and participate in a training session. Tim has been invaluable in the evalu-ation, use and troubleshooting of our tomographic equipment.
We had considered other imag-ing instruments such as the Fakopp Microsecond Timer™ (stress wave timer) and Tree Radar™. We have also used a Resistograph™ resistance drill for many years. Each technology has its own pros and cons and none of them are perfect. All of them require training in operating and “interpret-ing the data or images produced. This is not surprising, as wood is highly variable among species, and even between individual trees within a spe-cies. After critical review of the vari-ous technologies, we concluded that tomography provided the best, most
detailed and reliable information. Tomography also has a more impres-sive knowledge base behind it; having been the topic of many independent, peer-reviewed journal articles. I must also confess that I am an extremely fussy academically-oriented person who is not impressed by the newest technological gadget. I am suspect of technology when it is not supported by sound science.
ing, or supplemental support system, etc. Often these decisions are not clear cut; they depend upon many factors such as the tree owner’s perception and tolerance of risk, financial con-siderations and the value of the tree to the owner or community.
Prior to purchasing tomography equipment in 2010, I had followed its technological advancement for more than a decade. I have always thought that tomography was the most prom-ising of the available imaging meth-ods. I began to think that tomography might be useful to me in my consult-ing business. I asked my husband and business partner, Tim Ellis, a former electronic engineer, to research the
operation, performance, and accuracy of the instrument. Tim worked on bio-medical imaging equipment and soft-ware for 25 years and has an in-depth understanding of the various types of tomography and other imaging techniques – from both a hardware and software standpoint. In 2010 Tim traveled to Ontario, Canada to meet with the North American distributor of the PiCUS tomography equipment REE TOMOgRAPhy IS A
technology that can allow arborists to “virtually see” inside tree trunks or large branches to check for wood decay and/or deter-mine its location and extent within the stem or branch cross section where the scan is made. It can also be used to check for other defects that can affect mechanical stability. Most of the previous articles regarding tree tomography and other types of tree imaging systems have been written by the manufacturers or by research scientists testing the equipment or using it in their research. Therefore, I thought it would be helpful for arborists to read an article on the subject written by a consulting arbor-ist experienced in its use. I set out to present my experiences using this technology and to point out some of the pros and cons.
Like most tools, tomography works well in some instances but not so well in others. While it is not perfect, I’ve found that it can be very useful in imaging the interior mechanical structure of many trees, particularly large trees that pose a moderate or high risk to people. The use of tomography cannot determine whether a tree is “safe” or not, but it can provide useful information about the tree’s structural integrity. This information can then be used along with the information gathered during the basic risk assessment1 to help the consulting arborist provide a more accurate assessment of tree stability. This can aid in the decision process as to whether a tree should be removed or if the risk can be mitigated by
prun-Tree tomography— From a consulting
arborist’s perspective: Part 1
Deborah Ellis and Tim Ellis
The use of tomography cannot
determine whether a tree is ‘safe’
or not, but it can provide useful
information about the tree’s
1 Basic Assessment (of trees): a visual evaluation of the tree from the ground, using simple tools, without climbing into the tree
We purchased PiCUS™ sonic and electric resistance tomographic scanning instruments from Argus Engineering of germany in August of 2010. Since then we have been using the equipment to assess the structural stability of trees of various species. We chose the PiCUS equipment over other brands because PiCUS was the only manufacturer that offered both sonic and electric resistance tomo-graphic instruments. We have found that the use of sonic and electric resis-tance tomography together provides additional valuable information that is not possible using sonic tomogra-phy alone. We also like the fact that electric resistance tomography helps to overcome some of the limitations of sonic tomography, such as acoustic shadows that can be caused by cracks in the wood.
Tomography in a nutshell:
Tomography was developed in the early 1900s for use with X-rays. This technology is used to produce images of the interior of objects. In the 1970s with the advent of minicomputers the technology of Computerized Axial Tomography (CAT) and Magnetic Resonance Imaging (MRI) scans were developed. CAT scans use X-rays, while MRI uses strong magnetic and radio waves to “see” inside an object. Tomography produces three-dimen-sional images of an object constructed by computer via mathematical
algo-rithms, from a series of cross-sectional images constructed along an axis.
Sonic tomography of trees uses acoustic (sound) waves to measure relative differences in wood density. healthy wood is generally dense, while decayed wood is much less dense. hollow areas of a tree trunk have extremely low density because they are filled with air. The end prod-uct of tomography is the tomogram, a color-coded image showing the rela-tive density of the wood at the level of the scan. This is a representation of what a tree trunk or branch would look like if the section of wood was cut radially, and its relative density measured.
