Hair Replacement, Hair Transplants, Repair
110 East 55th Street, New York, NY
Telephone: 212.826.2400                        Bookmark This Page

Hair Restoration Home > Resources > Publications > Principles of Densitometry

PRINCIPLES OF DENSITOMETRY

[In preparation]

PRINCIPLES OF DENSITOMETRY

Robert M. Bernstein, M.D., New York, NY
William R. Rassman, M.D., Los Angeles, CA

ABSTRACT
Densitometry is a technique that analyzes the scalp under high-power magnification to give information on hair density, follicular unit composition and degree of miniaturization. It can be used to help evaluate a patient’s candidacy for hair transplantation and help predict future hair loss. Despite the utility of this technique, densitometry is still an art. The purpose of this paper is to lay the foundation for the scientific study of this important tool.

BACKGROUND
One of the earliest methods of measuring hair density was devised by Bouhanna, who used camera attachments to create a "phototrichogram,” an ultra close-up photograph of hair exiting the scalp. This method provided the capability to document the quality and quantity of hair shafts. However, the disadvantage to this innovation was that an assessment could not be done until after the film had been developed. [1-5] With more recent technology, digital trichograms allow the physician to take quantative measurements of hair shaft diameters and provide an immediate, permanent record of this information. However, digital equipment is expensive, particularly when a physician uses multiple consult rooms, and the instrument is too cumbersome for many clinical settings.

In 1993, Rassman introduced clinical densitometry to the field of hair restoration surgery, in order to quantify hair measurements, to increase the accuracy of the diagnosis of androgenetic alopecia, to anticipate its course, to define which patients are candidates for a hair transplant and to better assess a person’s donor hair supply when surgery is indicated. [6,7] The small, hand-held Densitometer makes densitometry easy to perform during a consultation.

The hair densitometer is a self-contained, portable, device that houses a magnifying lens and an opening of predetermined size. [Figure 1] The hair is clipped short (~ 1-mm) and the unit is placed on the scalp. An assessment is made from a standard 10mm2 field. Multiple measurements taken from different parts of the scalp are often helpful, particularly if there is significant variability from one location to another. [8] An advantage of the hand-held densitometer is that it is inexpensive and readily available to be used during the consultation, providing immediate information regarding a patient’s candidacy for surgical hair restoration.  

 
Figure 1. Rassman hand-held Densitometer
 
In 1993, Rassman, used densitometry to help determine the number of grafts that could be safely obtained from the donor area and showed that large sessions of small grafts could be performed on a practical clinical level [6,7]. In 1994, Dr. Limmer showed that the stereo-microscope could greatly improve one’s ability to dissect small micrografts from a donor strip without damage to hair follicles. [9] In 1995, Bernstein and Rassman used Densitometry to help define the new field of Follicular Unit Hair Transplantation by changing the focus of the surgeon from simply transplanting grafts of hair, to transplanting naturally occurring groups of follicular units [10]. [Figure 2]

In a series of publications on Follicular Unit Transplantation that followed over the next several years, Densitometry was used to evaluate a patient’s candidacy for the hair transplant procedure, to guide the surgical treatment of those with racially distinct hair characteristics, to improve the diagnosis and treatment of balding women, and to further define the conditions of diffuse patterned and unpatterned hair loss. [11-15]  


Figure 2. Natural occurring follicular units seen with a Densitometer


The widespread adoption of Follicular Unit Transplantation, with its ability to allow for large numbers of grafts to be safely transplanted in one session, dramatically changed the way modern hair transplantation could be performed. The procedure enabled thousands of follicular unit grafts to be transplanted in one session and complete hair restorations to be accomplished in just a few procedures. [10,16] However, with the ability of this new hair transplant technique to produce completely natural results, there is a tendency for doctors to become less discriminating in whom they are willing to treat.

As a result, patients are being transplanted at increasingly younger ages, those with only modest hair loss are being transplanted with megasessions and women with thinning hair are being recommended for hair transplantation almost as frequently as men (although much fewer are good candidates for surgery). Most disturbingly, some physicians are portraying surgery as an appropriate prevention for hair loss, rather than as a treatment to be used after more conservative measures have been tried. As a consequence, some doctors, or their non-physician representatives, are disregarding the normal protocols for proper diagnosis and are bypassing more conservative medical therapies.

