byte is 8 bits.
28. Concerning computed radiography (CR), which of the following is true?
A. Numerous, small solid-state detectors are used to capture the x-ray exposure patterns.
B. It has better spatial resolution than film.
C. It is ideal for portable x-ray examinations, when phototiming cannot be used.
D. It is associated with high reject/repeat rates.
E. The image capture, storage, and display are performed by the receiver.
Medical Physics/computers dig rad pacs physics exam 2003/pf Page 3
29. Modern digital computers utilize a number of system of zeroes and ones which is called the _____ system.
30. The images acquired as in question 3 above are to be lossless compressed by 3:1 and then archived on CDs. If
each CD holds 800 MB, each CD can hold ______ images.
31. Digital image consists of 2400 x 2400 pixels, which represents a 16" x16" (40 x 40 cm) image. What
What is the smallest possible line-pair resolution pattern that can be resolved, in line-pairs/millimeter?
A 0.17 lp/mm
B 1.7 lp/mm
C. 3 lp/mm
D. 6 lp/mm
E. 30 lp/mm
32. Steps involved in producing a computed radiography image include all of the following except:
A. loading CR cassette in a darkroom
B. exposing a cassette in a manner like film/screen cassettes
C. Identifying patient on image
D. Scanning (readout) of exposed CR cassette
E. QA of image before printing or forwarding to PACS
33. Compared to film/screen, the main advantage of computed radiography using photostimulable phosphors is ?
A. Wider dynamic range
B. Less quantum motttle
C. Lower radiation dose
D. Better spatial resolution than film
34. PACS systems include all of the following except:
A. Digital archive (tape or optical disk jukeboxes)
C. Computer network
E. Display workstations
35. Methods of obtaining directly digital radiographs (no cassettes) include all of the following except:
A. Scanned projection radiography
B. CCD cameras
C. Image storage phosphors/CR
D. Selenium plates with readoutS
E. Flat-panel (flat plate) digital receptors
Medical Physics/computers dig rad pacs physics exam 2003/pf Page 4
Computers, Digital Radiography, PACS – answers
1. E The term pixel is properly used in each of the contexts described by A,B, and C.
2. D There are 256 x 256 pixels, each with a gray scale. To display 256 shades of gray requires 8 bits (256 = 2 8).
Thus 256 x 256 = 524,288 bits total.
3. D Only the RAM (random access memory) is erased when the computer is turned off. The other devices are
either permanently (ROM) or semi-permanently magnetized.
4. C Each pixel requires 12 bits. Each images consists of 1024 x 1024 pixels. Since a Kbyte is 1024 bytes and a
Mbyte is a Kbyte times a Kbyte, each images requires 2 Mbytes of storage. There are 30 images times 10
seconds or 300 images. The total storage required is: 300 x 2 Mbytes = 600 Mbytes.
5. E Dynamic range is determined by the acquisition system. Post-processing cannot extend data not recorded.
6. C There must be a reduction by a factor of four in maximum frame rate because there will be four times as
much data. The data transfer rate in the system does not change. Doubling the matrix size can double the
resolution if the other components are adequate. It is similar to doubling film or screen resolution.
X-ray exposure per pixel must be maintained in order to maintain quantum mottle. Since there are four
times (1024/512)2 as many pixels, the exposure must be increased by a factor of four.
7. B The number of bytes needed is 512 x 512 x 2 = 524,288 bytes or 0.5 MB; 16 bits equals two bytes.
8. B Magnetic tape may take tens of second to wind through the tape, an optical disk up to a few seconds, a hard
magnetic disk milliseconds, RAM microseconds. A CPU is not a storage device.
9. C Optical disks typically store up to 2.5–5.2 GB, hard disks up to a few hundred GB, tape up to tens of MB,
and RAM typically up to a few hundred MB. However, these numbers are constantly evolving.
10. A RAM is active memory requiring constant electrical power; a CPU is not a storage device.
11. E Many laser optical disks are WORMS – Write Once Read Many.
12. D An icon is a symbol in a system used to identify programs. Parallel processing is performing two or more
sets of computations or analyses simultaneously on the computer. The Ethernet is coaxial cable that links
the various computers in a network. CD ROM is a CD disk on which data can be stored.
13. E A standard DSA system using a 525 line TV is around 1.6 lp/mm. Typical CT resolution is 0.6-1.0 lp/mm.
14. C A byte consists of 8 bits, each bit can store two possible values. Thus, a byte can store 2 8 = 256 values.
15. C Pixel size is defined as the field-of-view of the imaging device divided by the number of pixel across the
image. In this case, pixel size is 23 cm / 512 = 0.45 mm.