Sound waves are produced by tapping on nails inserted into the tree, through the bark and touching the wood. Sensors are attached to the nails. The nails and sensors are placed at specific intervals that correlate with the geometry of the tree (Fig 1a). Typ-ical spacing is about 10 centimeters apart on large trees with additional nails added to capture indentations and bulges along the periphery. To initiate the calculation of a tomogram, a sensor is removed from nail #1 and this nail is tapped with an electronic hammer (Fig. 1b). As this is done, all of the other sensors (e.g. #2, 3, 4 etc.) “listen” for the resultant sound wave. The time it takes for the sound waves to reach all of the sensors is
recorded. The process is repeated for nails #2, 3, 4 and so on. The process continues until all the nails have been tapped. Sound waves generally travel straight through and most quickly in sound (dense) wood, but around decayed wood or cavities, and thus take longer (Fig. 1c). The difference in time to reach the sensors is converted into an image of the density of the cross-section of the tree. Taps must be within a 10% velocity variance of each other; otherwise they will not be recorded. The instrument instructs us as to whether or not a tap is within the acceptable variance. The minimum number of recorded taps per nail is 3, but we have increased that to 5 for better accuracy. The instrument averages the velocity of the accepted taps and that is used in the tomogram calculation.
A limitation of sonic tomography is that cracks can cause “acoustic shad-ows” which reduce the accuracy of the portion of the image between the cracks. Sonic tomography measures variation in the density of the object, but it cannot tell you what caused the variation, such as reaction wood, decay, or a cavity. The training and experience of the operator combined with (in our case) the use of electric resistance tomography helps to clarify the nature of the defects. Post-mortem dissection of trees that we scanned that have been removed helps too. Despite the potential limitations of
Figure 1a. (Left) shows Tim placing sonic equipment on a severely leaning Italian stone pine in preparation of scanning. Figure 1b. (Center) shows a sensor nail ready to be tapped (circled). A sensor is attached to the nail to its left (arrow) and the sensor modules (boxes) are attached to a belt below.
Figure 1c. (Right) depicts the sound waves travelling through the tree after tapping. Each intersection of a sound wave produces a data point that is used in the calculation of the tomogram. Sound waves originated from only two nail tappings are shown.
cracks and acoustic shadows, we have found that the sonic tomograms still correlate surprisingly well with the actual interior of the trees that we have been able to dissect post-scan.
Electric resistance tomography
sends an electric current through the wood and measures electric resistance. This is affected by water content and ion concentration. high water content equals low electric resistance. Dry and hollow areas have high resistance as well. Normal, healthy trees of a particular species produce characteristic electric resis-tance tomogram “types”. Knowing a tree species electric resistance type can be used to differentiate between healthy and diseased and/or decayed trees. Electric resistance tomography can help to overcome some of the limitations of sonic tomography, such as acoustic shadows caused by cracks. For example, low electric resistance in an area that is questionable on a sonic tomogram means that there is materi-al in this area – because air (e.g. a cav-ity) has very high electric resistance. Electric resistance tomography can also help to clarify what a defective area in a sonic tomogram actually is – for example, high electric resistance in an electric scan combined with very low density in a sonic scan is probably a cavity.
We perform electric resistance tomography immediately after per-forming sonic tomography. The same tree geometry (Fig. 2a) is used and the sonic sensor nails remain in place. Electric resistance tomography requires more sensor locations how-ever, so we insert an additional nail between each sonic sensor nail. The number of electric resistance sensors is therefore double that used for sonic tomography. There is no nail tapping in electric resistance tomography – the electric sensor wires are hooked up to the sensor nails in pairs (a positive and a negative) the current is turned on and the instrument cycles through the sending of current from each pair of sensors and the reception of the electric signal from all of the other sensors. When the cycle has been completed the electric resistance to-mogram is calculated (Fig. 2b).
By comparing the tomograms to a photo of the cross-section of the coast live shown in Figure 4, we concluded the following:
Sonic tomogram: the majority of the interior of the trunk is much less dense than most of the perim-eter. In a sound coast live oak trunk the majority of the entire cross-section should be dense (black or brown). In old coast-live oaks (and
many other large-growing tree spe-cies) however, there may be a small amount of decay or cavity of the interior heartwood which does not affect the health or structural stabil-ity of the tree. In the field, we found that the interior of the cut cross-sec-tion was very soft and moist, much like florist’s foam. I was able to push my finger into it and pull it out, wet. The yellow lines indicate that there may be small cracks in the interior of the tree, which indeed there were. Although they are very small and do not extend as far outward as shown on the tomogram. As a default, the software places a crack symbol in the exact middle between the two sensors where an abrupt change in density, which could be caused by a crack, is detected.