In order to identify which candidates are suitable for surgery, which may respond best with medications alone, and who can be expected to have more stable hair loss – or become extensively bald – a careful history and physical examination should be performed. This evaluation should not be taken lightly, nor passed off to non-physicians, as the decision to proceed with a surgical solution for hair loss can have a profound impact on a person for the rest of his or her life. With its ability to offer more objective hair measurements, densitometry can play a useful role in this evaluation.

After the introduction of Finasteride (1mg O.D.) for the treatment of androgenetic hair loss in men in 1998, and its effectiveness in halting or reversing the miniaturization process, clinical densitometry took on added importance, as it provided a means for the physician to assess changes in the patient’s hair shaft diameter, rather than making judgments based solely upon changes in the patient’s appearance. About the same time, Propecia was gaining in popularity, digital trichograms started to appear on the market. However, instead of physicians taking advantage of this new technology, these “densitometer” devices were sold to hair salons and incorporated into a “sales pitch” for promoting healthy hair. These devices wooed the public, but lacked scientific metrics. They are now used mainly by nonprofessional’s in hair salons, rather than being incorporated into the diagnostic arsenal of the dermatologist.

In this brief paper, we wish to suggest that densitometry 1) has an important role in the diagnosis of hair loss, particularly in the doctor’s office, 2) can be helpful in monitoring the response to medications and 3) is useful in planning surgical hair restoration. [18] To date, no coherent or widely accepted body of information had been developed on the subject.

Presented are a number of assumptions and hypothesis regarding densitometry that seem to be clinically accurate, but have not yet been proven in rigorous scientific studies. For the purpose of this discussion, we will call these concepts the “Principles of Densitometry” with the understanding that these are not necessarily truths but ideas to be confirmed or disproved. We hope that this outline will stimulate further research and dialogue regarding this important subject

INFORMATION PROVIDED BY DENSITOMETRY

Objective

• Hair density – the # of hairs per unit area
• Follicular unit density – the # of follicular units per unit area
• Hairs per follicular unit – both the range (observed) and the average (calculated)
• Presence of miniaturization

Subjective with a Densitometer (Objective with a digital trichogram)

• Hair shaft diameter
• Degree of miniaturization
• Response to medication
• Growth after a hair transplant

Predictive

• Delineation of the permanent zone
• Stability of the “permanent zone”
• Extent of possible hair loss in androgenetic alopecia
• Rate of hair loss (via sequential measurements)

PRINCIPLES OF DENSITOMETRY

General

• Terminal scalp hair in man hair grows in follicular units i.e. “natural hair groupings” of one, two, three, four, and rarely five or more hairs. [10,17]
• The average follicular unit density is relatively constant and is approximately 1 unit/mm 2 (see racial variations below) [10,15,17,18]
• Hair density is correlated more with the number hairs per follicular unit than the absolute number of follicular units, so that an individual with high hair density has more hairs per follicular unit rather than a higher follicular unit density and those with low hair density have less average hairs per follicular unit, rather than follicular units spaced father apart. [10,15] [Figure 3]

Figure 3. Densitometry showing the natural follicular groupings in the scalp in a person with low density (left), average density (middle), and very high density (right). Note, that as the hair density increases, the average hair count in the follicular unit increases, but the density of follicular units remain relatively constant.

• Miniaturized hair can be differentiated from vellus hair.
• In patients with no miniaturization, there are two distinct populations of hair, terminal and vellus, with two distinct diameters.
• In patients with miniaturization there is a gradual diminution of hair shaft diameter from the full width terminal hair to the finest miniaturized hair at the end of it life cycle.

• “Mosaics” exist that have terminal hairs of varying thickness as a normal occurrence throughout the scalp.
• The number of small diameter hairs in a normal, non-balding adult male or female is less than 20% (this includes the vellus hair counts). [11]

Androgenetic Alopecia

• Under the influence of androgens, hairs randomly begin to miniaturize in each follicular unit, so that each unit will contain a combination of full terminal hairs, partially miniaturized terminal hairs and fully miniaturized hairs. Vellus hairs, when present remain unchanged.
• Miniaturization is the sine qua non of androgenetic alopecia. It is an inexorable process eventually leads to baldness.
• Focal miniaturization in the face of clinical thinning (to be differentiated from mosaics where there is no clinical thinning) is diagnostic of androgenetic alopecia and is not seen in other medical conditions or dermatologic disorders. [Figure 4]

Figure 4. Focal miniaturization in the donor area showing hair shafts of varying widths.