17. A The x-ray image is captured in a high resolution II with a CsI input phosphor.
18. B Because the difference signal contains noise determined by the sum of the noise in the mask and the
angiogram, a very low noise TV camera is normally employed (SNR 500:1 or higher) in order to prevent
additional noise from being added to the image.
19. B The analog video signals go to an analog-to-digital converter (ADC).
20. B Image manipulation and enhancement is best achieved if mask and run angiograms are separately stored.
Medical Physics/computers dig rad pacs physics exam 2003/pf Page 5
21. A The resolution is limited by the II (4 lp/mm), which has lower resolution than screen-film systems (6 to 10
lp/mm). However, contrast enhancement (window / level) is easily achieved.
22. E Digital systems usually require an entrance exposure into the II of 1.0 mR/frame, whereas cine requires
only 20 to 40 µR/frame. However, many more frames (30 to 60 per second) are used for cine compared to
digital radiography (about 1 per second).
23. C More pixels of a smaller size means better resolution. This also means increased pixel noise, because there
are less x-ray photons per pixel, but this may not necessarily mean increased noise for an area of interest,
because the eye integrates pixels. However, the increased resolution will make the noise appear more
prominent. Storage requirements increase from 256x256 to 512x512, i.e., 4 times greater.
24. B 128 colors (or shades of gray) require 27 numbers (or 7 bits) to keep track of the color for each pixel. Thus 1
byte (8 bits) is needed for each pixel, as it is generally very difficult to break up a byte. Thus the total
number of bytes required is the number of pixels, or 256 x 256 – 65,536 bytes (64 KB).
25. A ROM stands for Read Only Memory: any storage medium to which data cannot be written by the system in
which it exists. It is used for storing instructions that are used repetitively without modification.
26. D 12 bits at each pixel will require 2 bytes x 512 x 512 pixels = 524,288 bytes = 0.5 MB per image.
Therefore, a 2 GB disk can hold exactly 2 x 1024 MB/0.55 MB = 4048 images. (Note that 1 kB = 2 10 bytes
= 1,024 bytes, not 1000 bytes. Also 1 MB = 220 bytes = 1,048,576 bytes, 1 GB = 230 bytes = 1,073,741,824
bytes, and 1 GB = 1,024 MB.)
27. A 7 bits = 27 = 128 shades of gray. The pixel size in the image from a 10 inch II displayed on a 512 x 512
matrix is 254 mm/512 pixels = 0.5 mm/pixel. Line pairs/mm = (1/2) x (1/pixel size) = 1.0. In computer
language K means 1024, not 1000, to 64 K = 65,536 = 512 x 512.
28. C Computed radiography has a wide dynamic range, in effect wide latitude. It produces acceptable films with
sub-optimal technique, such as those obtained with portable x-ray units. It uses image plates covered with
photostimulable phosphor (Europium activated Barium Fluoride) as a receiver. Resolution with screen-film
systems (5 to 10 lp/mm) is better than that obtained with CR (2.5 to 5 lp/mm). The reject rate in CR is low
due to post processing techniques available electronically. Unlike film, in CR image capture, storage, and
display are all performed by different components of the system.
29. C In the binary system there are only two possible values: 0 and 1.
30. C Since each image is 8MB, 100 could be archived without compression. With 3:1 compression, each image
takes up 1/3 as much space. Thus 3x100 or 300 images can be stored.
31. C 400 pixels/400 mm = 6 pixels/mm. Two pixels are needed to resolve a line-pair (one to see the bar and one
for the space), so 3 line-pairs/mm are possible. In practice, the bar pattern will not line up perfectly with the
pixels, so effective resolution is reduced by an amount estimated by applying a 0.7 Kell Factor.
32. A Darkrooms are not needed with CR. Not only are the phosphors relatively insensitive to ambient light, the
cassettes are loaded in an automatic reader/loader.
33. A Wide dynamic range is a main advantage of CR. CR uses approximately the same number of captured
photons to form the image, so noise is similar. However, CR is Aslower@ than rare earth screens (~200
speed compared to ~400 for Lanex screens) so more radiation is needed (2x). CR has less resolution than
film/screen systems in most cases, due to the matrix size.
34. D PACS uses computer monitors to view images, not viewboxes.
35. C CR uses cassettes as in film/screen cassettes, and involves a similar of handling and processing steps
Medical Physics/computers dig rad pacs physics exam 2003/pf Page 6