Electric resistance tomogram: A healthy coast live oak should show a mostly black/red interior with mostly blue around the exte-rior. This indicates dead but intact heartwood in the interior, and a ring of healthy conducting wood around the exterior. In this case, the tomo-gram is quite blotchy, with most of the interior showing up as low resistance (wet and decayed) which indeed it was. There were some drier areas, for example the area between sensors #17 and 18. This is where the fruiting body (conk) of Ganoderma applanatum, a wood decay fungus, emerged. Decayed wood with fungal mycelium was found in this area. As indicated by the sonic tomogram, this area is drier than adjacent areas, and also very low in density. Another area which shows higher electric resis-tance is between sensors #7 and 8. Although the exterior of the tree at this spot appeared sound, the wood interior to it was decayed and a little drier. The dark red color along the perimeter was probably associated with reaction wood. This is also near where a large buttress root emerged from the trunk.
Comparing and interpreting
Figure 2a. (Left) Using electronic calipers to measure the geometry of the cross-section of the trunk of a valley oak tree (Quercus lobata) prior to scanning.
Figure 2 b. (Right) When the cycle has been completed the electric resis-tance tomogram is calculated.
ously a large interior defect in the trunk at the level of the scan, but no cavity. The reason we can tell that there is no cavity (even without using a resistance drill, increment borer, or dissecting the tree follow-ing removal, is that the majority of the interior of the tree has low elec-tric resistance, meaning that there is wood (wet material) present. Air filling a true cavity has very high electric resistance.
Combining the sonic and elec-tric tomograms with our basic field evaluation of the tree: The tree, located in a high school bike-parking area had basal trunk decay as indicated by mallet tapping and by a Ganoderma conk growing near the base. This was also verified by both tomograms. Furthermore, brown rot caused by sulfur fungus was evident in the large scaffold branches, where a large secondary branch had broken out long ago. Although tomography was not necessary to justify removal, we took advantage of the opportunity to scan the tree and then actually see what was inside as a part of our ongoing “dissection studies”. (Fig. 4 - 6)
nail from a starting point, using an electronic caliper (Figs. 7a-d). The instrument indicates which nails to measure and the distance between them. For a large tree this can take several different measurements. The shape of the cross-section is calculated
Tree geometry! Is very important!
The geometry (shape) of the cross-sectional area to be scanned must be accurately measured prior to gener-ating sound waves by tapping the nails. This is done by measuring the position and distance between each
Figure 3. (Left) Electric resistance sensors and sensor module box, running electric resistance scan of a large coast live oak. An arrow points to a fungal wood decay conk that has emerged near the base of the tree.
Figure 4. (Right) 4a shows the sonic tomogram of the trunk cross-section of a coast live oak shown below. Relative wood density is shown as decreasing from black (highest density) to brown, tan, green, violet, blue, and finally white. Figure 4b shows the corresponding electric resistance tomogram. Electrical resistance is highest at black, then red, orange, yellow, light blue and finally dark blue. In general, as moisture increases, electrical resistance decreases, but an increase in ion content can also decrease resistance. Figure 4c shows the cut trunk cross-section at the level of the scans.
Photos of the coast live oak that was tomographically scanned. Figure 5. (Left) The tree within the bike parking area.
Figure 6. (Right) Sulfur fungus conks can be seen emerging from an old branch failure wound with extensive decay.
Figure 4a RESISTANCEELECTRIC
Figure 4b Figure 4c
through the instrument’s software by triangulation. We also verify the calculated trunk outline in the field
trunk cross-section to make sure it matches up with the actual trunk geometry. Measurements are sent directly to the instrument via a blue-tooth interface which then calculates and produces the outline of the cross-sectional image. If the geometry of the tree is not accurate, then the tomogram will not be accurate either (Figs. 91-d). Large and irregularly shaped trees (which are usually what we scan) can take quite a long time to accurately measure.
Practical use for tomography in ar-boriculture— From our perspective, this technology is most useful for the
Large trees (those with a DBh greater than 24 inches)
High value trees
Trees where the consequence of failure are great
Trees that are considered “questionable” from a structural stability standpoint. For example, in cases where more information regarding structural stability is needed to determine what action needs to be taken.
In general, tomography and other interior tree imaging technologies are
not useful when:
it’s obvious during a basic tree assessment that a tree is structurally unstable and should be removed or the risk mitigated, or
the trees are of low value or pose little hazard to people or their property.
OUR KEY SPECIES:
Most of the trees that we’ve scanned are Coast live oaks. They are quite prevalent throughout much of the San Francisco Bay Area. Most com-munities here have been built within natural oak woodlands, so many still remain within the developed areas. Coast live oaks are also one of the largest commonly encountered trees in the area, and are often a safety con-cern due to their size and longevity. Tomography has allowed us to more accurately assess risk potential for our clients and avoid unnecessary removals.