Stages of Androgenetic Alopecia

• In a person susceptible to androgenetic alopecia, the balding area first thins by focal miniaturization i.e. miniaturization affects select terminal hairs in the follicular units in a random manner throughout the affected areas.
• As androgenetic alopecia progresses, miniaturization gradually affects all of the hairs in the thinning areas, before hairs actually disappear.
• Once all the hair in an affected area is miniaturized, hairs gradually disappear in a random focal pattern and follicular units become reduced in number.
• In androgenetic alopecia, actual loss of hair follicles is a late event, with diffuse miniaturization accounting for the majority of the thinning seen clinically.

Predictive Densitometry

• In all adult patients, the donor area contains both terminal and miniaturized hair, indicating that this zone is not truly permanent but will thin gradually over time
• Miniaturization, as observed by densitometry, is a more sensitive indicator of early hair loss than clinical thinning.
• In more advanced hair loss, the final pattern of hair loss can be better predicted by measuring miniaturization in the donor fringe than by gross clinical observation.
• The greater the rate of progression of hair loss and the younger the age of onset, the more extensive the final pattern of hair loss.
• Most patients, who eventuate in a Norwood Class 7 pattern, have extensive miniaturization or clinical evidence of hair loss by their late teens or early 20s.

Diffuse Patterned Alopecia (DPA)

• Diffuse Patterned Alopecia (DPA) is an androgenetic alopecia characterized by diffuse miniaturization in the front, top, and vertex of the scalp in conjunction with a stable (non-miniaturized) permanent zone. [11,19]

Diffuse Unpatterned Alopecia (DUPA)

• Diffuse Unpatterned Alopecia (DUPA) is characterized by generalized miniaturization and lack a permanent zone of non-miniaturized hair on the back and sides of the head. [11,19]
• donor density < 1.5 hairs/mm 2 (in Caucasians)
• donor miniaturization > 25%

• In most DUPA patients, the diffuse miniaturization can be detected using densitometry before the age of 20.
• Early donor miniaturization (< age 20) portends DUPA
• In DUPA (in contrast to DPA)
• The progression is often more rapid
• The hair loss will be much more extensive
• Over time, the donor area in many DUPA patients takes on a see-through look, due to the high miniaturization and low density.

Women

• Female pattern alopecia is characterized by generalized miniaturization that presents as diffuse thinning over the entire scalp.
• In women, diffuse thinning (DUPA) occurs over 5 times as frequently as DPA.

Racial Variations

• The average hair density varies significantly among different races
• The average follicular unit density varies significantly among different races
• The predominant follicular unit groupings in Caucasians are 2’s and 3s.
• The predominant hair grouping in Asians 1’s and 2s.
• The predominant hair grouping in Africans is 3s.
• In general, there a relationship between the # of hairs (density) and size of the hair shafts (thickness); so that races with the thickest caliber hair have the lowest density (i.e., Asians) and those with fine hair have the highest density (i.e., Scandinavians).

Senile Alopecia

• Senile alopecia is characterized by a diffuse, uniform miniaturization of the donor region that occurs at an older age (not true "androgenetic" alopecia).

Finasteride

• Miniaturization can be reversed by Finasteride
• Balding may be reversed by Finasteride
• The greater the degree of miniaturization, the less likely it is that Finasteride will reverse the balding/thinning process.
• The longer that hair has been miniaturized, the less likely that the process will be reversed by Finasteride.

Other Unanswered Questions

• Is the decrease in hair shaft diameter seen with senile alopecia actually miniaturization?
• Is miniaturization in women the same physiologic process as seen in men?
• Do Norwood Class 5A patients eventuate in regular Norwood Class 6s or 7s?
• Are there specific areas of the non-donor scalp that are resistant to miniaturization and are these areas race dependent?
• The frontal forelock
• The frontal juvenile hairline
• Can an early anagen hair, a miniaturized hair and vellus hair be differentiated using densitometry?
• What is the incidence of DUPA in men and women
• Is there a forme fruste of this condition?

CONCLUSION

Densitometry is an important tool for the evaluation of hair loss, for assessing the response to drug therapy, and for assessing candidacy for hair transplantation. It helps establish a diagnosis of genetic balding and provides a metric for determining the response to medical treatment. Densitometry is useful in determining which patients are candidates for a hair transplant by helping to predict the eventual extent of baldness and by facilitating the assessment of a patient’s donor hair supply. However, densitometry is still an art with few scientific studies to support many of the basic assumptions that the technique rests upon. We are hopeful that these “Principles” will stimulate further thought and ultimately result in research that will more precisely define criteria on which this diagnostic tool is based.

REFERENCES

1. Bouhanna P. The advantage of phototrichogram in hair surgery. International Advanced Hair Replacement Symposium. Birringham, AL, February, 1982.

2. Bouhanna P: Phototrichogram: a technique for the objective evaluation of the diagnosis and course of diffuse alopecia. In W Montagna et al. (eds). Hair and Aesthetic Medicine. Roma, Salus Ed. 1983: 277-280.

3. Van Neste D, Dumortier M, De Coster W: Phototrichogram analysis: technical aspects and problems in relation to automated quantitative evaluation of hair growth by computerassisted image analysis. In Van Neste D, Lachapelle JM, Antoine JL (eds). Trends in Human Hair Growth and Alopecia Research. Dordrecht, Kluwer Acad. Pub, 1989: 155-165.

4. Van Neste D, de Brouwer B, De Coster W: The phototrichogram: Analysis of some factors of variation. Skin Pharmacology, 1994; 7:67-72.

5. Hayashi S, Hiyamoto I, Takeda K: Measurement of human hair growth by optical microscopy and image analysis. Br J Dermatol 1991; 125:123-129.

6. Rassman WR, Pomerantz, MA. The art and science of minigrafting. Int J Aesthet Rest Surg 1993; 1:27-36.

7. Rassman WR, Carson S. Micrografting in extensive quantities; The ideal hair restoration procedure. Dermatol Surg 1995; 21:306-311.

8. Bernstein RM , Rassman WR, Seager D, Shapiro R, et al. Standardizing the classification and description of follicular unit transplantation and mini-micrografting techniques. Dermatol Surg 1998; 24: 957-63.

9. Limmer BL. Elliptical donor stereoscopically assisted micrografting as an approach to further refinement in hair transplantation. Dermatol Surg 1994;20:789-793.

10. Bernstein RM , Rassman WR, Szaniawski W, Halperin A: Follicular Transplantation. Intl J Aesthetic Restorative Surgery 1995; 3: 119-32.

11. Bernstein RM , Rassman WR: Follicular Transplantation: Patient Evaluation and Surgical Planning. Dermatol Surg 1997; 23: 771-84.

12. Bernstein RM , Rassman WR: The Aesthetics of Follicular Transplantation. Dermatol Surg 1997; 23: 785-99.

13. Bernstein RM . Measurements in Hair Restoration. Hair Transplant Forum Intl. 1998; 8(1): 27.

14. Bernstein RM , Rassman WR. Dissecting microscope versus magnifying loupes with transillumination in the preparation of follicular unit grafts. A bilateral controlled study. Dermatol Surg 1998; 24: 875-80.

15. Bernstein RM , Rassman WR: The logic of follicular unit transplantation. Dermatologic Clinics 1999; 17 (2): 277-95.

16. Bernstein RM , Follicular Unit Hair Transplantation. In: Robinson JK, Hanke CW, Siegel DM, Sengelmann RD, editors: Surgery of the Skin, Chapter 34. Elsevier Mosby Inc., London UK. 2005:549-574.

17. Headington JT: Transverse microscopic anatomy of the human scalp. Arch Dermatol 1984;120:449-456.

18. Jimenez F, Ruifernandez JM: Distribution of human hair in follicular units: A mathematical model for estimating the donor size in follicular unit transplantation. Dermatol Surg 1999; 25:294-298.

19. Norwood OT. Male pattern baldness: classification and incidence. So. Med. J 1975;68:1359-1365.