By dissecting trees we had previ-ously scanned, following their re-moval due to stability concerns, we learned that the tomograms correlate well with the interior status of the tree at the level of the scan. We have also cross-checked several scanned trees using a resistance drill and found that these two instruments correlate well for coast live oak as well as other tree species. The sonic tomogram, which uses sound waves to produce the tomogram, tends to correlate more obviously with the actual interior sta-tus of the tree. The electric resistance tomogram, which measures electrical
Species: Quercus agrifolia
Trunk DBH: 33.5 inches Size (height x canopy spread in feet, estimated): 37x25
Vigor: 70% (Fair/Good) Structure: 20% (Poor) Preservation suitability: Poor Action: Remove
Reason: High risk Figure 7. Placement of sonic sensors
around the root collar where the tomographic scans were performed. The arrow points to the remnants of a
Ganoderma applanatum conk emerg-ing from near sensor #9.
Figure 8a. shows the electronic calliper measuring the geometry of the cross-section of a tree.
Figure 8b. shows measurements using one baseline, which will work for circular or near-circular trees.
Figure 8c. shows measurements using three baselines, which produce more accurate geometry for irregularly-shaped trees. Figure 8d. shows an actual calculated cross section of a coast live oak as produced by the instrument.
Figure 8a Figure 8b Figure 8c Figure 8d
which may be useful in determining whether or not the user is encounter-ing a typical range of various mea-surement values for this species are presented below.
Deborah Ellis has worked as an independent consulting arborist and horticulturist since 1984, af-ter graduation with a B.S. degree in Horticulture from Cal Poly State University, San Luis Obispo in 1981 and an M.S. degree in Plant
resistance through the scanned sec-tion, is often more difficult to interpret and takes much more practice to use effectively. The use of both images together (sonic and electric resistance) however, provides more useful in-formation about the tree than using either method alone.
We have developed a database to collect and interpret the numeric data produced for different species as we work with tomography. For example, our current data for coast live oak,
Protection and Pest Management from U.C. Davis in 1983. She is also an ISA Board-Certified Mas-ter Arborist, an ASCA RegisMas-tered Consulting Arborist, and a Certi-fied Professional Horticulturist with the American Society of Horticultural Science. Since 2008, Tim Ellis has served as technical adviser and engineer in charge of operation, trouble-shooting and maintenance of the tomographic equipment and software.
Figure 9. The sonic tomograms above and the photo of the scanned trunk cross-section show why correct geometry is important. Figure 8a. If the geometry were shown as a circle, the defect would be incorrectly shown as much larger than it actually is.
Figure 9b. An elliptic geometry also shows the defect as being too large. Figure 9c. The precise geometry however, shows the defect in correct context. Figure 9d. The actual trunk cross-section at the level of the scan.
Figure 10a. (Left) Typical for a healthy, defect-free young tree (in this case a coast live oak) with a trunk diameter (DBH) of 18 inches. The entire cross-section is nearly black, indicating sound, defect-free wood all the way through.
Figure 10b. (Center) The electric resistance scan showing a mostly blue perimeter and a red interior. Between sensors #1 and 3, red extends to the exterior. The reason for this can be seen in the cross-section photo at lower right (10c). There is an in-dentation in the trunk that was not captured in the cross-sectional geometry of the image. The electric signal passed through air which has high resistance. This does not show up on the sonic tomogram because fewer sensors are used for sonic tomog-raphy, and this was not caught in the image.
Dr. Larry Costello, University of California Cooperative Extension Horticultural Adviser, Emeritus. For his review of this article and assistance in the use of tomography for arboriculture.
Lothar Goeke, Physicist and Engineer, Argus Electronic, Rostock, Germany. For his review of this article and his initial and continuing assistance in the use of the PiCus tomography equipment and interpretation of tomograms.
Don Hodel, University of California Cooperative Extension Horticultural Adviser, Emeritus. For his review of this article and assistance in the use of tomography on palms.
Max Pinedo, Stanford University, California. Grounds Maintenance Supervisor. For his help in providing palms and other tree species to scan for practice.
Jess Running, Davey Tree Expert Company, Redwood City, California. For helping us in our initial dissection studies, alert-ing us to trees to be removed, and providalert-ing cross sections at the level of the scans for our inspection.
Jim Lewis, The Tree Team tree service company, San Jose, California. For helping us in our initial dissection studies, alerting us to trees to be removed, and providing cross sections at the level of the scans for our inspection.
Argus Electronic GMBH. Rostock, Germany. January 15, 2010. PiCus Caliper™ User Manual. Program Version Q72. Argus Electronic GMBH. Rostock, Germany. January 15, 2010. PiCus Sonic Tomograph™ User Manual. Program Version Q72. Argus Electronic GMBH. Rostock, Germany. January 15, 2010. TreeTronic™ User Manual. Program Version Q72.
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